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Monday 18 March 2019

Are Visual images like Visual Perception?

Can people recognize patterns in mental images in the same way that they rec- ognize patterns in things they actually see? In an experiment designed to inves- tigate this question, Finke, Pinker, and Farah (1989) asked participants to create mental images and then engage in a series of transformations of those images. Here are two examples of the problems that they read to their participants:
● Imagine a capital letter N. Connect a diagonal line from the top right cor- ner to the bottom left corner. Now rotate the figure 90° to the right. What do you see?
● Imagine a capital letter D. Rotate the figure 90° to the left. Now place a capital letter J at the bottom. What do you see?
Participants closed their eyes and tried to imagine these transformations as they were read to them. The participants were able to recognize their com- posite images just as if they had been presented with them on a screen. In the first example, they saw an hourglass; in the second, an umbrella. The ability to perform such tasks illustrates an important function of imagery: It enables us to construct new objects in our minds and inspect them. It is just this sort of visual synthesis that structural engineers or architects must perform as they de- sign new bridges or buildings.
Chambers and Reisberg (1985) reported a study that seemed to indicate dif- ferences between a mental image and visual perception of the real object. Their re- search involved the processing of reversible figures, such as the duck-rabbit. Participants were briefly shown the figure and asked to form an image of it. They had only enough time to form one interpretation of the picture before it was removed, but they were asked to try to find a second interpretation. Participants were not able to do this. Then they were asked to draw the im- age on paper to see whether they could reinterpret it. In this circumstance, they were successful. This result suggests that mental images differ from pic- tures in that one can interpret visual images only in one way, and it is not possible to find an alternative interpretation of the image.
Subsequently, Peterson, Kihlstrom, Rose, and Gilsky (1992) were able to get participants to reverse mental images by giving them more explicit instructions. For instance, participants might be told how to reverse another figure or be given the instruction to consider the back of the head of the animal in their mental image as the front of the head of another animal. Thus, it seems apparent that although it may be more difficult to reverse an image than a picture, both can be reversed. In general, it seems harder to process an image than the actual stimulus. Given a choice, people will almost always choose to process an actual picture rather than imagine it. For instance, players of Tetris prefer to rotate shapes on the screen to find an ap- propriate orientation rather than rotate them mentally (Kirsh & Maglio, 1994).
■ It is possible to make many of the same kinds of detailed judgments about mental images that we make about things we actually see, though it is more difficult.
Visual perception & Visual Imagery
the same brain regions that are involved in visual perception are also involved in visual imagery. Such research has presumably put to rest the question raised at the beginning of the chapter about whether visual imagery really had a perceptual character. How- ever, although it seems clear that perceptual processes are involved in visual imagery to some degree, it remains an open question to what degree the mech- anisms of visual imagery are the same as the mechanisms of visual perception Evidence for a substantial overlap comes from neuropsychological patient stud- ies (see Bartolomeo, 2002, for a review). Many patients who have cortical damage leading to blindness have corresponding deficits in visual imagery. As Behrmann
(2000) notes, the correspondences between perception and imagery can be quite striking. For instance, there are patients who are not able to perceive or image faces and colors, but are otherwise unimpaired in either perception or imagery. Nonetheless, there exist cases of patients who suffer perceptual problems but have intact visual imagery and vice versa. Behrmann argues that visual perception and visual imagery are best understood as two processes that overlap but are not iden- tical, as illustrated in Figure 4.19. Perceiving a kangaroo requires low-level visual information processing that is not required for visual imagery. Similarly, forming a mental image of a kangaroo requires generation processes that are not required by perception. Behrmann suggests that patients who suffer only perceptual losses have damage to the low-level part of this system, and patients who suffer only im- agery losses have damage to the high-level part of this system.

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