The Depictive Nature of Visual Mental Imagery
Norman Yujen Teng
Page 2
Source: http://www.bu.edu/wcp/Papers/Mind/MindTeng.htm
Two Assumptions
That depictive representations require something more than just order-preserving is, as I take it, quite uncontroversial. Nevertheless, certain remarks are needed to clarify what is at issue and what is not. First, what is at issue is not about what qualifies as a representation, but whether or not there are depictive mental representations. Second, for the present purposes (A1) is strong enough to support the view that topographic representations are not depictive representations, though we still do not have a fully developed and complete theory of depictive representations. And I will not attempt one in the following discussion. That would be far beyond the scope of this paper. That being said, let me elucidate the distinction between depictive and topographic representations by the following example. Consider a square. The depiction of this shape normally places its sides parallel to the gravitational frame, that is, with its top side and bottom side parallel to the horizontal ground and the other two sides aligned with the vertical. Now rotate the square 45 degrees. Observe that the shape now looks like a diamond. This simple transformation preserves any spatial order of adjacent parts of the original shape, yet the resultant configuration depicts a diamond, rather than a square (Arnheim, 1974, pp. 98-103; Leyton, 1992, pp. 342-346; Mach, 1897/1959). This square-diamond phenomenon shows that depiction cannot be merely topographic representation. Consider now the second assumption that topographic representations in the auditory cortex are not depictive representations. In defending this assumption it will be helpful to make it clear at the outset that depiction is primarily a visual phenomenon. (1) If representing visual qualities is a necessary feature of depiction, then representations of sound frequencies cannot be depictive representations. However, the argument for (A2) cannot be so straightforward. We know that in synesthesia, music or voices may be perceived to have shapes, textures, and colors. We also know that people normally judge that low-frequency sounds express larger visual size than do high-frequency sounds, and may describe a soprano's tone as sharper than an alto's (Marks, 1996; Marks, Hammeal & Bornstein, 1987). Both synesthesia and the perceptual correspondence across visual and auditory modalities seem to suggest that, perhaps, an auditory representation can be a depictive representation via its associated link to visual perception. Notice, however, that the associated link between auditory and visual qualities is asymmetric in the sense that sounds are perceived to have, or judge to express, certain visual qualities, but the perceptual dimensions of visual experience are not perceived to have, or judged to express, auditory qualities. Which suggests that an auditory representation can become a depictive representation only with reference to its associated visual qualities. So, any auditory representation that can be realized in some medium with its representational character independent of visual representations is not a depictive representation. The topographic representations in the auditory cortex clearly satisfy this condition. Thus, I conclude, they are not depictive representations. So (A2) should be accepted. With (A1) and (A2) in hand, it is clear that one should take seriously the problems described in the last section. I turn now to Tye's theory and examine how he can respond to the problems (Q1) and (Q2).
Tye's Theory
Tye (1991, pp. 90-91) proposes the following account of what a visual mental image is: A mental image of an F (though no one F in particular) is a symbol-filled array to which a sentential interpretation having the content "This represents an F" is affixed. The array itself, which is very like Marr's 2 1/2-D sketch, occurs in a fixed medium resembling Kosslyn's visual buffer, and is generated from information in long-term memory that consists in part of viewer-centered information about the visual appearances of Fs and in part of information about the spatial structure of Fs. Let me unpack this formulation by analyzing its four components in the following order: (1) the visual buffer, (2) the viewer-centered representations, (3) Marr's 2 1/2-D sketch, and (4) mental images as interpreted symbol-filled arrays. I shall restrict my exposition to a number of basic points which concern us here.
The Visual Buffer
To understand what the visual buffer is, it will be helpful to start with the following analogy Kosslyn proposed. Visual mental images are conceived of on the model of displays on a cathode-ray tube screen attached to a computer; such displays are generated on the screen by a computer program (Kosslyn, 1980, pp. 5-9). In this model, the internal representations that underlie our experience of having an image are functionally analogous to the displays on the screen. And the visual buffer in which the internal representations are encoded is functionally analogous to the screen. The visual buffer in this functional analysis has at least the following two properties. (V1) Its components are individual cells capable of either being activated or not. Those cells, when activated, represent single spatial points on the surface of the imaged object. (V2) Those cells are structured into an array or matrix, with adjacent cells representing adjacent parts of the surface of the imaged object. (2) The properties (V1) and (V2) clearly make the visual buffer capable of supporting topographic representations discussed above. This result, buttressed by the empirical discovery that human visual cortex includes topographically mapped areas, shows that the visual buffer is indeed psychologically real (Kosslyn, 1994, pp. 12-20).
Viewer-Centered Representations
Topographic mapping in the visual system, as we have noted, preserves only spatial order of adjacent parts, which is not sufficient to support depictive representations. To remedy this situation, it is important to note that in visual perception the mapping is from the retinal image onto the brain. When one sees an object, one always sees it from a particular point of view. Which means that patterns of activation in retinotopic maps are viewer-centered. One can then postulate that after mapping the viewer-centered information is preserved. The information clearly cannot be preserved merely in terms of topographic relations. Yet the information can be preserved in the intentional contents expressed in the topographic maps. And the constraint of preserving viewer-centered information in visual information processing explains why a topographic representation can be a depictive representation. If this indeed is a case, we have the following scenario. For visual perception, the visual buffer is activated by processes that operate on information contained in the light striking the eyes. For mental images, the visual buffer is activated by generational processes that act on the viewer-centered information stored in long-term memory about the appearances of objects and their spatial structures. Note that this distinction between visual perception and visual mental imagery does not exclude the possibility that imagery is an integral part of how perception operates. To make the above points clearer, we need to examine the third component in Tye's formulation.
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