Are Theories of Imagery Theories of Imagination? An Active Perception Approach to Conscious Mental Content
Nigel J.T. Thomas
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Source: http://cogprints.org/5018/1/im-im-cp.htm
Kosslyn draws his theory from an analogy with computer graphics, and its basic form is illustrated in figure 1. Data from which images may be constructed are stored in long term memory in the form of "deep representations", structural descriptions not fundamentally dissimilar to those envisaged by description theorists3. But these are not directly available to consciousness. They are analogous to the files in which data is saved by a computer graphics program, and on the basis of which actual, viewable pictures are constructed on the computer's CRT monitor. Kosslyn's (1980) theory is explicitly based on this "CRT metaphor". He holds that "quasi-pictures" or "surface representations" are constructed on the basis of the information in deep representations. This construction takes place at a functionally defined neural locus that he calls the "visual buffer". Once the quasi-picture is established it is available to consciousness as an image, and, furthermore, information that was merely implicit in the deep representation (such as the pointedness of the fox's ears, in the depicted example) can be extracted from it by the postulated "mind's eye function". We should not think of this "function" as literally seeing the image, but it is needed to read and interpret the buffer's "surface display".
Clearly Kosslyn thinks of the visual buffer as also being a stage in perceptual information processing, and in more recent work he has explicitly identified it as composed out of the several retinotopic maps of the brain's occipital cortex (Kosslyn, 1994). The evidence regarding this claim, however, is conflicting. Although Kosslyn and others (Kosslyn et al., 1993; Damasio et al., 1993; Kosslyn, Thompson, Kim, & Alpert, 1995) have detected activity in the relevant brain areas during visual imagery, other researchers (Roland & Gulyás, 1994; Mellet et al., 1996) find no such activity, and argue that imagery is more consistently associated with activity in other, non-retinotopically organized regions. Neurological patients who have lost the retinotopically mapped regions in one cerebral hemisphere, leaving them blind in the corresponding half of their visual field, show certain impaired imagery abilities in the blinded hemifield (Butter, Kosslyn, Mijovic-Prelec, & Riffle, 1997; Farah, Soso, & Dasheif, 1992). However, other patients suffering from cortical blindness due to damage in these areas seem to have relatively normal imagery (Chatterjee & Southwood, 1995). Furthermore, some patients with localized damage in the retinotopically mapped areas experience vivid, well-formed "visual hallucinations" (i.e. imagery that is outside of conscious control--they do not typically mistake it for reality) precisely in the affected (blind or "blindsighted") parts of their visual fields (Ramachandran & Hirstein, 1997; Weiskrantz, Warrington, Sanders, & Marshall, 1974). This suggests that these brain areas cannot be essential for visual imagery.
However, even if Kosslyn proves to be mistaken in equating the occipital retinotopic maps with his imagery buffer, this will not in itself entail the wholesale rejection of his research program. It could still quite reasonably be argued that the buffer may be located elsewhere in the brain, mapped in a less obvious way. No particular claim about the buffer's neurological implementation should be taken as among pictorialism's core commitments.
2.1.2 Computational Implementation of Picture Theory
Kosslyn and Shwartz (1977) developed a computer model of Kosslyn's theory, and its instantiation of the "surface representation", the model for the mental image itself, is revealing. Figure 2 is produced by selectively filling cells in a rectangular array or matrix, which models the visual buffer of the underlying theory. Note that certain cells contain different letters. Kosslyn plausibly assumes that images fade over time, and successive letters of the alphabet represent older, successively more faded portions of the image. The cells, then, are not simply filled or unfilled, they containsymbols representing qualitative features of the image at that point. The theory implies that each cell might contain multiple symbols, representing such things as color, 3-D depth and the presence of edges (Tye, 1991; Kosslyn, 1994) (although these aspects have not been computationally implemented).
Simulated image ("surface representation") of a car, as printed out by the Kosslyn & Shwartz (1977) program.
Following figure 1, the array representation is constructed by following a stored description, and can thus readily be manipulated in various ways: being redrawn at various different sizes, positions and orientations on the buffer, and with varying amounts of detail included. This allows Kosslyn to account for the rotation, scanning, and size/inspection time effects mentioned above (§2).
It is important to note that the images of Kosslyn's theory are "quasi-pictures" or "functional pictures" rather than pictures in a literal sense (Tye, 1991). After all, the array representation in the computer (and, if it exists, in the brain) is not actually visible. The real model for the mental image is not the screen display but the underlying array representation in the computer's memory, physically instantiated as electronic states in RAM chips. Image manipulations are carried out on this array, and are merely mirrored on the screen or printout. Furthermore, the elements of the array in the memory need not be laid out in the actual spatial arrangement seen in figure 2. The representations of adjacent cells in the printout need not be in physically adjacent sections of RAM. All that matters is that they be treated as if they were adjacent by the computational routines which form, transform and inspect the array (Kosslyn, 1980).
Moreover, if, as suggested, each cell in the array holds several symbols, for various represented qualities, there is no need for even the symbols within a single cell to be represented in physically close memory elements. We might have multiple arrays, one for each sort of quality, so long as the accessing routines treat them as a single, superposed, array with multiple symbols in each cell. This is relevant, because in Kosslyn's (1994) neuropsychological version of the theory the visual buffer is taken to be composed of multiple arrays, instantiated in the multiple, specialized, retinotopically mapped areas of visual cortex.
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