Dual multiple-scale processing for motion in the human visual system

A number of psychophysical and physiological studies have suggested that first-and second-order motion signals are processed, at least initially, by independent pathways, and that the two pathways both consist of multiple motion-detecting channels that are each narrowly tuned to a different spatial scale (spatial frequency), However, the precise number and nature of the mechanisms that subserve first-and second-order motion perception in human vision remain both controversial and speculative. We sought to clarify this issue by conducting selective adaptation experiments, in which modulation-depth thresholds for identifying the direction of stimulus motion of first-order (luminance-defined) and second-order (contrast-defined) drifting gratings were measured both prior to and following adaptation to motion, The drift direction, spatial frequency and stimulus type (either first-or second-order) of the adaptation and test stimuli were systematically manipulated, When the adaptation and test stimuli were either both first-order gratings or both second-order gratings, robust elevations of direction-identification thresholds were found and, importantly, these aftereffects exhibited both direction-selectivity and spatial-frequency selectivity, Cross-over-adaptation effects between first-and second-order gratings were also sometimes observed, but were very weak and not spatial-frequency selective, These findings give direct support for the existence of multiple-scale processing for first-and second-order motion in the human visual system and provide additional evidence that the two varieties of motion are initially processed by independent pathways, (C) 1997 Elsevier Science Ltd.