Behavioural and electrophysiological chromatic and achromatic contrast sensitivity in an achromatopsic patient

OBJECTIVES--In cases of incomplete achromatopsia it is unclear whether residual visual function is mediated by intact striate cortex or results from incomplete lesions to extrastriate cortical visual areas. A patient with complete cerebral achromatopsia was tested to establish the nature of his residual vision and to determine the integrity of striate cortex function. METHODS--Behavioural contrast sensitivity, using the method of adjustment, and averaged visually evoked cortical potentials were measured to sinusoidally modulated chromatic and achromatic gratings in an achromatopsic patient and a normal observer. Eye movements were measured in the patient using a Skalar infrared monitoring system. RESULTS--The patient's chromatic contrast sensitivity was normal, indicating that despite his dense colour blindness his occipital cortex still processed information about spatial variations in hue. His sensitivity to achromatic gratings was depressed particularly at high spatial frequencies, possibly because of his jerk nystagmus. These behavioural results were reinforced by the nature of visually evoked responses to chromatic and achromatic gratings, in which total colour blindness coexisted with an almost normal cortical potential to isoluminant chromatic gratings. CONCLUSIONS--The results show that information about chromatic contrast is present in some cortical areas, and coded in a colour-opponent fashion, in the absence of any perceptual experience of colour.