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Abstract

To gain insight into the neural basis of colour constancy, we examined the colour vision of twenty-seven patients with defined unilateral lesions mainly located in the parieto-temporo-occipital cortex. Patients were tested with a battery of vision and colour vision tests. Detection and grouping thresholds for isoluminant chromatic and for luminance stimuli were measured to assess retinal and early cortical processing. To examine higher cortical functions, we tested colour memory, colour - object association, and colour constancy. For the evaluation of colour constancy, subjects had to adjust the colour of a test field until it appeared as neutral gray. The test field was embedded amid a set of coloured patches, and the illuminant was varied from trial to trial. A control group of healthy subjects was tested in the same tasks.

Five of the twenty-seven patients showed a selective deficit in the colour constancy tests. All five patients showed normal colour discrimination. A comparison with anatomical lesion data, based on CT- or MRI-scans, showed that one of the five patients had a lesion near the fusiform and lingual gyri, whose importance for colour constancy had been suggested in earlier studies. However, three other patients had overlapping lesions in a region of parieto-temporal cortex, which so far had not been associated with colour vision. These results indicate that the computations underlying colour constancy are mediated by specialised cortical circuitry, which is independent of the neural substrate for colour discrimination and for assigning colours to objects.