Temporal response characteristics of cells in monkey striate cortex measured with metacontrast masking and brightness discrimination.

After measuring metacontrast masking psychophysically in two monkeys, recordings from parafoveal striate cortex of the monkeys were made while they performed a simultaneous brightness discrimination and while they judged the apparent brightness of a stimulus masked by metacontrast. The size and orientation of the stimuli were held constant regardless of receptive field parameters. In both tasks, the single-cell activity immediately following the presentation of flashed discrimination stimuli reflected only stimulus parameters, and was independent of the monkey's behavioral choice. Later activity (up to 400 msec post-stimulus) was significantly greater if the monkey was about to press the correct panel in the discrimination, or if he pressed the unmasked side (with greater apparent brightness but identical intensity) in the masking paradigm. One quarter of the cells showed a change in firing rate during the 250 msec preceding the behavioral response, though the difference in overall firing level between correct and incorrect brightness discrimination trials was diminished in this epoch, and the corresponding firing difference in metacontrast trials was not significant. The temporal pattern of firing also differed between correct and incorrect trials in the pre-response interval. The results suggest an iterative or recurrent coding of visual information, where the same cells participate in early, late, preresponse coding in different ways.