Global and local influence of form information on human heading perception

Abstract

We have previously reported that the static focus of expansion (FOE) in a radial Glass pattern influences human heading perception (Cheng, Khuu, & Li, VSS, 2008). Here we investigate the underlying mechanism. In Experiment 1, we presented observers with an integrated form and motion display in which the dot pairs in a radial Glass patterns were oriented toward one direction on the screen (the form FOE) while moving toward a different direction in depth (the motion FOE) and a non-integrated display in which a static radial Glass pattern was superimposed on a regular optic-flow stimulus. Heading judgments were strongly biased towards the form FOE for the integrated but not the non-integrated display (form weight: 0.78 vs. 0.27), indicating that the form influence on heading perception is not a decision bias. In Experiment 2, we manipulated the global form information in the radial Glass pattern by randomly orienting some dot pairs. The heading bias towards the form FOE decreased as the global form signal was degraded, suggesting that the bias is mediated by the global form percept. In Experiment 3, we examined whether observers combined the form and motion FOEs for heading perception in a statistically optimal way. The motion FOE was weighted less than its variance warranted, suggesting that the local orientation of each dot pair in the radial Glass pattern disturbed its perceived motion direction, thus affecting the reliability of the estimated motion FOE in optic flow. By approximating the level of motion direction noise for which integration would be optimal, we found that the strength of the effect of local dot-pair orientation on its perceived motion direction was at about 50%. We draw the conclusion that the influence of the form FOE on heading perception is due to both global and local interactions between form and motion signals.