The regional cortical basis of achromatopsia: a study on macaque monkeys and an achromatopsic patient

Previous experiments have revealed total loss of colour vision following removal of all inferior temporal cortex, a condition akin to complete cerebral achromatopsia in humans. Whether less extensive ablation genuinely impairs colour perception without abolishing it or retards learning involving coloured stimuli is contested. We therefore tested macaque monkeys, with total removal of temporal areas TEO and TE but sparing rostral and perirhinal temporal cortex and the upper bank of the superior temporal sulcus. Compared with three monkeys with lateral parietal ablations, the monkeys with TEO/TE lesions were impaired at learning and retention of simultaneous two-choice colour discriminations and with a nine-choice oddity discrimination whether the coloured target was embedded among grey distracters of the same luminance or among isoluminant coloured distracters. However, their performance was superior to that of an achromatopsic human subject and to that previously measured in monkeys with much larger temporal lobe ablation. They were only mildly impaired at nine-choice oddity discrimination for grey stimuli where the grey target was brighter than the grey distracters. The impairment could be exacerbated or alleviated by altering the colour of the background of the displays and by static and dynamic luminance masking of the entire display in a manner that indicates that the colour deficit reflects a change in perception rather than a disorder of learning and memory. It resembles central dyschromatopsia in human subjects but falls short of achromatopsia.