Velocity gradient information influences optical flow processing in human observers

Abstract

The human visual system can discriminate among different kinds of optical flow patterns (e.g. expansion, contraction, rotation). These stimuli contain velocity gradients (VG) that are differentially oriented relative to their arrangement of motion directions (e.g. parallel or accelerating in expansion and anti-parallel or decelerating in contraction). Whether humans use VG information to distinguish among different kinds of optical flow patterns remains unclear. Here we clarified this issue by determining human subjects' ability (n=4) to discriminate among different kinds of optical flow patterns with and without VG. In one set of trials subjects were asked to determine whether an expanding/contracting pattern was rotating clockwise or counterclockwise. In a second set of trials subjects were asked to determine whether clockwise/counterclockwise-rotating patterns were expanding or contracting. Patterns could either contain a VG (VG-trials) or all the dots moved equally fast (without-VG-trials). All trials were randomly intermixed. Discrimination thresholds were defined as the amount of change in the VG-orientation relative to the motion direction (e.g. 0 deg for pure expansion and 90 deg for pure clockwise rotation) needed for the subjects to perform correctly in 75% of the trials. On average thresholds were 35% lower for expansion/contraction and 25% lower for rotations in VG-trials relative to without-VG-trials. In a second experiment, we further tested the intensity of a VG after-effect evoked by selective adapting the visual system to the same absolute orientation of a VG (i.e. upwards) while changing the direction of a translational motion. We found a VG after-effect that was dependent of the VG-orientation and relatively independent of motion direction. Our results suggest that humans use VG information to discriminate among different kinds of optical flow patterns and that the visual system possesses mechanisms selective for VG-orientation in such patterns