CORTICAL COLOR-BLINDNESS IS NOT BLINDSIGHT FOR COLOR

Cortical color blindness, or cerebral achromatopsia, has been likened by some authors to ''blindsight'' for color or an instance of ''covert '' processing of color. Recently, it has been shown that, although suc h patients are unable to identify or discriminate hue differences, the y nevertheless show a striking ability to process wavelength differenc es, which can result in preserved sensitivity to chromatic contrast an d motion in equiluminant displays. Moreover, visually evoked cortical potentials can still be elicited in response to chromatic stimuli. We suggest that these demonstrations reveal intact residual processes rat her than the operation of cover? processes, where proficient performan ce is accompanied by a denial of phenomenal awareness. We sought evide nce for such covert processes by conducting appropriate tests on achro matopsic subject M.S. An ''indirect'' test entailing measurement of re action times for letter identification failed to reveal covert color p rocesses. Ln contrast, in a forced choice oddity task for color, M.S, was unable to verbally indicate the position of the different color, b ut was surprisingly adept at making an appropriate eye movement to its location. This ''direct'' test thus revealed the possible covert use of chromatic differences. (C) 1998 Academic Press.