Isolating Excitatory and Inhibitory Nonlinear Spatial Interactions Involved in Contrast Detection **Part of this paper was presented at the 17th ECVP conference, Eindhoven, The Netherlands (September 1994).

Interactions between filters tuned to different orientations and gpatial Iocatiom were investigated with a masking paradigm. Targets were masked by pairs of Gabor signals presented either at a different orientation (*A(I)or at a different spatial location (&Ay).The two mask components were either of equal phase or of opposite phase to each other. Detection thresholds of the target were measured as a tbnction of mask contrast. Typically, the curves obtained showed the following behavior: for increasing mask contrast the threshold first decreased, then reached a minimum and then increased linearly on a log-log scale reflecting a power-law behavior. Mask pairs of equal phase as well as pairs of opposite phase were shown to facilitate detection. Facilitation by mask pairs of equal phase was larger (up to 0.4 log units) and decreased for increasing A(3and Ay. The facilitation for mask pairs of opposite phase (W0.1 log units) was observed only for larger AOand Ay. Phase independent suppression was observed with higher mask contrasts at smaller Ad and Ay. The strength of this suppression was shown to decrease with practice. We account for the observed facilitation with an accelerating transducer timction applied on a second-stage filter. Suppression is modeled with an additional inhibitory second stage filter that divides the output of this transducer. Selective reduction of the inhibitory gain accounts for the practice effects.