Spatial characteristics of the second-order visual pathway revealed by positional adaptation

The visual system is thought to process luminance (first-order) and contras t (second-order) information by dedicated cortical streams. To explore the spatial characteristics of the second-order pathway, we examined the effect of adaptation on spatial localization in human subjects. We show that, unl ike first-order adaptation, second-order positional adaptation via cortical mechanisms transfers across orientations but not across spatial frequencie s. These results support physiological evidence that these two processing s treams are distinct and suggest that the cortical mechanism mediating secon d-order positional adaptation maintains spatial frequency information but s ums signals across orientations.