LUMINANCE AND COLOR EFFECTS ON LOCALIZATION OF BRIEFLY FLASHED VISUAL-STIMULI

Visual localization was studied by flashing small stimuli on a green b ackground and requiring observers to press keys to indicate whether th e stimulus appeared to the left or right of fixation. The results sugg est that, for small (0.25 deg) briefly flashed (17 ms) stimuli at an e ccentric location (10 deg), color contrast is not useable and localiza tion presumably must rely on the magnocellular pathway. When stimulus size and duration were increased at 10-deg eccentricity, isochromatic stimuli could be localized at less than 10% luminance contrast (again suggesting use of the magnocellular high sensitivity luminance-contras t system), but isoluminant color-contrast stimuli could also be locali zed (suggesting use of the color-contrast sensitive parvocellular syst em). Thus, the results indicate that, dependent on stimulus conditions , both magnocellular and parvocellular pathways were utilized by norma l observers in this localization task.