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. Eac h of the bars had an intensity profile of one-half cycle of a sinusoid al wave subtending 2.26 x 0.45 arc deg and each bar was presented twic e at two positions with a duration of 10 msec. The spatial separation was defined as the distance between the adjacent banks of two flickeri ng stimuli, while the temporal separation was determined as the time-l ag between the offset of the first flickering stimulus and the onset o f the second. We found that the thresholds increased asymptotically wi th the spatial separation in such a way as to suggest that the spatial extent over which inhibitory interaction could be effective was as la rge as about 2 arc deg. We also found that the threshold gradually dec reased with greater temporal separation; this indicated that the tempo ral proximity of successive stimuli effects less' suppression on the t emporal response. These two effects were seemingly additive. These fin dings suggest that the visual system involves not only local spatial i nteraction, but also a global mechanism capable of spreading inhibitio n over several local units after a delay of several msec.