Figure 915

Diagram of the relative amounts of the visual arc that are taken up by different-sized animals. Adapted from Ghosts in the mind's machine: Creating and using images in the brain by Stephen Kosslyn. Reproduced by permission of the author. Copyright © 1983 by Stephen M.Kosslyn.

completely the visual arc and may even overflow it. That is, it may stretch beyond our field of view. Kosslyn employed the same idea to test the limited extent of the spatial medium. If one assumes that the spatial medium has a limited extent and has a similar visual-image arc, then one way of measuring the size of an imaged object is in terms of the arc it subtends. At some point an object of a certain size should overflow the medium (see Figure 9.15). To test this prediction, subjects were asked to close their eyes and to image an object (usually an animal again) far away in the distance. They were then asked to "mentally walk" towards the image until they reached a point where they could see all the object at once (i.e., the point just prior to overflow). Finally, they were asked to estimate how far away the animal would be if they were seeing it at that subjective size. If the spatial medium has a limited extent of a constant size then the larger the object, the farther away it would seem at the point of overflow. This was the result found by Kosslyn. In general, the estimated distance of the point of overflow increases linearly with the size of the imaged object.

As we have seen throughout this book, one strong test of a theory is to see whether it is consistent with neuropsychological evidence from the study of individuals with brain injuries. As we shall see in the next section, Kosslyn's theory has also been applied to understanding the patterns of behaviour manifested by brain-damaged patients.

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