Figure 215

A cross-section of the visual cortex of the macaque monkey. From Zeki (1992). Reproduced with permission. ©

1992 by Scientific American, Inc. All rights reserved. 13% more blood flow within area V4 with the coloured stimuli, but other areas were not more affected by colour (Figure 2.16). Zeki (1993) carried out a similar study, and found that V1 and V4 were both activated more by the coloured displays.

If area V4 is specialised for colour processing, then patients with damage mostly limited to that area should show little or no colour perception, combined with fairly normal form and motion perception. This is the case in some patients with achromatopsia. However, many of them do have problems with object recognition as well as an inability to identify colours by name.

In spite of the fact that patients with achromatopsia complain that the world seems devoid of colour, some aspects of colour processing are preserved. Heywood, Cowey, and Newcombe (1994) studied MS, a patient with achromatopsia. He performed very poorly on an oddity task, on which he had to select the odd colour from a set of stimuli having the same shape. However, he performed well on a similar task on which he had to select the odd form out of a set of stimuli (e.g., one cross and two squares), a task that could only be performed accurately by using colour information. As Kehler and Moscovitch (1997, p. 326) concluded, "MS is able to process information about colour implicitly when the actual perceptual judgement concerns form, but is unable to use this information explicitly when the judgement concerns colour."

Shuren et al. (1996) studied EH, a man who had developed achromatopsia as a result of a stroke. Use of MRI confirmed that area V4 was damaged. However, Shuren et al. (1996) were mainly interested in testing

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.

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