Referee #1 anon

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".

2. neglect

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" (Russell 1927, 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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