Spatial selectivity in the anterior ectosylvian visual area of the cat

Pattern reversal visually evoked potentials (PR-VEPs) were recorded in the anterior ectosylvian visual area (AEV) of the cat, and the dependence of the PR-VEPs on the spatial frequency (SF) and the mean luminance of the stimulation was studied. Six SFs between 0.025 and 1 cycles per degree (c/d) and four light density levels between 2 x 10(-2) and 117 cd/m2 (candela per square meter) were tested. The latency of the first positive component (P1) and the amplitude measured between P1 and the first negative (N1) component were analyzed. The dependence of the PR-VEP latencies on the SF displayed a significant relationship, with a minimum at around 0.05 c/d. This relationship was consistent at all luminance levels tested. The dependence of the amplitudes on the SF gave a "low-pass" tuning curve at every luminance level. The higher the luminance level, the steeper was the curve. The cut-off SF of these curves was at around 1 c/d under photopic conditions and around 0.6 c/d under scotopic conditions. The contrast dependence of the PR-VEP amplitudes exhibited saturation at 0.5, which was much higher than that of either the striate cortex or other extrastriate visual cortical areas, but similar to that of the superior colliculus. These results provide further evidence in support of the notion of a tecto-extrageniculo-extrastriate visual pathway functioning separately from the geniculo-striate mechanisms.