Abstract
Several visual variables are known to influence motion perception, including motion energy (1st-order motion) and tracking the positions of features (3rd-order motion) (Lu and Sperling 1995), but their contribution to locomotor control is unknown. Previously, we found that 3rd-order motion dominated locomotor responses in a crowd following task. Here, we manipulate the strength of 1st- and 3rd-order motion and measure their effects on heading and speed responses during crowd following. Human participants wore a head-mounted display (101ºH x 105ºV, 90 Hz) and were asked to “walk with” nine virtual objects textured with Julesz pattern in a mid-gray environment. The objects initially moved forward at 1.2 m/s, then their heading direction (±20º) or speed (± 0.2m/s) was perturbed for 2s. During the perturbation, the texture within object boundaries was rendered with a 4-stroke reverse-phi illusion, such that it moved laterally (heading perturbation) or radially (speed perturbation). The direction of the 1st-order texture motion and the 3rd-order boundary motion were either the Same or Opposite direction. Locomotor responses were always in the same direction as object boundaries. When both texture and boundaries are clearly defined (Experiment 1), heading responses were actually larger in the Opposite condition than the Same condition (p<0.05), suggesting a motion-contrast effect. When the strength of the boundary feature was reduced by blurring with a Gaussian filter (Experiment 2), both heading and speed responses were reduced (p<0.001), revealing an influence of motion energy. When the strength of motion energy was reduced by blurring the texture (Experiment 3), the motion contrast effect in the heading condition was eliminated. The results indicate that human locomotor responses are dominated by (3rd-order) tracking of object features, but are slightly modulated by (1st-order) motion energy, depending on their relative strength. This suggests that 1st- and 3rd-order motion signals are integrated prior to locomotor control.