Abstract

A realistic radial flow field contains a range of local velocities, yet global movement is perceived at a single speed. The present experiments explore the contribution of complex motion sensors to this velocity percept, by recording changes in speed perception and speed sensitivity after adaptation to "scrambled", or to coherently expanding/contracting radial flow. A large-scale drifting concentric sine grating, conveying motion in depth, was confined to non-abutting display sectors, defining different global patterns of flow. After adapting to a flow pattern in one display region, observers compared its speed to that in a non-adapted region. Velocity aftereffects (VAEs) from continuous unchanging motion were independent of the pattern of flow: apparent speed was reduced and speed discrimination improved in inverse proportion to the speed of the test. Sensitivity to speed differences, however, was pattern specific, and superior for expansion. Also, adapted expansion recovered its apparent speed when tested against non-adapted contraction, and direction reversals of the adapter attenuated scrambled, but not coherent VAEs. No VAEs were recorded for test motions opposite to the adapted direction. It is concluded that higher-order optic flow mechanisms are not uniquely involved in velocity estimation per se, but modulate velocity judgments in response to changes in the ongoing flow. Independent expanding and contracting velocities rival and do not suppress one another when juxtaposed in space or over time. This unique motion opponency appears to be transient and depends on the 3D quality of the flow. It ensures that the speed of approaching objects is correctly perceived, regardless of stimulus history.