The influence of threshold variability on the response of visual cortical neurons

Orientation-selective neurons in monkeys and cats show contrast saturation and contrast-invariant orientation tuning (Albrecht and Hamilton, J. Neurop hysiol. 48 (1982) 217-237). Recently proposed models for orientation select ivity predict contrast invariant orientation tuning but no contrast saturat ion at high strength of intracortical recurrent couplings, whereas at lower coupling strengths the contrast response saturates but the tuning widths a re contrast dependent (Hansel and Sompolinsky, Methods in Neuronal Modeling , MIT Press, Cambridge, MA, 1997, pp. 499-567; Stetter et al., Visual Neuro sci., Submitted for publication). In the present work we address the questi on, if and under which conditions the incorporation of a stochastic distrib ution of activation thresholds for cortical neurons helps resolving that pa radoxon. We find that both phenomena occurs naturally if two different clas ses of inhibitory inter-neurons are combined. Low-threshold inhibition keep s the gain of the cortical amplification finite, whereas high-threshold inh ibition causes contrast saturation. (C) 2000 Elsevier Science B.V. All righ ts reserved.