Form processing

Several areas are involved in form processing in humans, including areas V3, V4, and IT (see Figure 2.13). However, the cognitive neuroscience approach to form perception has focused mainly on IT (inferotemporal cortex). Tanaka (1992) took recordings from individual neurons in IT while numerous objects were presented to monkeys to discover which objects produced the greatest response. After that, he presented features of the most effective stimulus in order to find out the crucial features to which each neuron was responding. Tanaka found that there were elaborate cells in IT that seemed to respond maximally to simple shapes.

Several other researchers have followed this line of research. For example, Sary, Vogels, and Orban (1993) found that the responses of elaborate cells in IT were unaffected by the size and orientation of the visual stimulus. The cells in IT are organised into functional columns, with all the cells in any one column responding to similar stimuli. It seems as if the simple shapes involved may form a "visual alphabet" of perhaps 600 shapes, from which object representations can be constructed. However, neuronal responding in IT may be more complex than has been suggested so far, with some cells responding best to their preferred shape plus the absence of some other shape (Young, 1995).

Some of the most interesting research has focused on responses to faces. There are numerous cells in IT that are responsive to faces, but show virtually no response to other stimuli. For example, Rolls and Tovee (1995) carried out a study on monkeys in which 23 faces and 45 other stimuli were presented. Any one cell showed strong responses to a few faces, coupled with little responding to the other faces or to the non-faces.

It might be expected that some brain-damaged patients would suffer from severely impaired form vision but fairly normal colour and motion processing. However, Zeki (1992, p. 47) claimed that, "no one has ever reported a complete and specific loss of form vision". He argued that the reason for this might be that a lesion that was large enough to destroy areas V3, V4, and IT would probably destroy area V1 as well. As a result, the patient would suffer from total blindness rather than simply loss of form perception.

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