The spatiotemporal range of inhibitory interaction in flicker detection

The flicker thresholds of luminous bars were measured as a function of the spatial and/or temporal separation of two flickering stimuli. Each of the bars had an intensity profile of one-half cycle of a sinusoidal wave subtending 2.26 x 0.45 arc deg and each bar was presented twice at two positions with a duration of 10 msec. The spatial separation was defined as the distance between the adjacent flanks of two flickering stimuli, while the temporal separation was determined as the time-lag between the offset of the first flickering stimulus and the onset of the second. We found that the thresholds increased asymptotically with the spatial separation in such a way as to suggest that the spatial extent over which inhibitory interaction could be effective was as large as about 2 arc deg. We also found that the threshold gradually decreased with greater temporal separation; this indicated that the temporal proximity of successive stimuli effects less suppression on the temporal response. These two effects were seemingly additive. These findings suggest that the visual system involves not only local spatial interaction, but also a global mechanism capable of spreading inhibition over several local units after a delay of several msec.