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Margaret Livingstone, Bevil Conway; Responses of V1 neurons to reverse phi stimuli. Journal of Vision 2002;2(7):127. doi: 10.1167/2.7.127.
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Anstis (1970) showed that 2-stimulus apparent motion appears to go in the reverse direction if the two stimuli are of opposite contrast. He termed this reverse phi. Neural correlates of reverse phi have been reported in cat V1 (Emerson et al., 1987), primate MT (Livingstone et al., 2001), and wallaby pretectum (Ibbotson & Clifford, 2001). Here we present a quantitative evaluation of responses to phi and reverse phi apparent motion stimuli in alert macaque V1. Of a population of 118 V1 cells (60 complex, 38 simple, 20 S1) ALL showed direction-selective responses to 2-bar apparent-motion stimuli, and All of them showed reversed direction preference when the two bars were of opposite contrast. The reversal was not complete: the slope of the same-contrast direction index vs the inverting-contrast direction index was −.76, −.72 and −.67 for the 3 cell types, respectively, and the r2 was .96, .8 and .83.
This result implies that direction-selective cells are generated by combining spatially and temporally offset inputs that are linear with respect to contrast; that is they change their firing rate in opposite directions for stimuli of opposite contrasts, and it further implies that the direction-generating mechanism combines linear inputs of like contrast selectivity. This combination has been modeled as a sum (Energy model) or a product (Reichardt model). The Reichardt model predicts complete reverse phi. The Energy model predicts complete reverse phi after a squaring nonlinearity. The observed difference from complete reverse phi can be accounted for by rectification.
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