Abstract

After prolonged adaptation to a moving pattern, a subsequently presented static or dynamic test pattern will appear to drift in the opposite direction. This illusory drift is referred to as the motion aftereffect (MAE). It is well accepted that a MAE is not elicited after adaptation to second-order motion if tested with a static test (sMAE), although a clear MAE is perceived if tested with a dynamic pattern (dMAE), which is believed to access higher levels of motion processing. These second-order MAE properties, in part, have led to motion-detection models postulating that first- and second-order motion are processed, at least initially, by separate mechanisms. The purpose of the present study was to define the MAE properties of second-order motion in the periphery in order to evaluate the exclusiveness of the second-order MAE properties to central vision. Observers were adapted to non-scaled second-order patterns at fixation and in the periphery (up to $12\sp\circ)$ and tested with both static and dynamic test patterns. Results showed that for all observers, a same was perceived after second-order adaptation in the periphery, although none was elicited after central adaptation. This suggests the existence of different motion mechanisms operating in central and peripheral visual fields; separate mechanisms sensitive to each motion type in the central visual field and a common, low-level mechanism mediating both first- and second-order processing in the periphery. Additional results obtained from preliminary studies comparing first- and second-order motion sensitivity across eccentricity also provide evidence that first- and second-order motion processing may be mediated by a different mechanism in the periphery than at fixation. The possible effects of spatially scaling the patterns as well as more direct methods to address the underlying peripheral mechanisms are discussed.