Hierarchy of spatial interactions in the processing of contrast-defined contours

Both psychophysical and neurophysiological evidence suggest that there are two visual cortical processing streams, a linear stream that processes firs t-order stimuli and a nonlinear stream that also processes second-order sti muli. This evidence also suggests that before the extraction of the second- order signal, the nonlinear pathway broadly but not completely pools signal s across initial linear filters that encode the orientation of the carrier of the second-order signal. The evidence suggests that such pooling does no t occur across carrier spatial frequencies. We show that similar results ar e obtained with repulsion tilt illusions but not with attraction effects. A ttraction effects exhibit complete orientation crossover (while retaining s patial frequency selectivity), perhaps indicating higher-level processing; an experiment on interocular transfer of the effects supported this conclus ion. (C) 2001 Optical Society of America.