Gestalt Isomorphism and the Primacy of the Subjective Perceptual Experience

Steven Lehar

Open peer review commentary response to Pessoa et al. 1999 published in Behavioral & Brain Sciences


The Gestalt principle of isomorphism reveals the primacy of subjective experience as a valid source of evidence for the information encoded neurophysiologically. This theory invalidates the abstractionist view that the neuro physiological representation can be of lower dimensionality than the percept to which it gives rise.


I see no difference between the original Gestalt formulation of isomorphism, and what Pessoa et al. call "analytic isomorphism" except for the secondary issue that in the former, the "bridge locus" need not imply a physical location anchored to the neural substrate, but can refer to dynamic patterns of energy in that substrate, as in Köhler's Field Theory. The authors are mistaken when they claim that Köhler's concept of isomorphism did not extend to sensory qualities such as color. Köhler clears up this confusion ( Köhler 1947 p. 60) by explaining that by similarity of "structural properties" he means that "physiological events must be variable in just as many directions or `dimensions' as the [perceived] colors are," and he specifically cites Müller's axiom.

The key insight of Gestalt theory is that when we view the world around us, what we see is not the world itself, but in fact it is primarily a percept, that is, an internal data structure active within the brain, and only in secondary fashion is this data structure also similar in certain respects to external objects and surfaces, just as an image on a television screen is first and foremost a pattern of glowing phosphor dots, and only in secondary fashion is it representative of a remote scene. Based on this insight, it is valid to examine the properties of the world we see before us, not as a scientist observing the physical world, but as a perceptual scientist observing a rich and complex internal representation.

We cannot yet say with certainty exactly how information is encoded in the brain; neither can we say with certainty what the neural correlate of any perceptual experience might be. What we can say with certainty however, according to isomorphism, is that when we see a filled-in percept, whether real or illusory, that the information apparent in that subjective experience is exactly representative of the information coded in the mechanism of the brain. Hence, whenever we find, as in the many manifestations of filling-in, a disparity between what we perceive and our notions of neural representation, we cannot argue, as Dennett (1992) suggests, that what we perceive is somehow more explicit than the representation on which it is based, instead we must revise our notions of neurophysiological representation to account for the properties of the subjective experience as observed.

The argument that filling-in is not logically necessary, because the world we perceive around us can be encoded in a compressed or abstracted representation, seems to be supported by computer image compression algorithms that eliminate redundancies in images by encoding only higher order regularities. However the compressed representation is of lower dimensionality than the spatial world we perceive, in violation of Müller's axiom. Furthermore, as in image processing, a compressed representation is useless without a complementary image decompression algorithm to unpack the compressed code and express it again in the ecologically useful form of filled-in surfaces and objects. In fact, the many manifestations of filling-in implicate exactly such an explicit completion mechanism because the percept is observed to contain more explicit spatial information than the retinal stimulus on which it is based.

In response to the objection of "Cartesian materialism," there is no need for a miniature observer of the filled-in scene, for that scene is a data structure, like any data in a computer, except that this data structure is expressed in explicit spatial form.

Dennett argues that the neural representation of a filled-in percept need not involve an explicit filled-in representation, but can entail only an implicit encoding, such as the edge image of the retinal ganglion cells, from which the filled-in percept could in principle be computed although this calculation need not, according to Dennett, actually be performed. However if such an implicit representation were sufficient to account for perception, then there would be no need to posit any further processing beyond the retinal image, since that image already implicitly encodes everything in the visual scene.

The notion of perception by "ignoring an absence" (Dennett 1992, p. 48) is meaningful only in a system that has already encoded the scene in which the absent feature can be ignored. In the case of the blind spot, that scene is a two-dimensional surface percept, every point of which is experienced individually and simultaneously at distinct locations. The onus is on Pessoa et al. to explain how this 2-D structure can be encoded as a non-spatial "absence."

O'Regan (1992) argues that visual saccades sample the external world like a data access of an internal data structure, and that therefore the structural representation of the visual world need not be encoded internally, because that information is immediately available in the world itself. However the three-dimensional spatial information of the external world is by no means immediately available from glimpses of the world, but requires the most sophisticated and as-yet undefined algorithm to read that spatial information from the world. Indeed, isomorphism suggests that the required algorithm involves the construction of a fully spatial three-dimensional internal model of that external world as observed subjectively. The fact that O'Regan's argument seems at all plausible to him is explained by the fact that when O'Regan thinks he is observing the external world, he is actually accessing an internal spatial model of the world, because the external world itself is beyond direct experience. The reductionist arguments accepted by Pessoa et al. are plausible only in the context of a Naive Realism that confuses the subjectively perceived world with the objective external world.


Köhler W. (1947) Gestalt Psychology. New York: Liveright.

Dennett D. C. (1992) "Filling in" versus finding out: a ubiquitous confusion in cognitive science. In Cognition: Conceptual and Methodological Issues, Eds. H. L. Pick, Jr., P. van den Broek, & D. C. Knill. Washington, D. C.: American Psychological Association.

O'Regan, K. (1992) Solving the `real' mysteries of visual perception: the world as an outside memory. Canadian Journal of Psychology 46: 461-488.