Investigation of center-surround interaction in motion with reaction time for direction discrimination.

How motion onset asynchrony (MOA) alters the effects of stimulus size on reaction time (RT) for direction discrimination of a drifting grating was examined. MOA is a delay from the stimulus onset to the onset of motion. Without MOA, RTs were found to increase as the stimulus size was increased at high contrast, but decrease with it at low contrast or at high noise levels. With MOA, however, RTs did not increase as the stimulus size increased even at high contrast. These results suggest that sudden stimulus onset evokes the increase of RTs with the increase of stimulus size at high contrast. RTs for direction discrimination of a drifting Gabor patch (the target) surrounded by a different drifting or a static grating as well as RTs for the target that was not surrounded by an additional grating were measured. The RTs for the target moving in the same or opposite direction as the motion of the surrounding grating were larger than those for the target with the static grating or no additional grating at moderate or high contrast. There was no significant difference between the RTs for the target moving in the same direction as the surrounding grating and the RTs for the target moving in the opposite direction. At low contrast and without MOA, however, the RTs for the target moving in the same direction as the surrounding grating were larger than those for the target moving in the opposite direction. These results suggest surround suppression at low contrast under some conditions. They also suggest that the decrease of RTs for discriminating motion direction of a drifting single Gabor patch with the increase of stimulus size at low contrast does not necessarily mean the absence of surround suppression.