The In uence of Threshold Variability on theResponse of Visual Cortical

Orientation{selective neurons in monkeys and cats show contrast saturation and contrast{invariant orientation tuning (1). Recently proposed models for orientation selectivity predict contrast invariant orientation tuning but no contrast saturation at high strength of intracortical recurrent couplings, whereas at lower coupling strengths the contrast response saturates but the tuning widths are contrast dependent (3; 7). In the present work we address the question, if and under which conditions the incorporation of a stochastic distribution of activation thresholds for cortical neurons helps resolving that paradoxon. We nd that both phenomena 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.