On the In uence of Threshold Variability in a Model of the Visual Cortex

Orientation{selective neu-rons in monkeys and cats show contrast saturation and contrast{ invariant orientation tuning (Albrecht and Hamilton, 1982). Recently proposed models for orientation selectiv-ity predict contrast invariant orientation tuning but no contrast saturation at high strength of recurrent intracor-tical coupling, whereas at lower coupling strengths the contrast response saturates but the tuning widths are contrast dependent (Hansel and Som-polinsky, 1997; Bartsch, Stetter and Obermayer, 1997). In the present work we address the question, if and under which conditions the incorporation of a stochastic distribution of activation thresholds of cortical neurons leads to the saturation of the contrast response curve as a network eeect. We nd that contrast saturation occurs naturally if two diierent classes of inhibitory inter-neurons are combined. Low threshold inhibition keeps the gain of the cortical ampliication nite, whereas high threshold inhibition causes contrast saturation.