Extract from my paper:
The phenomenal world is composed of solid volumes, bounded by colored surfaces, embedded in a patial void. Every point on every visible surface is perceived at an explicit spatial location in three-dimensions (Clark 1993), and all of the visible points on a perceived object like a cube or a sphere, or this page, are perceived simultaneously in the form of continuous surfaces in depth. The perception of multiple transparent surfaces, as well as the experience of empty space between the observer and a visible surface, reveals that multiple depth values can be perceived at any spatial location. I propose to model the information in perception as a computational transformation from a two-dimensional colored image, (or two images in the binocular case) to a three-dimensional volumetric data structure in which every point can encode either the experience of transparency, or the experience of a perceived color at that location. The appearence of a color value at some point in this representational manifold corresponds by definition to the subjective experience of that color at the corresponding point in phenomenal space. If we can describe the generation of this volumetric data structure from the two-dimensional retinal image as a computational transformation, we will have quantified the information processing apparent in perception, as a necessary prerequisite to the search for a neurophysiological mechanism that can perform that same transformation.
This "picture-in-the-head" or "Cartesian theatre" concept of visual representation has been criticized on the grounds that there would have to be a miniature observer to view this miniature internal scene, resulting in an infinite regress of observers within observers (Dennett 1991, 1992, O'Regan 1992, Pessoa et al. 1998). In fact there is no need for an internal observer of the scene, since the internal representation is simply a data structure like any other data in a computer, except that this data is expressed in spatial form (Earle 1998, Singh & Hoffman 1998). For if a picture in the head required a homunculus to view it, then the same argument would hold for any other form of information in the brain, which would also require a homunculus to read or interpret that information. In fact any information encoded in the brain needs only to be available to other internal processes rather than to a miniature copy of the whole brain. The fact that the brain does go to the trouble of constructing a full spatial analog of the external environment merely suggests that it has ways to make use of this spatial data. For example field theories of navigation have been proposed (Koffka 1935 pp 42-46, Gibson & Crooks, 1938) in which perceived objects in the perceived environment exert spatial field-like forces of attraction and repulsion, drawing the body towards attractive percepts, and repelling it from aversive percepts, as a spatial computation taking place in a spatial medium. If the idea of an explicit spatial representation in the brain seems to "fly in the face of what we know about the neural substrates of space perception" (Pessoa et al. 1998 author's response R3.2 p. 789), it is our theories of spatial representation that are in urgent need of revision. For to deny the spatial nature of the perceptual representation in the brain is to deny the spatial nature so clearly evident in the world we perceive around us. To paraphrase Descartes, it is not only the existence of myself that is verified by the fact that I think, but when I experience the vivid spatial presence of objects in the phenomenal world, those objects are certain to exist, at least in the form of a subjective experience, with properties as I experience them to have, i.e. location, spatial extension, color, and shape. I think them, therefore they exist (Price 1932, p. 3). All that remains uncertain is whether those percepts exist also as objective external objects as well as internal perceptual ones, and whether their perceived properties correspond to objective properties. But their existence and fully spatial nature in my internal perceptual world is beyond question if I experience them so, even if only as a hallucination.
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