Differential roles of contrast polarity reveal two streams of second-order visual processing

Humans can easily segregate texture regions based on differences in contrast, orientation, and contrast polarity. It has been suggested that these abilities can be inclusively modeled by 2nd-order visual mechanisms that detect changes in the half-wave rectified outputs of orientation-selective filters. Using a subthreshold-summation paradigm, however, we show that modulations of contrast polarity are detected by mechanisms that pool signals of different orientations while modulations of orientation are detected by mechanisms that pool signals of different contrast polarities. The results support the existence of two streams of 2nd-order processing, one that receives the full-wave rectified inputs from oriented filters, the other separate half-wave rectified outputs from on-center and off-center filters pooled across all orientations. The two-stream model is shown to predict the perceptual effects of changes to the skewness statistics of natural-image textures, and to solve a contradiction among previous data concerning the detection of contrast modulation.