FORM AND MOTION FROM COLOR IN CEREBRAL ACHROMATOPSIA

Patients with cerebral achromatopsia, resulting from damage to ventrom edial occipital cortex, cannot chromatically order, or discriminate, h ue. Nevertheless, their chromatic contrast sensitivity can be indistin guishable from that of normal observers. A possible contributor to the detectability of chromatic gratings is the subadditive nature of cert ain colour combination such that mixtures of, for example, red and gre en (yielding yellow) appear dimmer than expected from the simple addit ion of luminances. This subadditivity is believed to reflect colour-op ponent interactions between the outputs of long- and medium-wavelength cones. We performed a first-order compensation for such subadditivity in chromatic gratings and demonstrated that their detection was still not abolished in an achromatopsic patient. In addition, we used a two -alternative forced-choice procedure with an achromatopsic patient, wh o was required to judge the apparent relative velocity of two drifting gratings with different degrees of compensation for subadditivity. It is well known that isoluminant gratings, constructed by adding a red and green sinusoidal grating of identical peak luminances in antiphase , appear to drift substantially slower than an achromatic grating with the same velocity. Adding 2f luminance compensation to an isoluminant grating of spatial frequency f, resulted in an identical minimum of p erceived velocity at a compensation contrast of 5% in both achromatops ics and normal observers. Furthermore, while compensation for subaddit ivity did not substantially compromise grating detection at low contra sts, such correction severely affected motion detection. Saccadic eye movement accuracy and latency were also measured to uncompensated chro matic, compensated chromatic and achromatic targets. We conclude first that subadditivity, resulting from colour-opponent P-channel processe s, influences motion judgements. The ability to extract motion from ch romatic differences alone is little, if at all, different in achromato psic and normal vision. Second, the paradoxical detection of sinusoida lly modulated chromatic gratings in achromatopsic patients is not mere ly a result of subadditivity. Third, saccadic latency, but not accurac y, to chromatic targets is affected by luminance compensation. Finally , and more generally, wavelength processing continues to contribute to several aspects of visual processing even when colour is not perceive d.