Directional tuning properties of 89 neurones sampled in the visual wulst. (A-D) Responses of two representative direction-selective neurones to 45% contrast sinusoidal gratings drifting at optimal spatial and temporal frequencies. Left panels: Responses are shown as average peristimulus time histograms (PSTHs) for 16 different stimulus directions (as indicated on top of each histogram) with 10 repeated measures at each using a randomized order of stimulus presentation (bin size ¼ 10 ms). Spontaneous activity was collected before the onset of the stimulus (first 500 ms of the PSTHs). Gratings were presented from 500 to 2500 ms, as indicated by the bars beneath the first column of PSTHs. The grey-shaded rectangles delineate the 1800 ms period considered for the analyses, after the exclusion of the 200 ms period following stimulus onset. Right panels: Direction tuning curves in which mean firing rate (spikes ⁄ s), computed over the whole stimulation period minus the initial 200 ms, is plotted as a function of stimulus direction. The open circles indicate mean response amplitude in the preferred and antipreferred direction. Solid lines represent the fitted models used to determine the half width at half height of the model peak response, which was our measure of the cell's directional tuning bandwidth (BW). Bars indicate the confidence interval of the mean at the 95% level. The models were based on the sum of two von Mises functions, as described in Materials and methods. The grey horizontal line indicates the level of spontaneous activity. (E) Distribution of direction indices (DIs). The DI has positive values that increase from 0 to 1 as mean response in the preferred direction increase relative to that in the antipreferred direction, with values above 1 if the response is inhibited (below spontaneous activity level). The dashed line indicates the boundary between orientation-selective (DI < 0.5) and direction-selective (DI ‡ 0.5) neurones. (F) Distribution of direction bandwidths. Mean and median values are represented for both DI and bandwidth distributions by the filled and empty triangles, respectively.