Blurring Boundaries: Weakening 3rd-order Motion Reduces Locomotor Responses When Following A Crowd

Abstract

Collective motion in human crowds can emerge from pedestrians locally following their neighbors. Previously we showed that the visual-motor system relies on 3rd-order motion (tracking boundary features), not 1st-order motion (motion energy) (Lu & Sperling, 1995), to follow a crowd. We used the reverse-phi illusion to put 1st- and 3rd-order motion in conflict. Here we show that weakening 3rd-order motion by blurring object boundaries reduces both heading and speed responses, when in conflict with 1st-order motion. Participants (n=12) wore a wireless head-mounted display (101ºH x 105ºV, 90 Hz) and were asked to “walk with” a crowd of virtual objects as if they are real people. The ‘crowd’ consisted of 9 rectangles (1.6mV x 0.5mH) mapped with a Julesz texture. The ‘crowd’ initially moved forward at 1.2 m/s, then was briefly perturbed (heading perturbation: ±20˚, translation left or right; speed perturbation: ±0.2 m/s, optical expansion or contraction). During the perturbation, object texture underwent constant 4-frame reverse-phi motion so 1st-order texture motion was in the same or opposite direction as 3rd-order boundary motion (Same/Opposite Condition). Object boundaries were blurred at three different levels (Zero, Small, Large). The participant’s walking direction and speed were recorded. In the Opposite condition, locomotor responses progressively decreased as object boundaries were more blurred (heading: 15.3˚, 13.2˚, 12.1˚; speed: 0.072, 0.041, 0.039 m/s, respectively), consistent with reliance on 3rd-order motion. In the Same condition, on the other hand, responses increased with more blur (heading: 16.4˚, 17.1˚, 18.3˚; speed: 0.105, 0.119, 0.121 m/s, respectively), likely because texture motion was faster than boundary motion. Linear mixed effects models found significant main effects and interactions for all comparisons. The results confirm that following a crowd is dominated by 3rd-order motion of bounded ‘figures’, although 1st-order motion can also influence responses when 3rd-order motion is weak.