Interaction of spatial and temporal factors in psychophysical estimates of surround suppression

Recent psychophysical work has shown that performance on a direction discrimination task decreases with increasing stimulus size, provided the stimulus is high in contrast. This psychophysical surround suppression has been linked to the inhibitory spatial surrounds that have been observed throughout the primate visual system. In this work we have examined a temporal factor that may also contribute to psychophysical surround suppression. Consistent with previous work, we found that psychophysical surround suppression is strongest when a high-contrast motion stimulus is presented very briefly so that the appearance of the stimulus coincided with its motion. However, when a brief delay was inserted between the stimulus onset and the onset of motion, the counterintuitive effects of stimulus size disappeared. The effect of the motion onset asynchrony (MOA) was strongest when the stationary stimulus immediately preceded the stimulus motion and when stimulus orientation during the MOA was very similar to that during the motion presentation. We conclude that psychophysical surround suppression is partially linked to the temporal structure of the stimulus, more precisely to a masking effect caused by sudden stimulus onsets (and to a smaller degree stimulus offsets).