INSIGHTS INTO THE DIFFERENT EXPLOITS OF COLOR IN THE VISUAL-CORTEX

A new method that allows controlled masking of luminance contrast has been developed to study the use of chromatic signals in human vision. The method also makes it possible to examine the different uses of chr omatic signals (e.g. the generation of perceived colour, or the constr uction and representation of object structure and form). By using this technique, we studied the threshold detection of chromatic signals in normal trichromats. The results show that chromatic signals are virtu ally unaffected by ongoing, randomly varying, luminance contrast chang es. These findings suggest that chromatic signals are either processed independently or can be separated completely from any confounding lum inance contrast components in the stimulus. Thresholds for detection o f colour changes only, and for extraction of stimulus structure from c hromatic signals in normal trichromats, in subjects with single cone r eceptor deficiency (i.e. dichromats) and in three subjects with abnorm al colour vision caused by bilateral damage to ventromedial, extra-str iate visual cortex (i.e. subjects with cerebral achromatopsia) have al so been measured. No significant difference in thresholds for the two conditions was observed either in normal trichromats or in dichromats. Subjects with cerebral achromatopsia, however, reveal markedly differ ent thresholds. The results suggest that chromatic signals are process ed independently to generate perceived object colour or to construct s patially structured objects, and that these functions involve differen t neural substrates. The results help to explain, at least in part, wh y cerebral achromatopsia is a het and why there can be significant dif ferences in the effective use of chromatic signals in subjects describ ed as cerebral achromatopsics.