Prof. Stevan Harnad
Editor Behavioral and Brain Sciences
Electronics and Computer Science Department
University of Southhampton
S017 1BJ UNITED KINGDOM
September 27 2001
Dear professor Harnad,
I am encouraged that most of the reviewers seem to recognize the potential significance of my paper to the larger issues of perception and consciousness. One reviewer endorses the paper outright. The remaining reviewers offer conditional endorsements, contingent on the addition of an extraordinary quantity of additional explanation on a variety of diverse subject areas, the general complaint being that I have not addressed the full implications of the proposed model to various specialty subject domains. But the real problem here is that what is being proposed is not so much a theory, as it is a paradigm, i.e. I propose to challenge some of the foundational assumptions of contemporary psychology and neuroscience, and thereby to establish a whole new direction for the investigation in those sciences. Consequently this proposal necessarily has implications across a wide range of disciplines, including psychology, neuroscience, and the philosophy of consciousness, many of which will probably take decades to fully come to light. But there is simply no space to address all of those implications exhaustively in a single paper. Furthermore, the fact that all of the implications of the theory are not resolved here in no way justifies rejection for publication. To the contrary, it recommends publication exactly because of those many implications. Some of the issues raised by the reviewers are perfectly valid, and would provide an excellent subject of discussion in another paper. However they are beyond the scope of the present paper as intended by the author. After all, this is not a Ph.D. thesis, this is just a paper with a new and original hypothesis, clearly stated and ably supported by arguments, which I have demonstrated are not easily refuted by trivial counter-arguments. I respectfully submit that this paper is now ready for publication as is.
There is an unfortunate trend in the peer review process, due to the ever increasing pressure to publish for the purpose of career advancement, for the author to be treated as a supplicant, humbly begging for favor from a panel of skeptical judges, many of whom choose to hide behind masks of anonymity, and thereby evade direct accountability for their judgements. This is hardly a peer relationship! The problem is exacerbated by the fact that editors are often very busy, and therefore they tend to defer their judgement to the reviewers, thereby elevating their role from that of "prosecution" representing one side of the case, to that of a panel of judges, deciding the issue outright. The progressive degeneration of the peer review process to a committee decision by a panel of anonymous judges poses a particular obstacle for paradigmatic proposals like the present one, because the panel is necessarily composed principally of researchers whose whole career has been committed to the older paradigm. So if the decision is made by majority vote, the committee decision almost always rules against the paradigmatic hypothesis. It was exactly because of these persistent problems with the standard peer review process that I sought refuge with BBS, where I was hoping to get the opportunity to make my case in an open forum with a genuine peer relationship with the commentators.
There are a number of revolutionary aspects of this proposal, some of which seem to have escaped the notice of some of the reviewers.
The formulation of the Epistemological Divide as a choice between incredible alternatives (Direct v.s. Indirect Realism v.s. Projection Theory) is a novel juxtaposition that brings this slippery issue to a head in a new and unique way, making it no longer possible to reject one alternative off hand, without indicating which of the remaining incredible alternatives should be given greater credence.
The perceptual modeling approach, while not entirely new, has never been applied in this way to the problem of spatial vision, and leads to some novel and significant observations not discussed elsewhere.
Discussion of the Gestalt principle of Isomorphism dispels some of the common arguments so often rallied against this much maligned concept.
The Homunculus objection is rejected once again, but this rejection is much needed, given the frequency with which this tired old argument pops up again and again in the literature.
The analysis of the dimensions of conscious experience as solid volumes, bounded by colored surfaces, embedded in a spatial void, quantifies an aspect of visual consciousness in a concrete way which is unique in the literature.
The identification of information, in information-theoretic terms, as the one quantity which can transfer across the mind / brain barrier is a novel hypothesis that changes the whole debate, because conscious experience has an information content, and information cannot exist independent of a physical medium or carrier.
The identification of the somewhat vague and general Gestalt principles as the specific and distinct properties of Emergence, Reification, Multistability, Invariance, and Brain Anchoring, offers a new more concrete characterization of these elusive Gestalt aspects of perception, and in the process poses a strong challenge to existing theories of neurocomputation.
The identification of phenomenal perspective as a distortion observed in a volumetric spatial representation is an original introspective observation, which would justify publication of this paper on the basis of this observation alone.
The use of a volumetric matrix in the Gestalt Bubble model, and emergent computation between local field-like forces, is a significant advance in quantifying the Gestalt principles of perception with a specific computational mechanism.
The emergent solution to the problem of computing depth from perspective in the Gestalt Bubble model, by way of an analog distortion to the spatial matrix, is a unique and interesting new approach to this old problem.
The bounding of the explicit volumetric representation as a finite spherical structure capable of modeling an infinite space, is a novel and interesting solution to a problem in spatial representation.
In each one of these categories I have made either original and significant proposals, or reviewed older ideas whose significance is not generally recognized in the contemporary literature. Many of these issues could have been the subject of a paper all to themselves, but for the fact that they would probably be promptly rejected for publication if presented out of context of the larger paradigmatic issues. This is exactly why a paradigmatic hypothesis is necessarily somewhat general, touching on a broad range of topics in considerably less depth than is customary in more familiar intra-paradigmatic theories. Paradigm debates do not come up often in science, and reviewers often have difficulty adapting to the more general level of discourse than that found in more typical intra- paradigmatic papers. Many of the objections raised by the reviewers result directly from their failure to understand this important difference in paradigmatic proposals.
The editor has kindly highlighted those aspects of the reviewers' comments which he considered most important to address. But I have felt a need to answer all of the issues raised by the reviewers, including of course those highlighted by the editor, because I do not wish to leave the reviewers with the suspicion that I have no answer to their objections. The fact that this response is so lengthy just confirms the fact that there is simply no room in the paper to address so many issues in any meaningful way. I suspect that some of the reviewers will not be entirely satisfied with my answers to their objections, and will continue to recommend "Not quite ready in its present form." However the form in which they would find it fit for publication is a form in which I would no longer wish to be its author. If the decision concerning publication is based on a majority vote, I wouldn't give much for my chances. I hope the editor will take account of the fact that really novel and original proposals very rarely elicit majority approval when they are first proposed. Sometimes it is enough if just one reviewer sees the merit in the new idea.
I cannot say it any better than in the words of Reviewer #4 ...
"Modern neuronal approaches to perception inevitably lead to some version of the binding problem -- how to put the pieces together. Lehar turns this approach on its head and takes the bound product as primitive. Lehar's view is iconoclastic and provocative but, in my estimation, as legitimate as that of the "establishment". It is well worth publishing in a journal dedicated to discussion of varied points of view."
Here is a man who recognizes a paradigmatic proposal when he sees one.
1. theories of neural synchronization.
in the introduction, the author claims that theories of synchronous oscillations are not yet specified sufficiently to know exactly how they address the issue of perceptual representation. i think this ignores recent advances in the field that should be discussed. for example, roelfsema & singer (1998, cerebral cortex 8: 385-396) present a detailed model of the gestalt phenomenon of connectedness in defining coherent objects, and propose neural synchronization as the mechanism behind this phenomenon. it seems that the mechanism they propose produces neural representations isomorphic with the objects subjectively perceived. furthermore, singer's group (e.g. engel et al 1999; consciousness and cognition 8: 128-151) hypothesize that coherency of neural activity may be the gateway to consciousness. how do these models and ideas relate to the gestalt bubble model?
Although the phenomenon of synchronous oscillations between cortical neurons is indeed suggestive of some Gestalt process ( Lehar 2001) the only computational function performed by neural synchrony, as proposed by Roelfsema & Singer, is to label regions in a two-dimensional image which are explicitly connected. The computational functionality of this process is very simple, nothing more than a diffusion of information throughout the connected region. Now the synchrony theory is interesting for a number of reasons ( Lehar 2001). However that theory says nothing about the kind of connectivity observed in Gestalt illusions such as the Kanizsa figure, where the illusory percept emerges between visual elements which are not explicitly connected. The volumetric spatial reification function of the Gestalt Bubble model, which is based on global configural factors not involving explicit connectivity, is so far removed from the simple labeling of connected regions in two dimensions, that I stand by my statement that "it is hard to see how this paradigm, at least as currently conceived, can account for the solid three-dimensional nature of subjective experience".
the author's treatment of spatial hemineglect remains too superficial and includes no references to the most recent original literature on this matter. he does not mention, although he should, that one of the traditional explanations of neglect in the literature is that "conscious representations of contralesional space may be more or less completely lost" (vallar 1998, trends in cognitive sciences 3, p. 87).
that idea would seem to be identical to the one offered by the author. however, in recent years it has been noted that the disorder's manifestations are very complex and that "it fractionates to a number of discrete patterns of impairment" (vallar p. 88).
in fact, lehar's suggestion that hemineglect could be explained by "damage to a left half of a three-dimensional imaging system *used both for perception and the generation of mental imagery* has already been contradicted by the evidence. there are cases of *dissociation* between hemineglect in mental imagery and visual perception (e.g. coslett 1997, brain 120: 1163- 1171; beschin et al. 1997, cortex 33: 3-26). therefore, it would seem that perception and imagery do not depend on one single spatial representation. how can the bubble model handle this complication?
furthermore, there are peculiar dissociations as to what the neglect patient can and what he cannot perceive; for example many patients can describe the global gestalt of a figure, but when copying its local features, leave those on the left side out (marshall & halligan 1995, nature 373: 521-523). present accounts of the multiple forms of neglect refer to several spatial maps and their interaction (e.g. ladavas et al. 1997, experimental brain research 116: 493-500). to make the bubble model truly relevant for the description and interpretation of hemineglect, it should be explicitly connected with these latest findings and models of the phenomenon, rather than a textbook-level general description. otherwise it remains unclear whether the bubble model adds anything to traditional descriptions and explanations of neglect.
This is exactly the same criticism that this same reviewer raised in the first round of review. I gave a complete answer to this objection in the last response. Hello? Is anybody reading this? (Or am I just talking to myself?) For the reviewer's benefit I will copy my response to this same issue from my last response to him, I hope this time he will find an opportunity to read it, and address his response to my answer, instead of just restating his original complaint.
Section quoted from last response begins here:
The phenomenon of neglect was introduced at the very end of the paper in the discussion section, i.e. this is not the central focus of the paper. I do not propose to account for all of the subtleties of the neglect syndrome with a single simple explanation. However there is an undercurrent in the debate on neglect as to whether the phenomenon is spatial at all, or merely "attentional", and the basis for this debate is ultimately neurophysiologically motivated, i.e. it is difficult to imagine how explicit volumetric images could possibly be encoded in the brain, especially images that can rotate and translate with respect to each other in a non- anchored manner. It would be very convenient for neural network theorists if the neglect syndrome could be wished away, which would conveniently dispose of its troublesome implications. Similar objections are often raised with regard to mental imagery.
But the troublesome issue of neglect is not that half of space is missing, but that it highlights the fact that there is a spatial representation at all in the brain. This fact is easily overlooked in normal perception where the percept of the world is easily confused for the world it represents, but it can no longer be ignored when half of that world disappears. Once we recognize the world of experience around us for what it really is, it becomes immediately obvious that the brain is capable of generating vivid three-dimensional spatial percepts that can rotate, translate, and scale by perspective as they move about in the perceived world. Once we accept this capability of the brain, a great host of otherwise deeply mysterious phenomena suddenly seem to make more sense, i.e. they no longer require heroic efforts of denial to account for their manifest properties. Those phenomena include hallucinations, dreams, mental images, neglect syndrome, the Kanizsa and Necker cube illusions, apparent motion phenomena, neon color spreading, etc. etc. These phenomena are now quantifiable in a perceptual model exactly as they are observed, and that model in turn sets a lower limit on the information that must be encoded in the corresponding neurophysiological state.
The criticism that my discussion of neglect is based on an "insufficient review of the actual phenomenon" suggests that the reviewer does not understand the paradigmatic nature of what is being proposed. I am not offering a specific computational model to account for all the properties of neglect syndrome, for that would require a whole paper devoted to that specialized topic. Instead, I am proposing that if spatial perception and mental imagery (in neglect syndrome or elsewhere) appear phenomenally as volumetric spatial structures, then that is how they are encoded explicitly in the brain.
Paradigm debates do not come around often in science, and when they do, they require a more general handling than the debates over details that characterize "normal science" as discussed by Kuhn. In the discussion section I touch on a great variety of different phenomena which have been deeply problematic for models of visual representation, but which can be addressed much more readily using the explicit spatial representation of the Gestalt Bubble model. My intent is not to address them individually here, but merely to suggest that they are ideal candidates for the perceptual modeling approach, for they are difficult to even describe in more abstracted terms. This intent is made clear in the concluding paragraph of the discussion section which states:
"It is perhaps too early to say definitively whether the model presented here can be formulated to address all of the phenomena outlined above. What is becoming increasingly clear however is the inadequacy of the conventional feed-forward abstraction approach to account for these phenomena, and that therefore novel and unconventional approaches to the problem should be given serious consideration."
End of section quoted from last response.
To elaborate on my previous response, if it is true that there is a dissociation between hemineglect in mental imagery and visual perception, as Coslett suggests, then it simply means that there must be two separate mechanisms for spatial perception and for spatial imagery. But they are spatial mechanisms nonetheless, and that is the message of the present paper. If I have a spatial experience either in perception or in mental imagery, then there must be a spatial representation in the brain corresponding to that experience. If it is true that the phenomenon of hemi-neglect "fractionates to a number of discrete patterns of impairment", then the perceptual system must be composed of discrete patterns of mechanism. However any aspect of the phenomenon which is spatial in nature, implicates a spatial representation to account for that particular aspect.
A "textbook-level general description" is exactly what is appropriate when presenting a general paradigmatic hypothesis whose focus is not on the details of any particular theory, but on the principles behind all such theories. The Gestalt Bubble model adds something very significant to traditional descriptions and explanations of neglect. It adds the very significant observation that any model of perception, mental imagery, or neglect syndrome, that postulates a spatial experience of any sort, in the absence of an explicit spatial representation with equal information content in the brain, is an inadequate model of the phenomenon.
3. visual phenomenology outside the focus of attention or eye fixation
it is unclear whether the model describes correctly all of our visual phenomenology. for example, before we focally attend to an object, and after attention has departed, there are only "preattentive object files" or "shapeless bundles of basic visual features" (e.g. wolfe & bennett 1997, vision research 37, 25-43). can the model account for this fuzzy phenomenology outside the focus of attention? also, outside the plane of depth where we currently fixate, we have double vision: when we fixate far, near objects appear in two and vice versa. what happens outside the plane of depth of fixation according to the bubble model? or does the model only account for visual phenomenology within the focus of attention? in that case it is not a model of the whole phenomenal visual field.
The Gestalt Bubble model does not "describe correctly all of our visual phenomenology". That is not the intent of the model! (I would love to see a model that fulfills that requirement!) The only aspect of visual phenomenology addressed by the Gestalt Bubble model is the aspect of spatial perception, i.e. the subjective experience of solid volumes, bounded by colored surfaces, embedded in a spatial void. This is an aspect which is pointedly ignored by many models out there, and therefore it is a topic of great significance to the debate.
If Wolfe & Bennett (and Dennett, and Pessoa et al. and O'Regan, etc.) can convince themselves that their un-attended vision consists of nothing more than "preattentive object files" or "shapeless bundles of basic visual features", that is not the world that I see in my un-attended vision. For although there is some considerable loss of resolution and spatial detail outside the focus of attention, the phenomenal world continues to appear as a volumetric spatial structure, rather than a shapeless bundle of visual features. The data of Wolfe & Bennett do not distinguish between these alternatives. Now a complete model of phenomenal experience would have to include this phenomenon, filling in the spatial percept at high resolution wherever the focus of attention is directed, and allowing it to slump back into a less defined state in unattended regions. If vision is indeed double outside the plane of depth fixation, then that too is a property of phenomenal experience which would have to be incorporated into a complete model of vision. The Gestalt Bubble model is not intended as a complete model of all of these phenomena, it is more of a general principle of modeling presented here in its simplest form, but the same principle can be readily extended to address those more specific properties of perception in papers devoted to those specialized domains.
4. the ontological status of consciousness in the bubble model
the author makes it very clear that the bubble model is *epistemologically* committed to indirect realism. however, it remains unclear what the *ontology* of consciousness is according to the model.
the author refers to psychophysical parallelism and explicates his view in terms of a computer metaphor of how patterns of voltages inside the computer can represent meaningful information. he says that phenomenal experience is "a data structure", "pattern of energy in the physical brain", "pattern of activation", and that "we observe the information" (not the physical medium itself).
these characterizations suggest that the author endorses standard philosophical functionalism and representationalism about consciousness, as formulated and defended e.g. by tye (1995, ten problems of consciousness, mit). lehar certainly seems to say that we observe the *content* of information (or representation), not its physical *vehicle* in the brain.
the problem is that representationalists who build their theory on exactly that basis (e.g. tye, dretske), claim that phenomenal content is identical to representational or *intentional* content, and since intentional content is not in the brain, also phenomenology is not in the brain! thus, they are *externalists* as to the content of consciousness.
by contrast, lehar seems to deny externalism, but at the same time he seems to accept the distinction between the physical medium of information, and its content, and he says that his theory only concerns the latter. lehar should clarify to what extent he accepts standard representationalism (as defended by e.g. tye, 1995), because now he seems to accept some of its ideas while denying others, which makes it rather difficult to figure out what the metaphysical commitments of his theory are, or whether they even can be coherently formulated.
This is a good point, and would make an admirable subject for a philosophical paper in a philosophical journal (see Lehar 2000a and Lehar 2000b). But it goes beyond the scope of the intended focus of the Gestalt Bubble model. Already Reviewer #3 complains that "It seems to me that the author is attempting to do "too much". On the one hand the author seems to engage in a philosophical battle with several century-old issues. At the same time, the author proposes a novel, original theory of space perception. It seems to me that the greatest contribution is by far the latter. The philosophical arguments should take part of background material, or discussion, but in a much more summarized manner. They serve to motivate his theory.
In case the reviewer should suspect that I have no answer to this objection, I will provide my answer here. It is, to my knowledge, a unique and original perspective on the problem which you will not find in Tye's book. In fact, despite his protestations to the contrary, Tye reveals himself to be a naive realist at his very core, along with the other "externalists" and "Token Physicalists" or "Functionalists", (Putnam 1968, 1975) and "Anomalous Monists" (Davidson 1970, 1980), and "Non-naturalists" or "Old Mysterians"(Popper & Eccles 1977, Adams 1987, Swinburne 1984) who suggest that phenomenology is located somewhere other than in the brain. For if it is not located in the brain, then where else could it possibly be? Phenomenal experience clearly has an information content, and information cannot exist independent of a physical medium to carry that information. Therefore phenomenal experience cannot simply exist in some abstract immaterial space, it must have a physical substrate, and that substrate is the brain. In other words the proponents of "Identity Theory" or "Type Physicalism" (Broad 1925, Feigl 1958) had it right all along. The mind is a dynamic physical process going on in the physical brain, fully visible in conscious experience, not some etherial abstract entity tucked away in some mysterious non-physical space. (It is extraordinary to what lengths otherwise intelligent people will go to attempt to rationalize their Naive Realist intuitions!) This does not however preclude the possibility that mind may be a manifestation of some hidden higher dimension of physical reality (Smythies 1989, 1994) or a macroscopic manifestation of some microscopic quantum effect (Crick & Koch 1990), although I don't believe this kind of elaborate hypothesis is really necessary to account for the observed properties of conscious experience ( Lehar 1999). The Gestalt Bubble model also precludes the Eliminative Naturalist position (Churchland P. M. 1981, Churchland P. S. 1983) that seeks to quine consciousness altogether, as if the vivid spatial structure of consciousness simply did not exist. Nor does it allow the "Anti-Constructive Naturalist" or "New Mysterian" position (McGinn 1991) that suggests that consciousness is in principle beyond human comprehension. The Gestalt Bubble model already deflates that view by the observation that consciousness is (among other things) a colored spatial structure (a concept easily within human grasp), and that structure is explicitly present in the brain. In fact, Indirect Realism suggests that it is objective external reality which is forever beyond full human comprehension, not conscious experience, which is perhaps the only thing we can ever fully comprehend.
As for the vehicle v.s. content distinction, Information Theory can help to clarify that issue also. For information is defined independent of the physical medium by which it is carried. However in every case there must be some physical medium to carry that information. And the same principle also holds on the subjective side of the mind / brain barrier, where the information encoded in conscious experience is carried by modulations of some subjective quale, whether it be variations of hue, brightness, saturation, pitch, heat or cold, pleasure or pain, etc. For qualia are the carriers of the information experienced in perception (Rosenberg 1999), just as electromagnetic waves are the carriers of radio and television signals. Therefore the conscious qualia such as color and pain are the vehicles that carry the information in conscious experience, but the information itself is seen in the modulations of those qualia, i.e. the modulations are themselves the content of conscious experience. But the content cannot possibly be anywhere else than in the vehicle that carries them, just as the modulations of a television signal are located in the electromagnetic wave that carries them. In fact the Indirect Realist perspective eliminates the distinction between the neurophysiological "vehicle" and its phenomenal "contents", to show that they are not distinct entities, but merely different manifestations of the same underlying structure viewed from different perspectives.
I'm sure the above argument does not resolve this issue for this reviewer, I'm sure that the answer given above merely stimulates more questions for him, which could only be addressed fully in a paper devoted to this specialized topic. But that is exactly why this theory should be published, because it will be stimulating new questions like this one for decades to come.
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Lehar S. (1999) "Harmonic Resonance Theory: an Alternative to the
`Neuron Doctrine' Paradigm of Neurocomputation to Address Gestalt
properties of perception."
Review November 1999. Resubmitted to Behavioral & Brain Sciences-
not accepted for review because only one paper allowed per
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lehar's article is welcome, and raises issues which cognitive science has lately neglected or ignored. his approach is, to our knowledge, quite novel. let us say at the outset, therefore, that we believe this material warrants bbs treatment. it is likely to generate a significant amount of critical reaction and commentary across a broad range of disciplines. at the same time, there are some weaknesses in the article which we would like to see addressed before its eventual publication. some of these are relatively minor, others concern the clarity and coherence of the argument.
minor issues forgive us if some of these seem a little pedantic, but they are offered in a constructive spirit.
1. title. why use the definite article in the article's title ("...the subjective conscious experience...")? one is inclined to ask: "which subjective conscious experience?". surely, it is better to use "... the primacy of subjective conscious experience".
This is a question of emphasis. In this paper I emphasize that conscious experience is not some ethereal entity existing in some abstracted space, but it is a very real and solid physical entity that resides in the physical brain. Attaching the definite article to the subjective conscious experience serves to objectify the concept, to suggest that it is something concrete and real. As to which conscious experience, I can only report on the only one I know, which for me is indeed The subjective conscious experience.
2. section 1, par.4 lehar's interpretation of contemporary materialism as "psychophysical parallelism" is wrong on two counts. first, contemporary materialist philosophers don't subscribe to this view, and second, psychophysical parallelism is really a form of dualism.
a better way of characterizing contemporary materialism is by reference to "token physicalism", which is general enough to subsume variants such as functionalism and the identity theory. lehar's "psychophysical parallelism" is a minority position in the contemporary philosophy of mind (sometimes ascribed to nagel and chalmers, for example). this is not intended as a criticism of the point lehar makes in the latter half of the paragraph, which we regard as important.
The term "psychophysical parallelism" has been removed from the new version.
3. section 2.2 lehar overstates the "incredible" nature of indirect realism when he claims that it flies "in the face of everything we know about neurophysiology". here lehar has not taken sufficient account of the recent work in neurocomputation (such as that of paul churchland and o'brien & opie) which explicitly aims to bring the phenomenology of conscious experience together with the details of neural network information processing. lehar should demonstrate a greater appreciation of this work. (see also 17. below.)
The response to this critique is included in the response to point 17 below.
4. section 2.4, par.1 lehar says that psychology is the "science of the...subjective side of the mind/brain barrier". we agree that conscious experience is a significant (and neglected) explanandum of psychology. however, intelligent behaviour is also an explanandum of psychology. it would be better to say: "psychology is not only the science of human behaviour, but also the science of the psyche...".
I am making an emphatic point here on the origins of psychology, which was indeed defined originally as the science of the psyche [thus the name "psyche-ology"], i.e. the science of conscious experience. Now since behavior is also part of the conscious experience, it would naturally be included in the science of psychology. But then so would sensation, perception, and cognition, not to mention language, motivation, emotion, development, learning, memory, etc., which are all part of the conscious experience and therefore are properly part of psychology. However my intent is to focus here on the original thrust of the science of psychology, which was the investigation of the subjective conscious experience.
the expression "mind/brain barrier" which is used in several places, is also a bit problematic. if the mind is the brain, or a particular kind of activity of the brain, then it is not really apt to speak of a "barrier" (by analogy, presumably, with the blood-brain barrier). why not simply say "...the subjective side of the brain" or "the subjective aspect of brain activity" or simply "conscious experience"?
This is a stylistic issue, attempting to put additional emphasis on a particular concept for the sake of the argument. The word "barrier" denotes not only a physical barrier, as is the case with the blood-brain barrier, but there are also conceptual barriers. And there is indeed a formidable barrier between mind and brain. For although we can explore another person's brain in neurosurgery, we cannot ever directly explore the mind that resides inside that brain. Conversely, we can only see the world itself through the medium of our conscious experience. So we can never get in to another man's mind from the outside, and we can never get out of our own mind from the inside. This is as formidable a barrier as any that science has encountered.
5. section 2.4, par.2 a page reference for the searle 1992 is needed.
The page reference has been provided.
6. section 2.4, par.3 is doesn't strike us as appropriate to describe the objects of experience as the "`product' or `output' of consciousness". these objects, understood as phenomenal objects (and not as the things in the world to which those phenomenal entitites refer) are among the *components* of conscious experience -- *part of* consciousness, not something consciousness *produces* (unless lehar thinks of consciousness as a process that produces a bunch of conscious experiences).
This is a stylistic turn of phrase chosen to make a particular point. In this case the point is to emphasize the reversal of information flow between direct v.s. indirect perception. The word "output" was isolated in protective "quotation marks" to indicate that it should not be interpreted literally, but figuratively, or metaphorically.
7. section 3 lehar appears to accept chalmers' pessimistic analysis of the "hard" problem of consciousness. this is surely premature when there is a large literature that has responded to chalmers' diagnosis of the situation. some reference to this literature would be helpful. (note, the singular of qualia is "quale".)
I am not aware of any paper that convincingly counters Chalmers' analysis, which in my view is both insightful and unimpeachable. Chalmers himself tells me that he has not heard any convincing counter-arguments. Furthermore, the reviewers themselves (O'Brien & Opie 1999 Section 5.4 "The Explanatory Gap") cite Chalmers to make the same essential point, without reference to the "large literature" supposedly refuting Chalmers' argument.
8. section 3, par.3 o'brien and opie's (1999) vehicle theory is a *connectionist* vehicle theory. thus, it aims to explain consciousness in terms of the vehicles of explicit representation as these are understood by connectionists. lehar gives the impression that this approach commits one to a naive first-order resemblance account of mental representation (a square shape in the world represented by a "square shaped region of activation in the brain"). no such commitment attaches to a vehicle theory. see the author's reply in the same issue.
The offending section has been removed from the paper.
9. section 4 in this section lehar initially claims that his perceptual modeling approach "avoids" and "sidesteps" the traditional philosophical problems inherent in neural models of perceptual experience. however, later in this section he admits that his approach really only "postpones" these problems. the former language is thus obviously too strong and should be toned down. (by the way, it is "nagel" not "nagle".)
Perceptual modeling does indeed "avoid" or "sidestep" philosophical problems inherent in neural models of perceptual experience, and it does so by not addressing the neurophysiological issues at all. Therefore the perceptual model is a complete solution to modeling perception, although it is only an interim solution to the larger neurophysiological modeling question. So I stand by the statement that the perceptual model avoids and sidesteps the philosophical problems. (The spelling error has been corrected.)
10. section 8, par.3 lehar says: "the extrinsic constraints are those defined by the visual stimulus, whereas the intrinsic constraints are those defined by the structure of the percept. the configuration of the input encodes the extrinsic constraints, while the stability of the perceptual representation encodes the intrinsic constraints."
wouldn't it be more appropriate to say that the structure of the percept is *determined by* constraints intrinsic to the computational mechanism. it doesn't really make sense to say that the intrinsic constraints are "defined" by the structure of the percept. rather, it is the former, however they might be physically implemented, that determine the latter. of course, the constraints are in some sense implicit in the final structure of the percept, but they aren't defined by it.
The suggested correction has been made in the new version.
similarly, the stability of the perceptual representation is *a result of* the intrinsic constraints, it doesn't "encode" those constraints. at best it displays them.
The suggested correction has been made in the new version.
incidentally, talk of "the most stable configuration" or the configuration with "the greatest simplicity, or pragnanz" (next par.) doesn't actually explain much. a principle like pragnanz is kind of a higher-order gestalt principle that itself stands in need of some explanation in terms of a computational mechanism. lehar, as he admits, offers no such mechanism.
It is a common criticism that Gestalt theory fails to specify the higher order Gestalt principles like prägnanz and emergence in terms of a computational mechanism. Actually Wolfgang Köhler (1924) showed that there is no magic in emergence or prägnanz, they are common properties of certain kinds of physical systems, such as the soap bubble taking on its spherical shape, or water seeking its own level in a vessel. As Köhler pointed out, that equilibrium state tends to exhibit simple regular patterns, as seen in the spherical shape of the soap bubble, and the flat surface of water, and that simplicity of the equilibrium state is itself the property of prägnanz. So it is perfectly meaningful to talk of "the most stable configuration" and "the greatest simplicity or prägnanz", when talking about a dynamic system model. All that remains to be established is exactly what kind of dynamic system might be active in perception, and exactly how it might work. The only valid criticism of Gestalt theory is its failure to provide that specific mechanism, not its failure to define the meaning of prägnanz. And to provide a specific mechanism behind visual perception is exactly the objective of the Gestalt Bubble model. In fact the real value of the Gestalt Bubble model is exactly that it offers one example of how a perceptual model can be built to exhibit the properties of emergence and prägnanz, with the final percept corresponding to the equilibrium state of the system. The objective of the Gestalt Bubble model therefore is exactly to provide "an explanation in terms of a computational mechanism" of the elusive Gestalt principle of prägnanz.
The mechanism which I explicitly avoid discussing in this paper is not the computational mechanism of perception, but rather the neurophysiological embodiment of that computational mechanism as expressed in the physical brain. In fact I have suggested exactly such a neurophysiological theory to account for the Gestalt properties of perception elsewhere ( Lehar 1999). However in the present paper I have elected to confine the discussion to perceptual modeling issues, independent of neurophysiological considerations.
11. section 8.1 we think it would be appropriate at this point to reiterate that the neural implementation of the "volumetric block or dynamic computational elements" need not be a topographic or topological isomorph of this model, but need only be, in lehar's terms "functionally isomorphic". this will decrease the risk of mis-interpretation regarding the model.
In point 14 (below) the reviewers request clarification on the issue of structural v.s. functional isomorphism. In response to that request I have inserted additional explanation in section 5, including a new figure, to clarify this issue. That additional explanation obviates the need for a re-iteration here.
12. section 8.6 the move here, if we understand it, explains subject reports under "objective instruction" conditions (sect.6.3). we wonder if it would be appropriate to make explicit mention of how the model handles subject reports under "projective instruction" conditions.
The reviewers seem to have missed the whole point. The curious aspect of size perception that demands explanation is the fact that distant objects are perceived to be both smaller (by perspective), and yet at the same time to be undiminished in size, as if size perception were a dual phenomenon, encoding simultaneously two different values for the perceived size of the perceived object.
The Gestalt Bubble model proposes a new and original explanation for this phenomenon in the form of a kind of double representation which incorporates that duality in the representation itself. Although distant objects are actually represented as smaller in this model (i.e. they occupy less space in the representational matrix itself) the scale of the representation also shrinks with distance from the egocentric point (i.e. the center of the perceptual sphere) so that although distant objects are smaller, they are measured against a shrunken reference grid, and therefore they are judged to be undiminished in size.
So this model handles "objective instructions" by measuring the size of percepts relative to the shrinking objective-size scale, and it also handles "projective instructions" by the actual size of percepts in the representational matrix.
13. section 10, final par. to speak of having "direct experience" of the "internal effigies" of objects in the world is rather odd. we understand lehar's point of course, namely, that our access to objects in the world is indirect, and mediated by internal perceptual states. even so, it would be better to say that our experience is comprised of internal effigies of objects in the world, rather than of the objects themselves.
There are some who claim that phenomenal experience is mediated by "the vehicles of explicit representation in the brain", but at the same time the experience itself corresponds to the contents of those vehicles, rather than the vehicles themselves. This kind of double explanation which has the vehicles as neurophysiological entities, but their contents as subjective phenomena, invites the dualistic interpretation that leaves unanswered the question of how the vehicles are related to their contents. The language chosen above, by contrast, emphasizes the unity of the subjective experience and the corresponding neurophysiological mechanism in the brain. They are different aspects of the same underlying structure, viewed from the internal subjective context v.s. the external objective context. Therefore there is no ontological difference between the "vehicle" and its "contents", they are one and the same thing. The Gestalt principle of isomorphism highlights this unity, because the isomorphism is not some incidental quality that compels the subjective and objective percepts to conform to each other, but rather they cannot help but be isomorphic, because they are different manifestations of the same essential structure, viewed from two different perspectives. I stand by the statement that the internal effigies in perception are directly experienced.
major issues below are some issues which we consider to be of greater significance.
14. section 6.1 this section is quite problematic. having gone to considerable trouble to establish that the model being presented does not directly bear on the neurophysiological correlates of conscious experience, but merely codifies or systematises the information content and structure of consciousness, lehar here raises what appear to be problems for a theory of the neural correlates.
the reply to the homunculus problem is fine, as far as it goes. lehar rightly notes that this is just as much a problem for a symbolic conception of mental representation as for an analog conception, but is in fact no problem at all, because the threatened regress can be contained by assuming mental processes that are sensitive to the symbolic or analog structure of the representations.
the trouble is, lehar in effect admits here that his theory is (to some extent) a theory of the neural vehicles of consciousness. he describes them as "full spatial analog[s]" of the environment and "explicit spatial representation[s]". those assertions appear to speak to the nature of the vehicles, rather than their contents.
to make matters worse, in the latter part of the paragraph lehar reverts to talking about the contents of the vehicles in defense of his view. he says that the "existence and fully spatial nature [of percepts] in my internal perceptual world is beyond question". this is surely a claim about the contents of visual experience.
talk of "explicit spatial representation" and "spatial nature of the perceptual representation" now becomes quite ambiguous. is it the vehicles that are spatial (and hence topologically isomorphic with what they represent), or do they merely *represent* spatial properties.
Some sorting out and clarification needs to be done here. Lehar will have to SHOW HOW TALK OF "SPATIAL ANALOGS" IS CONSISTENT WITH HIS EARLIER INSISTENCE ON FUNCTIONAL ISOMORPHISM, as opposed to structural or topographic isomorphism. And he will have to SHOW HOW TALK OF "EXPLICIT SPATIAL REPRESENTATION" IS CONSISTENT WITH HIS CLAIM TO HAVE SIDESTEPPED "THE PROBLEMS OF EXPLICIT V. IMPLICIT REPRESENTATION" (Section 4, Par.2).
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A whole new section has been added to section 5 in the paper, clarifying the relation between structural and functional isomorphism, complete with the new Figure 1 to illustrate the concept. I suspect these reviewers will be unhappy with this explanation, as it highlights the fact that I do indeed advocate a "Picture-In-The-Head" theory of spatial representation, a position that they argue against in their own paper.
O'Brien & Opie (1999 Sect. 5.3) argue that "We don't expect the green of grass to be represented by green-colored neural vehicles. Why, therefore, should we expect spatial properties of the world to be represented by corresponding spatial properties of the brain?"
But if "greenness" is not a property of the brain, where does that green color come from? It cannot be a property of the world itself, because it is a subjective quale, i.e. a kind of arbitrary mapping used in the internal representation of the brain to represent light of median wavelengths. The green color is itself a property of the physical mechanism of the brain, not of the external world. Although the neurophysiological correlate of this green quale has yet to be identified, we already know for a fact that it is green, because we can see it "from the inside". That does not mean it will look green to an electrode that measures the activation of particular cells in the brain, nor that those cells would appear green under microscopic examination. The greenness can only be experienced internally, but that does not make it any the less green, or any the less a characteristic quality of certain physical processes in the physical brain.
On the question of the spatial properties of the phenomenal world, if the representation subserving that phenomenal experience is not itself spatial, then why would it appear spatial phenomenally? There are many non-spatial ways to represent spatial information. For example it can be expressed in the form of mathematical equations that describe spatial structures, or spatial structures can be expressed in a Fourier code. But if our brain were to employ either mathematical equations, or Fourier descriptors as its representation of spatial structure, then our subjective experience would itself necessarily appear in the form of mathematical equations, or of Fourier descriptors. The fact that the world of experience takes the form of a spatial structure is direct evidence for a spatial representation in the brain.
I do not expect the reviewers to be convinced by this argument, this is a central paradigmatic issue around which the whole paper revolves. It is the inevitable consequence of taking an Indirect Realist view of perception, which shows that the properties of the phenomenal world are properties of the brain first and foremost, and only in secondary fashion are those properties also reflective of certain properties of the external world. It is curious that O'Brien & Opie join Atkins (1996) in calling Isomorphism a "Naive Theory of Perception". In fact it is their own view which is the one consistent with Naive Realism, because they seem to accept without question that the world appears spatial, although they deny a spatial representation in the brain, as if we were viewing the world directly, instead of by way of the medium of the representational mechanism in the brain.
If, at the end of the day, Lehar does subscribe to a "PICTURE-IN-THE-HEAD" APPROACH to visual perception, he MUST DO MORE TO DEFEND IT AGAINST THE NUMEROUS OBJECTIONS IT FACES. There is A VOLUMINOUS PSYCHOLOGICAL/ PHILOSOPHICAL LITERATURE ON THIS PROBLEM, with which Lehar should show at least some familiarity. A good place to go, for example, is the volume "IMAGERY" EDITED BY NED BLOCK. Of particular interest to Lehar will BE FODOR'S WELL- KNOWN REPLY to the "infinite regress of observers within observers" criticism that Lehar ascribes to Dennett, O'Regan and Pessoa et al.
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As far as I can tell, the "numerous objections" faced by the Picture-In-The-Head theory amount to one single objection, and that is the tired-old homunculus argument, which has already been dispatched many times over. This is the worst kind of critique to receive in a review because it is so open-ended, it is hard to address. If there really were so many objections, why don't the reviewers mention one or two of them so I would know what they are talking about? I understand that the reviewers may think that there are numerous objections, but that is only because this alternative is so rarely given any serious consideration that the reviewers haven't heard it defended before. The reviewers casually point me to Ned Block's "Imagery". But that book contains eight different articles by eight different sets of authors! Any hint as to which of these authors supposedly delivers the coup-de-grace for the Picture-In-The-Head hypothesis? Or do I have to go through the entire book to address this reviewers objection?
Ned Block himself offers the answer right in the introduction, on page 2:
"But no one writing in this book (nor any other serious participants in the debate) thinks that people can literally see and manipulate real internal pictures. Brain scientists have found no pictures in the brain, and even if they had, the presence of pictures wouldn't explain the phenomena unless the brain contained an internal eye to view them and an internal flashlight and internal hands to manipulate them etc. (And even if we postulate an internal eye, would there still be another eye in that eye's brain? So we have a problem: the obvious explanation is blocked."
So the "formidable objection" by Ned Block is nothing more than the tired old homunculus argument! Block does however explain why this alternative seems so incredible, and that is because "brain scientists have found no pictures in the brain". This either means that there are no pictures in the brain, or that modern neuroscience is in a state of serious crisis, because it cannot find the pictures in the brain that we know must be there. And the evidence for the presence of those pictures is right before our eyes, if only we can see beyond the naive realist illusion and recognize the phenomenal world for what it really is.
Chapter 1, by Roger Brown & Richard Harrnstein offers no arguments against the Picture-In-The-Head hypothesis.
Chapter 2, by Daniel Dennett, offers the following objection: (page 52)
"For an image to work as an image there must be a person (or an analogue of a person) to see or observe it ... we shall have to design a perceiver- analogue to sit in front of the image, and yet another to sit in front of the image which is the end product of perception in the perceiver-analogue, and so forth ad infinitum."
So its the tired old homunculus argument being dragged out again! On page 56 Dennett discusses the question of hallucinations, and the phenomenal space in which they are observed, using the example of a "freak" visual experience caused by electrical stimulation of the cortex...
"Having a visual hallucination is ... just being aware of the content of a non- veridical visual `report' caused by such a freak discharge. And where is this report and what space does it exist in? It is in the brain and exists in the space taken up by whatever event it is that has this non-veridical content, just as my description of hallucinations takes up a certain amount of space on paper. Since spatiality is irrelevant to descriptions, freak descriptions do not require ghostly spaces to exist in.
If the neurophysiological "description" of the hallucination is not expressed as a spatial pattern in a spatial medium, what is it that transforms the phenomenal correlate of that description into the spatial pattern that we experience? How did phenomenal experience settle on a spatial format for presenting the non-spatial information in the brain? Why would it not, for example, appear phenomenally as a Fourier descriptor, or as a mathematical formula? And where does the transformation from non-spatial to spatial take place if not in the brain? And what is the algorithm of that transformation? And where is the spatial percept stored or registered? In truth, the fact that the phenomenal world appears as a spatial pattern is direct evidence for a spatial representation in the brain.
In Chapter 3 page 63 Jerry Fodor offers his own "fatal blow" to the Picture-In-The- Head theory by announcing that he is...
"unsympathetic about the empirical basis for the existence of stagelike changes in modes of internal representation ... because I think it would be appalling if the data really did somehow require us to endorse that kind of view. I am, in fact, strongly inclined to doubt the very intelligibility of the suggestion that there is a stage at which cognitive processes are carried out in a medium that is fundamentally nondiscursive."
So Fodor's whole argument is that he finds the idea incredible! He then goes on to present a caricature of a simplistic image-based "language" which he calls "Iconic English", and demonstrates how this simplistic concept is inadequate as a model of thought. For example if "John" is represented by an image of John, and "green" is represented by a green tile, then the sentence "John is green" could not be meaningfully constructed out of those elements. So Fodor succeeds in shooting down his simplistic straw man theory of imagistic thought, and thereby supposes he has put an end to the notion altogether.
Chapter 4 is written again by Daniel Dennett. This time he "defeats" the notion of mental imagery by constructing a fanciful analogy. He asks the reader to imagine that anthropologists have discovered a native tribe that believes in a hitherto unheard-of God of the forest, named "Feenoman" (this will stand in for the supposedly fictitious "phenomenal experience" whose existence Dennett so forcefully denies). Some of the anthropologists begin to believe in Feenoman as an objectively real deity (the "Feenomanists"), while others retain their scientific rigor, and study the native religion while remaining agnostic themselves (the "Feenomanologists"). On page 102 Dennett concludes that:
"Phenomenal space is Mental Image Heaven ... [mental images] can reside, with Santa Claus, in the logical space of fiction."
But analogies only work when the analogy is analogous. In this case nobody has ever actually seen Feenoman, and therefore it is reasonable to question His existence. The phenomenal world on the other hand is right before our eyes, if only we can recognize it for what it is. Dennett's argument works better against mental imagery than perception, because it is easier to deny the existence of those...
"...entirely mythical species of mental images: the various non-physical, phenomenal or epiphenomenal, self-intimating, transparent to cognition, unmisapprehensible, pseudo-extended, quasi-imagistic phantasms that have often been presented as mental images in the past." (page 104)
However this argument does not work so well against perception. For to deny a spatial representation in the brain is to deny the vivid spatial structure so plainly evident in the world we see around us.
Chapter 5 is provided by Robert Schwartz, who launches into a minute analysis of exactly what images might be, and by what principles they differ from other forms of representation. His only serious challenge to the Picture-In-The-Head theory is his complaint that the concept of isomorphism is too vague, because it uses the concept of "similarity" between the picture and the object it represents. But since similarity is itself a rather vague term, Schwartz objects that
"the unqualified claim that a picture resembles its referent is vacuous."
Does Schwartz suggest that any statement about the similarity between items is also vacuous? Surely scientific discourse can meaningfully employ terms such as similarity even in the absence of a rigorous definition. In any case this objection is only valid when the claim is unqualified. In my own discussion of isomorphism I have qualified the concept of similarity by invoking information theory, and specifying that the information content of the subjective experience cannot be any greater than the information content of the corresponding neurophysiological state. The use of information theory to quantify the otherwise vague notion of similarity is a significant and original formulation of the issue of isomorphism, and this one step makes my own discussion of isomorphism impervious to many of the objections raised against this much-maligned notion.
Chapter 6 is provided by Kosslyn, Pinker, Smith, & Schwartz. As mental imagery advocates they offer no challenge to the Picture-In-The-Head hypothesis.
Chapter 7 is contributed by Zenon Pylyshyn, who is an ardent opponent of mental imagery. His argument is that what people report as properties of images, are actually properties of the objects they represent, not of the images themselves. For example when a mental image of a table is observed to have spatial extent, Pylyshyn argues that spatial extent is not a property of the mental image, it is a property of the table itself. Only a Naive Realist could possibly accept this argument because an Indirect Realist realizes that none of the properties of the objective external world can possibly penetrate into conscious experience except by way of explicit representations of them in the brain, mediated by sensory input. If the mental image did not have spatial extent, then no spatial extent would be experienced for that image. Pylyshyn argues as if the quality of spatial extent can somehow bypass the sensory interface to the world and penetrate directly into the experience of the imaging subject, without leaving an impression in the subject's brain.
The final chapter, Chapter 8, is written again by Stephen Kosslyn, who as a strong advocate of mental imagery offers no argument against the Picture-In-The-Head theory. However he does hazard to say on page 207 that
"Although no serious researcher today maintains that images are actual pictures in the head, some still find it reasonable to posit quasi-pictorial representations that are supported by a medium that mimics a coordinate space."
I find this statement puzzling, since the notion of quasi-pictorial representations supported by a medium that mimics a coordinate space is exactly what I consider to be "actual pictures in the head". So I am at a loss as to whether I am in agreement or disagreement with Kosslyin on this point. But in any case he offers no explanation for why no serious researcher believes in pictures in the head.
So, after poring through the "voluminous literature" in the recommended tome, all I have come up with is the homunculus argument! This is what I really hate about these journal reviews. At the casual mention of a "voluminous literature" and a vague pointer at a book, the humble supplicant for publication has to rush off to the library and plough through a mass of irrelevant material only to find that the reviewers did not know what they were talking about! I can understand why they may have had the impression that this book contained "numerous objections" to the notion, and that is because the notion was never even considered as a serious alternative by any of the parties to the debate. But that is all the more reason why it deserves that consideration now.
Daniel Dennett makes the insightful observation that although this issue remains unresolved, and that nobody really knows with any certainty whether there are pictures in the brain, (page 30)...
"A curious feature of the debate is the passion it evokes, which is unlike the normal passion of scientific controversy ... everyone, it seems, has a fiercely confident opinion about the nature and existence of mental images. This manifests itself in remarkable ways: in unhesitating predictions of the results of novel psychological experiments, in flat disbelief in the integrity of recalcitrant experiments, in gleeful citation of "supporting" experimental evidence, coupled with bland imperviousness to contrary evidence."
In his summary of the debate to date, Kosslyn et al. offer the observation that...
"Not surprisingly neither arguments nor counter-arguments have been definitive, and neither seems to have had enough force to sway most people from whatever position they found most congenial in the first place."
This is a sure sign of a paradigmatic debate. The reason for the supreme confidence of the opposing camps is that they are not debating the facts of the case, as they would in a debate over more specific theories. Instead, the issue involves the paradigmatic question of whether the subjective conscious experience is a valid source of evidence for the nature of the neurophysiological representation employed in the brain. For if it is, then the existence of mental images can be easily confirmed by inspection. And that choice in turn depends on the epistemological question of whether the world we see around us is the real world itself, or whether it is merely a replica of that world in an internal representation. The only way to break the endless cycle of fruitless debate is to cut through to the paradigmatic issue hidden at the core of the debate, and settle once and for all the question of the epistemology of conscious experience. The fact that none of the contributing authors in Ned Block's book even considered a Picture-In-The- Head theory even as a theoretical possibility, and the fact that the homunculus objection crops up so often without challenge, highlights the urgent need for this paper to be published without further unnecessary delay.
15. Section 6.2, Section 8.6 Same problem. WHAT DOES "LITERAL VOLUMETRIC REPLICA OF THE WORLD MEAN"? This talk HAS TO BE MADE CONSISTENT WITH THE ASSUMPTION OF FUNCTIONAL ISOMORPHISM.
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This issue is now clarified in the new extended Section 5 in the paper.
16. Section 8.6 Again, Lehar says "The most significant feature of this concept of perceptual processing is that the result of the computation is expressed not in the form of abstract variables encoding the depth and slope of the perceived rectangle, but in the form of an explicit three-dimensional replica of the surface as it is perceived to exist in the world." ARE WE TO TAKE THIS LITERALLY? OR IS THE "REPLICA" HERE SOME KIND OF FUNCTIONAL ONE?
This issue is now clarified in the new extended Section 5 in the paper.
17.On PDP models.
In Section 9 Lehar claims that "the computational transformations observed phenomenologically are implausible in terms of contemporary concepts of neurocomputation and even in terms of computer algorithms".
In view of the VERY LIMITED DISCUSSION OF CONTEMPORARY COMPUTATIONAL APPROACHES TO CONSCIOUSNESS, AND TO GESTALT PHENOMENA in particular, THIS CLAIM HAS HARDLY BEEN ESTABLISHED.
Even so we grant that there is some prima facie plausibility to Lehar's claims where "neuron doctrine" style theories are concerned. We are not so convinced when it comes to PDP approaches. Indeed, we would offer the suggestion that the PDP APPROACH TO NEURAL COMPUTATION, SUITABLY INTERPRETED, HOLDS OUT SOME HOPE OF *IMPLEMENTING* THE VERY PERCEPTUAL MODEL LEHAR DEFENDS.
The PDP approach takes seriously the intrinsic structural properties of the brain, and attempts to develop an account of both cognition and perception consistent with these properties. It identifies as a principal computational mechanism a style of processing known as relation search (or constraint satisfaction). This, we suggest, is just the kind of mechanism required to implement the kinds of dynamic, reciprocal interactions, and emergent phenomena that Lehar postulates in sections 8.2, 8.3, 8.4, 8.5 and 8.6. We think Lehar might consider this suggestion, and, at the very least, MAKE SOME COMMENT AS TO THE POTENTIAL OF THE PDP APPROACH to implement his perceptual model.
18. On originality. This follows from the previous point. We accept that PDP theorists haven't said much about the representation of space, but there have been some tentative steps in the direction of applying PDP thinking to gestalt perceptual phenomena. We commend the following papers to the author:
PALMER, S.E. (1992) Modern theories of Gestalt perception. In: G.W.Humphreys (ed.) Understanding Vision. Blackwell. (See particularly the latter part of the paper.)
READ, S.J., VANMAN, E.J. & MILLER, L.C. (1997) Connectionism, Parallel Constraint Satisfaction Processes, and Gestalt Principles: (Re)Introducing Cognitive Dynamics to Social Psychology. PERSONALITY AND SOCIAL PSYCHOLOGY REVIEW Vol.1(1):26- 53 (Not focussed on perception, but a nice discussion of PDP and gestalt theory).
OPIE, J. (1999) Gestalt theories of cognitive representation and
processing. Psycoloquy 10(021)
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The reviewers are obviously advocates of PDP models and therefore naturally reluctant to accept the limitations of their paradigm without specific proof that it is wrong. But it is impossible to prove a negative- i.e. I cannot demonstrate that no possible future PDP model could ever be formulated to address the Gestalt phenomena discussed in the paper. The onus is on the PDP advocates themselves to demonstrate the capabilities of their paradigm. However there is good reason to believe that the neural network paradigm has some serious limitations with respect to the phenomena specifically addressed by the Gestalt Bubble model.
The Gestalt Bubble model was formulated as a perceptual model, but the computational functionality that it performs can be readily expressed in PDP terms. However the result is a combinatorial explosion of receptive fields so great as to seem completely implausible to the author. That is why I chose to present it as a perceptual model rather than a neural network model, and that is why I stand by my statement that "the computational transformations observed phenomenologically are implausible in terms of contemporary concepts of neurocomputation". Now this is admittedly a personal judgement, as plausibility judgements necessarily are, so I cannot prove definitively that PDP models are implausible. But there is good reason to believe that they are implausible for the class of perceptual computation addressed by the Gestalt Bubble Model.
The reviewers point out correctly that PDP models of Gestalt phenomena have been proposed, that exhibit the Gestalt properties of emergence and prägnanz. That's all well and good, but those models happen to be irrelevant to the specific perceptual phenomena addressed by the Gestalt Bubble model, i.e. the perceptual transformation from a two-dimensional stimulus to a three-dimensional volumetric perceptual structure.
In fact some of the models the reviewers cite actually highlight the limitations of the PDP approach by the parts of the problem which they pointedly avoid addressing. For example Palmer (1990 p. 316) presents a PDP model to account for the ambiguous percept of an equilateral triangle as an arrow pointing in one of the three directions indicated by its corners. Palmer presents a simple dynamic neural network model in the form of three nodes with mutually inhibitory connections, resulting in a winner- take-all behavior. The activation of the winning node represents a percept of the triangle as an arrow pointing in the corresponding direction. This is an excellent demonstration of Gestalt dynamics in a perceptual model. But now consider what kind of architecture would be required to make those three nodes operate as advertised in response to a given visual stimulus. For it is here at the lower end of the visual hierarchy that neural network theory runs into a combinatorial mess. Let us begin with a functional description of what that processing must do. We need a circuit which can read a visual image, and identify the presence of one or more equilateral triangles in the stimulus. For each equilateral triangle the system must set up a triangle of mutually inhibitory nodes with dynamic behavior as described by Palmer. Forget the rest of the problem, like the question of how the final activation of the winning node for each triangle becomes a percept of a directed arrow. Even the computational functionality outlined above already poses a formidable challenge for PDP theories. We know how to make edge detectors out of neurons with spatial receptive fields, so we begin with edge detectors at every location across the visual field, replicated at every orientation. But the next problem of finding the triangles already begins to leap into combinatorial territory. For we now need "angle detectors" tuned to respond to pairs of lower-level edge detectors spanning a 60 degree angle. Like the edge detectors themselves, the angle detectors must be replicated at every location across the visual field, where they are also replicated at every possible orientation, and wired to the appropriate pair of edge detector cells. There are several ways to proceed from this point, but all of them are ugly. We could now posit "triangle detectors" tuned to respond to the presence of three "angle features" in a triangular configuration. These "triangle detectors" would also have to be replicated at every location across the visual field, at every orientation, and through a range of spatial scales, so as to be able to detect triangles independent of their rotation, translation, and scale. We could then equip each of these innumerable triangle detectors with the three nodes of Palmer's circuit to perform the competition between perceptual interpretations. If the reviewers do not find this architecture already completely implausible, consider the problem of generalizing the model to detect other types of triangles besides equilateral triangles, or other shapes such as squares or rectangles, each of which would require a whole different combinatorial set of feature detectors for each different shape!
There are neural network models out there which address lower level visual processing in Gestalt terms, for example Grossberg & Mingolla (1985, 1987) and Zucker et al. (1989). But these models themselves run headlong into the combinatorial problem as soon as they dare to tread beyond simple collinear completion. Zucker et al. (1989) posit curvature detectors at every location, orientation, and through a range of curvatures across the visual field. Grossberg & Mingolla (1987) briefly court with the idea of cells with receptive fields configured to respond to right-angled corners in the stimulus. But no mention is made of the combinatorial explosion hidden in that pandora's box, because explicit curvature or corner detectors suggest also detectors for other detectable features, such as acute and obtuse angle detectors, not to mention detectors tuned to vertex types composed of more than two edges, for example "T" or "Y" or "X" vertex detectors. For each of those specialized detectors would also have to be replicated at every location, orientation, and through a range of spatial scales across the visual field. Note that this is a problem not only with "feature detector" or "grandmother cell" models of visual representation, for the models of Grossberg & Mingolla, and of Zucker et al. are specifically dynamic neural network models, i.e. models that employ dynamic patterns of activation across the neural substrate to encode perceptual structure. But all those different receptive field types are still required just to control or channel the patterns of activation required for the functionality of the model, as I explain in detail in Lehar (2001). And the Gestalt Bubble model extends the required functionality into three dimensions, requiring three- dimensional volumetric receptive fields replicated at every location in three dimensions, and at every orientation in three dimensions and through a range of spatial scales.
I have explained elsewhere ( Lehar 1999, 2001) that the Achilles heel of neural network theory lies in the principle by which it encodes spatial structure, i.e. the neural receptive field that is anchored to the tissue of the brain. For the receptive field is no different than a template theory, a concept whose limitations are well known. And yet this idea is so deeply entrenched in contemporary neuroscience that it will take a profound paradigmatic revolution to uproot it.
But it is impossible to prove a negative- plausibility arguments are necessarily based on intuitive appeal, or on the perceived promise (or lack of it!) of a theoretical approach which has not yet been fully demonstrated. So I do not expect the reviewers to be convinced by this plausibility argument, because the plausibility issue cannot be resolved definitively by argument and evidence alone. And that is why I felt that a discussion of the limitations of neural network theory would only distract from the principal message of the paper, which is not on the limitations of the older paradigm, but on the promise of the new one.
To conclude, let us reiterate that we think this is an insightful and valuable paper. We look forward to seeing it in print.
I would like to see how these reviewers review a paper they do not like!
Atkins K. (1996) Lost the Plot? Reconstructing Dennett's Multiple Drafts Theory of Consciousness. Mind & Language 11 1-43.
Grossberg, Stephen, & Mingolla, Ennio (1985). Neural Dynamics of Form Perception: Boundary Completion, Illusory Figures, and Neon Color Spreading. Psychological Review, 92, 173-211.
Grossberg, Stephen, & Mingolla, Ennio (1987). Neural Dynamics of Surface Perception: Boundary Webs, Illuminants, and Shape-from-Shading.. Computer Vision, Graphics and Image Processing, 37, 116-165.
Grossberg, Stephen, & Todorovic, Dejan (1988). Neural Dynamics of 1-D and 2-D Brightness Perception: A Unified Model of Classical and Recent Phenomena" Perception and Psychophysics 43, 241-277.
Köhler W. (1924) Die Physischen Gestalten in Ruhe und im stationären Zustand: Eine naturphilosophische Untersuchung. Erlangen: Verlag der Philosophichen Akademie.
Lehar S. (1999) "Harmonic Resonance Theory: an Alternative to the
`Neuron Doctrine' Paradigm of Neurocomputation to Address Gestalt
properties of perception." Rejected Psychological
Review November 1999. Resubmitted to Behavioral & Brain Sciences-
not accepted for review because only one paper allowed per
customer at a time (!) Available at
Lehar S. (2001) "Directional Harmonic Theory: A Computational
Gestalt Model to Account for Illusory Contour and Vertex
Formation". Submitted Perception & Psychophysics, August 2001. Also
O'Brien G. & Opie J (1999) A Connectionist Theory of Phenomenal Experience. Behavioral & Brain Sciences 22, 127-148. http://www.bbsonline.org/documents/a/00/00/05/07/bbs00000507-00/bbs.obrien.html
Palmer S. E. (1990) Modern Theories of Gestalt Perception. Mind & Language 5 (4) 289-323)
Zucker, S. W., Dobbins, A., & Iverson, L. (1989). Two Stages Of Curve Detection Suggest Two Styles Of Visual Computation. Neural Computation, 1, 68-81
The present article is quite a difficult one to review. I suspect that referees with expertise in Philosophy have already been contacted, but I would strongly encourage you to contact experts in space/form perception, such as Jan Konderink, James Todd, or Ennio Mingolla. Researchers in these fields could more adequately evaluate the theory advanced by the author. I provide my own review below.
Lehar's manuscript has great potential to become a BBS target article. IN IT'S PRESENT FORM, HOWEVER, I FIND IT INADEQUATE for publication. Below I outline some of the reasons.
It seems to me that the author is attempting to do "too much". On the one hand the author seems to engage in a philosophical battle with several century-old issues. At the same time, the author proposes a novel, original theory of space perception. It seems to me that the greatest contribution is by far the latter. The philosophical arguments should take part of background material, or discussion, but in a much more summarized manner. They serve to motivate his theory. I find the present "AGGRESSIVE TONE" INADEQUATE, AND TO BE FRANK, NOT VERY PERSUASIVE.
[Note: Caps above indicate sections highlighted by the editor for particular importance to address.]
Much of the philosophical discussion to which this reviewer objects was originally not part of the paper, and was added by demand of Reviewer #1 in the first round of review. I agree with this criticism, consequently I have removed some of that added material.
As for the "aggressive tone" which this reviewer finds "unpersuasive", this is an attitude I have developed as a consequence of a series of encounters with reviewers like this one. He says he is "not persuaded" but does not bother to explain of what he is not persuaded, or which arguments specifically he found unpersuasive, and exactly why they failed to persuade.
Perhaps it is the philosophical discussion which makes him so uncomfortable, because it forces the reader to make a choice between three incredible alternatives. The "aggressive tone" emphasizes the fact that this is a choice which cannot be conveniently avoided, because rejecting one alternative simply commits one to one of the remaining incredible alternatives. In my discussions with colleagues I have discovered that many feel very uneasy about having to make this choice, and would prefer to just avoid the issue altogether. The reviewer does not give us the benefit of his own thoughts as to whether it is Indirect Realism of which he remains unconvinced, or whether he is unconvinced of the validity of the argument which forces him to a choice in the first place. Unless he informs us otherwise, we can only guess at his motivations. But given the abundance of closet Naive Realists out there, the most likely explanation is that this reviewer is a Naive Realist at heart, his whole career has been committed to theories based implicitly on Naive Realist assumptions, and he feels uncomfortable at now being challenged to defend the indefensible. Like many psychologists this reviewer probably considers philosophical debates to be outside of his specialty, and therefore irrelevant to his branch of psychology. This is exactly why an "aggressive tone" is needed to wake these people up to the fact that psychology cannot be so neatly compartmentally insulated from philosophy, because every science is built upon a foundation of philosophical assumptions, and those assumptions are not always "testable" by the normal rules of evidence. Paradigmatic choices require the exercise of a kind of broad-minded judgement or intuition, which this reviewer probably considers to be unscientific.
And yet the choice must be made, because scientific theories which are built upon the wrong paradigmatic foundations are like castles built on sand, impervious to assault on the grounds of theory and evidence, but vulnerable to a blow below the paradigmatic belt. Given this reviewer's reluctance to share with us the reasons for his discomfiture, we can only guess what is motivations might be. But even if I have this reviewer's motivations all wrong, there are many more closet Naive Realists out there who are in need of an "aggressive tone" to wake them up to their responsibility as scientists to justify the foundations of their theoretical stance.
An EXPLICIT SECTION ON HOW THE MODEL DEALS WITH IMPORTANT FINDINGS IN THE PSYCHOPHYSICS OF FORM PERCEPTION IS REQUIRED. I find THE PRESENT DISCUSSION SECTION INADEQUATE. Some of the discussion is indeed suggestive (e.g., mental imagery), but some is TOO TANGENTIAL. For example, the HEMI-NEGLECT ARGUMENTS ARE WEAK, they hardly helps us understand the nature of this important condition. The DISCUSSION OF THE LITERATURE SHOULD BE BOTH MORE COMPREHENSIVE AND IN-DEPTH.
The author must provide a MORE STRUCTURED COMPARISON OF HIS PROPOSAL WITH OTHER THEORIES OF SPACE/FORM PERCEPTION. It is only against the background of existing proposals that his contribution can be assessed. And here I mean NOT GENERAL LINES OF INVESTIGATION, SUCH AS NEURAL NETWORK MODELS, BUT SPECIFIC *PERCEPTUAL* THEORIES.
Is it the volume of referenced material which the reviewer finds inadequate, or is the reviewer refering to some specific literature? Would the reviewer care to specify which "important findings" in the psychophysics of form perception he considers to be so relevant to the Gestalt Bubble Model? Or does he merely suspect that there may be some such findings and that the author should seek them out? But the real issue here involves the general nature of the proposal. The reviewer is unhappy that this paper is not a detailed and specific model that makes testable predictions which can be matched against experimental data. And this reviewer will not consider the present paper suitable for publication until it is revised accordingly. What he has failed to understand is the paradigmatic nature of what is being proposed. What I propose is not a detailed model of some specific perceptual effect, but a whole new class of model, motivated by the Indirect Realist perspective that perception involves the construction of a volumetric spatial replica of the external environment in an internal representation. What makes this a paradigmatic idea is the fact that if it should turn out to be right, then it would necessarily have implications on our interpretation of a large volume of psychophysical data. In fact, should this idea be proven right, virtually no aspect of psychology will be entirely unaffected by this new perspective on the problem. So the volume of psychophysical data which are relevant to the model would include much of the literature in psychology. Unfortunately that data cannot be definitive in determining the paradigmatic choice itself, because in paradigmatic debates, both sides often cite the same evidence to support their opposite conclusions. That is because each side interprets the data from the perspective of their own paradigm. For example the most convincing evidence for a spatial representation in the brain is the fact that consciousness exhibits a spatial structure. The structural nature of experience is an indisputable and undisputed fact. But that evidence is convincing only to the Indirect Realist. The Direct Realist interprets that self-same factual evidence as a property of the world rather than of the brain. The two paradigms draw opposite conclusions from the self-same evidence. And they also draw opposite conclusions from the evidence of mental imagery, neglect syndrome, visual illusions, Gestalt phenomena, and virtually every other domain of psychology.
The reviewer suggests a "more structured comparison" with other theories of space / form perception. "Not general lines of investigation such as neural network models, but specific perceptual theories". Here the reviewer clearly reveals his misunderstanding of the scope of the present proposal. For the competing hypotheses which the present model proposes to challenge are not specific perceptual theories, but exactly more general lines of investigation, such as neural network models. Models can be validly defined at many levels, from general concepts to specific mechanisms. Consider for example Marr's and Biederman's models of vision by abstraction of features; Selfridge's Pandemonium model; Triesman & Gelade's spotlight theory of attention; Collins & Quinlan's Spreading Activation model, McClelland & Rummelhart's PDP approach, to name just a few. Some of these models are far more general and conceptual than mine, but are valid models nonetheless. And then of course there is the example of Gestalt theory itself, a concept so general that it can hardly even be called a model. And yet despite its vagueness as to specific mechanism, the Gestalt view of perception serves as an invaluable reminder to resist the temptation to consider only simpler aspects of perception that can be described by specific models. It would have been a great loss if the Gestalt ideas had been denied publication because the concept was not sufficiently specified! General models are appropriate in a new field where much remains to be discovered, while more specific models are derived as more exact specification of general models, as a science matures. It is unfortunate that there are so many in the scientific community who consider general discussion of paradigmatic issues to be beyond the bounds of science, which they would reduce to a pedantic pursuit of minute details in hermetically insulated narrow specialty domains. That is a very impoverished view of the enterprise of science!
The set of PREDICTIONS PRESENTED SHOULD CONCERN PERCEPTUAL FACTS. After all this is a perceptual theory, not a theory of consciousness. For example, the last two predictions concern the nature of subjective experience, and are hardly predictions of the model.
Is it not the prerogative of the author to determine whether his paper is a theory of consciousness or a perceptual theory? What this reviewer fails to see is that every theory of perception is also a theory of consciousness, because the two are inseparable.
The "last two predictions" to which the reviewer objects are the fact that an illusory figure is experienced as a solid spatial surface at high resolution, and that the reversal of a multistable percept is vividly experienced as an inversion of a perceptual data structure. What he means is that I have not specified the computational algorithm of the model sufficient to perform computer simulations that reproduce those phenomena in detail. He is right, I have not. But the vast majority of models out there do not even consider it necessary for a model of the phenomenon to produce a volumetric spatial output. The novel and significant message of the present paper is to point out that an adequate model of the phenomenon must produce exactly such an output. This is the nature of paradigmatic hypotheses, they outline a general approach to a problem, not its detailed solution. It is only after the paradigm has been established that others will feel the need to construct those specific models and perform those simulations. If this reviewer had his way, the paradigm would never even be published as a theoretical possibility for other researchers to consider, so those models would never be built and the predictions never tested.
In summary, the paper has novel, interesting elements that have great potential to become a BBS paper. Nevertheless, the author needs to REORGANIZE THE PAPER SO AS TO HIGHLIGHT THE ACTUAL CONTRIBUTIONS IN A MORE STRUCTURED MANNER.
This is the kind of review that I hate most of all! In the first place the reviewer does not bother to summarize the paper, so we have no idea whether he read it or understood it to any depth, or more importantly, whether he understood the principal arguments as intended by the author.
Secondly his criticisms are so vague as to be essentially meaningless. He complains that the philosophical discussion is "not very persuasive", but he does not explain of what he is not persuaded, and why. He demands discussion of "important findings in the psychophysics of form perception" but does not specify what those findings might be. He says that the discussion of the literature should be "more comprehensive and in-depth", but he does not specify what issues should be discussed. He demands "more structured comparison" with other theories, but he does not specify which theories he considers relevant to this comparison. This kind of criticism is such a carte-blanche, there is no way that these vague requirements could ever be shown to have been met.
This reviewer appears to offer a qualified endorsement of the paper, i.e. revise & resubmit. But make no mistake about it, this is actually an outright rejection. Because the reviewer has made it clear that the kind of revision that he requires is the kind of revision which would transform the paper from a paradigmatic hypothesis to a simple perceptual theory. And as a perceptual theory it would be promptly rejected for publication because the volumetric filling-in operations would seem unnecessary and neurophysiologically implausible in the absence of the discussion of the paradigmatic issues. If this paper were revised to meet with this reviewer's satisfaction, I would no longer wish to be its author.
I haven't changed my opinion since the first review I sent to you. Essentially this is an updated view of the questions raised by the gestaltists some of which have yet to be answered and are well worth presenting. Modern neuronal approaches to perception inevitably lead to some version of the binding problem -- how to put the pieces together.
Lehar turns this approach on its head and takes the bound product as primitive. Lehar's view is iconoclastic and provocative but, in my estimation, as legitimate as that of the "establishment". It is well worth publishing in a journal dedicated to discussion of varied points of view.
Here is a man with the vision to recognize a paradigmatic issue when he sees one. This is also a man who apparently shares my belief that a novel idea, clearly stated, and supported by reasonable arguments deserves to be released to the larger community without unnecessary fuss and delay, even if it topples a number of sacred cows in the process!