Figure 216

PET scans use radioactively-labelled substances introduced into the blood to view metabolic activity in three-dimensions, and this is a PET scan of the brain seen from below during visual activity. The frontal lobe is at lower centre. The most active area is the visual cortex within the occipital lobe at the back of the brain (at upper centre), showing the brain's visual centre. Photo credit: Montreal Neurological Institute/McGill University/CNRI/Science Photo Library.

Farah's (1989) hypothesis that imagery involves use of the same stored representations as visual perception. This hypothesis led them to predict that EH would have impaired performance on imagery tasks involving colour (e.g., working out which two out of three named objects had the same colour). In fact, EH performed at normal levels on these imagery tasks. Shuren et al. (1996) concluded that EH probably had intact stored colour representations of objects, which permitted him to use colour imagery However, connections between the visual input and these stored representations had been destroyed by the stroke, and so he had no colour perception.

It has often been assumed that the area of human cortex involved in colour processing corresponds to V4 in monkeys. However, a cautionary note may be in order. Schiller and Lee (1991) found that lesions to monkey V4 do not produce the permanent great impairment of colour perception seen in human patients with achromatopsia. Such findings led Tovee (1996, p. 110) to conclude: "Although an area in human cerebral cortex has been located that is selective for colour, it may not be the homologue [same structure] of monkey V4."

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