This paper examines how the responses of cells in area 17 of the cat vary as a function of the vernier offset between a bright and a dark bar. The study was prompted by the finding that human vernier acuity is reduced for bars or edges of opposite contrast sign (Mather & Morgan, 1986; O'Shea & Mitchell, 1990). Both simple and complex cells showed V-shaped tuning curves for reverse contrast stimuli: i.e. response was minimum at alignment, and increased with increasing vernier offset. For vernier bars with the same contrast sign, γ-shaped tuning curves were found, as reported earlier (Swindale & Cynader, 1986). Sensitivity to offset was inversely correlated in the two paradigms. However, complex cells with high sensitivity to offsets in a normal vernier stimulus were significantly less sensitive to offsets in reverse contrast stimuli. A cell's response to a vernier stimulus in which both bars are bright can be predicted by the shape of its orientation tuning curve, if the vernier stimulus is approximated by a single bar with an orientation equal to that of a line joining the midpoints of the two component bars (Swindale & Cynader, 1986). This approximation did not hold for the reverse contrast condition: orientation tuning curves for compound barswere broad and shallow, rather than bimodal, with peaks up to 40 deg from the preferred orientation. Results from simple cells were compared with predictions made by a linear model of the receptive field. The model predicted the V-shaped tuning curves found for reverse contrast stimuli. It also predicted that absolute values of tuning slopes for vernier offsets in reverse contrast stimuli might sometimes be higher than with normal stimuli. This was observed in some simple cells. The model was unable to explain the shape of orientation tuning curves for compound bars, nor could it explain the breakdown of the equivalent orientation approximation.
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