The Depictive Nature of Visual Mental Imagery
Norman Yujen Teng
Page 3
Source: http://www.bu.edu/wcp/Papers/Mind/MindTeng.htm
Marr's 2 1/2-D Sketch
Marr's (1982) notion of the 2 1/2-D sketch is very complicated, but the basic points which concern us here are not. I shall restrict my discussion to those basic points. According to Marr, a 2 1/2-D sketch is a representation of visible surfaces in three dimension. This notion can be illustrated in the following way. Suppose I am looking at a circular object, say a coin on my desk, which is tilted away from me. The appearance of this object as it presents to me is dependent upon the point of view from which I see it. And I see the object as being circular and as being tilted away from me. But let us specify the appearance of the object without mentioning the fact that it seems to be tilted away from me, that is, without any reference to its outward distance and orientation in my visual field. The apparent shape of this object then looks elliptical. The information of this apparent shape, presumably, can be encoded in an image-based representation in the form of an array in my visual system, with adjacent cells in the array representing the adjacent parts of the apparent shape. How this can be done needs not concern us here. The important point is that the function of the 2 1/2-D sketch is to assign portions of the image to the surfaces in the environment and to specify the distance and orientation of those surfaces relative to the viewer. This can be done by letting the cells in the array contain symbols representing such features as orientation and depth of the patch of surface relative to the viewer (Tye, 1991, pp. 81-83). Having said this, it is clear now how the viewer-centered information can be preserved in the intentional contents expressed in the topographic maps. Activity in any given cell in the maps can be conceived of as containing descriptive labels, which represent the following local features: presence of a tiny patch of surface, orientation and depth of the patch of surface, determinate shade of color, texture, and so on. This result, in my view, can in principle explain why a topographic representation can become a depictive representation, though Tye didn't directly address this issue.
Mental Images As Interpreted Symbol-Filled Arrays
Tye is quite aware of the fact that mental images are pre-interpreted and are composed of organized units rather than arbitrary parts. That is why he introduces the notion that descriptive labels that provide a more specific content are appended to the array at different levels of groupings. For example, some labels are attached to single cells representing orientation of the patch of surface and so on, as described above. Further labels are attached to groups of cells representing nonlocal features, such as a circle or a square, and in more complex cases, a duck or a rabbit. He argues that the labels are linguistic in form (Tye, 1991, pp. 90-102; see also Raffman, 1997, p. 190). But he also argues that the groupings and how the descriptive labels are appended to the array can be done at a nonconceptual level (Tye, 1995, pp. 122-123). A problem immediately arises concerning the linguistic nature of the labels and its relation to the nonconceptual activity of appending the labels to the array. I will return to this problem in the next section.
The Imagery Format and Depictive Representation
Now is the time to answer the two problems set out at the beginning of this inquiry. Consider first the problem (Q1). The format of image representation specifies what types of representational elements can be used as its components and the way in which these components can be arranged and processed. The format can be conceived of as constituted by topographically structured arrays or matrices to which labels of certain types can be attached. The groupings of the cells and the labelling activities form an overlapping and nested hierarchy, depending on how the patterns of activation of the cells are formed. In the case of visual mental imagery, the labels are of the type which can be interpreted as having the intentional contents which make the labels stand for certain determinate surface features from a particular point of view. In the case of auditory mental imagery, the labels are of the type which can be interpreted as having the intentional contents which make the labels stand for certain sound properties. We don't need the so-called depictive format of mental imagery, which is beset by problems of the sort we have discussed (cf. Kosslyn, 1980, pp. 31-35; 1983, p. 32-37; 1994, pp. 12-20). Consider the problem (Q2). It should now be clear that topographic representations can become depictive representations by virtue of the intentional contents of certain appropriate labels that are attached to them. Tye argues that the labels are linguistic in form, but the grouping and labelling activity can be done at a nonconceptual level. This is possible once we note that the internal resources that are deployed when one is using linguistic entities need not be parts of a linguistic system (Bechtel & Abrahamsen, 1991, Ch. 7; Clark, 1993; Rumelhart, 1992; Rumelhart et al., 1986; Teng, forthcoming). It seems to me, however, that Tye glosses over the different levels of grouping and labelling activity by saying that all the labels are linguistic in nature. If the image is a rabbit, perhaps a linguistic label, say, `This is a rabbit' will be attached to it. The linguistic labels, presumably, constitute a representational scheme, independent of the topographically organized format of image representations. But for labels that indicate presence of a tiny patch of surface, orientation of the patch of surface, and so on, it is the topographic medium, rather than some linguistic system, that accounts for their organization and how they can be grouped into consistent parts of an image representation. Those labels do not constitute a representational scheme independent of the activity of the cells in the topographically organized array. They are embodied in the activity. It is under certain functional analyses that we talk of the activities and the groupings of the cells as if some descriptive labels were attached to them. And what happens when one attaches a descriptive label to an image representation is that one has the representation whose content is then brought under the given concept expressed by the descriptive label. Consider an image of a square, for example. The image has its content encoded in the topographically organized array; the pattern of the activities of the cells indicates that a square is represented. The pattern and the activity of each cell, presumably, are nonconceptual. One can further bring the pattern under the concept expressed by a linguistic label, say, `This is a square', for further information processing.
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