Abstract
Humans can distinguish between contours of similar orientation, and between directions of visual motion. There is consensus that both of these capabilities depend on selective activation of tuned neural channels. The bandwidths of these tuned channels are estimated here by modelling previously published empirical data. Human subjects were presented with a rapid stream of randomly oriented gratings, or randomly directed motions, and asked to respond when they saw a target stimulus. For the orientation task, subjects were less likely to respond when two preceding orientations were close to the target orientation but differed from each other, presumably due to a failure of summation. For the motion data, by contrast, subjects were more likely to respond when the vector sum of two previous directions was in the target direction. Fitting a cortical signal-processing model to these data showed that the direction bandwidth of motion sensors is about three times the bandwidth of orientation sensors, and that it is the large bandwidth that allows the summation of motion stimuli. The differing bandwidths of orientation and motion sensors presumably equip them for differing tasks, such as orientation discrimination and estimation of heading, respectively.
Highlights
Orientation selectivity and motion direction sensitivity are two fundamental properties of the mammalian visual system
Bandwidths have been measured from single neurons in the primate: the median orientation bandwidth for a population of primary visual cortical cells was 40u [7], and the mean motion direction bandwidth in area MT ranged from 125u to 147u [8]
Gratings in the orientation experiment had a spatial frequency of 2 cycles/deg, a contrast of 0.998, and a diameter of 3u
Summary
Orientation selectivity and motion direction sensitivity are two fundamental properties of the mammalian visual system. A critical feature of such sensor arrays, has been measured in the orientation domain using techniques such as masking [2,3]. Estimates of motion direction bandwidth [4,5,6] are varied, 62u–100u, but have a mean substantially larger than that for orientation. Bandwidths have been measured from single neurons in the primate: the median orientation bandwidth for a population of primary visual cortical cells was 40u [7], and the mean motion direction bandwidth in area MT ranged from 125u to 147u [8]. Motion direction bandwidth substantially exceeds orientation bandwidth
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