Cross-orientation suppression is proportional to the square-root of speed for flickering Gabor stimuli

Abstract

A property of many striate cells is cross-orientation suppression (XOS): the response to an optimal grating is suppressed by an orthogonal and superimposed mask. Models of this nonlinear phenomenon have been motivated by physiological constraints (pre-synaptic depression), engineering solutions for components with poor dynamic range (contrast normalization) and fundamental coding strategies for natural images (redundancy reduction). These accounts often make tacit assumptions about the scale invariance of XOS, but this has not been investigated. We addressed this by measuring psychophysical masking functions for flickering horizontal Gabor stimuli (full-width at half-height of 1.67 cycles) over wide ranges of spatio-temporal frequency (SF=0.5–8c/deg; TF=0.5–15Hz) and mask contrast (0–45%). Gabor masks were identical to the target, but with orthogonal orientation. We found substantial levels of XOS (∼12dB) that predominated at fast speeds (where speed=TF/SF) and small levels (∼3dB) of cross-orientation facilitation (XOF) that predominated at slow speeds. Little or no XOS or XOF were found at slow and fast speeds respectively. The data were normalized by detection threshold, and well fit by a functional model of contrast gain control. In this model, the suppressive weight of XOS (w) was a free parameter for each of 15 masking functions, and the modulatory weight of XOF was constant (=16 DF for 90 points). A power function (exponent=0.48) accounted for 89% of the variance in a plot of w against speed. These results (i) provide new constraints for general models of suppression and (ii) suggest that the process underpinning XOF is widespread, but often hidden by suppression.