Local biases drive, but do not determine, the perception of illusory trajectories

Tamara N Gheorghes,John Reidy,Paul Richardson

doi:10.1038/s41598-020-64837-0

Abstract

When a dot moves horizontally across a set of tilted lines of alternating orientations, the dot appears to be moving up and down along its trajectory. This perceptual phenomenon, known as the slalom illusion, reveals a mismatch between the veridical motion signals and the subjective percept of the motion trajectory, which has not been comprehensively explained. In the present study, we investigated the empirical boundaries of the slalom illusion using psychophysical methods. The phenomenon was found to occur both under conditions of smooth pursuit eye movements and constant fixation, and to be consistently amplified by intermittently occluding the dot trajectory. When the motion direction of the dot was not constant, however, the stimulus display did not elicit the expected illusory percept. These findings confirm that a local bias towards perpendicularity at the intersection points between the dot trajectory and the tilted lines cause the illusion, but also highlight that higher-level cortical processes are involved in interpreting and amplifying the biased local motion signals into a global illusion of trajectory perception.

Highlights

When a dot moves horizontally across a set of tilted lines of alternating orientations, the dot appears to be moving up and down along its trajectory
Cesàro and Agostini[1] have described the slalom illusion, whereby the straight horizontal trajectory of a dot appears to be undulating when moving across a sequence of alternating tilted inducer lines (Fig. 1)
The perpendicularity bias has often been proposed to be rooted in lateral inhibition of orientation-selective neurons, and as a consequence, lines intersecting at acute angles appear to repel each other towards a more perpendicular angle[10]

Summary

Introduction

When a dot moves horizontally across a set of tilted lines of alternating orientations, the dot appears to be moving up and down along its trajectory. When the motion direction of the dot was not constant, the stimulus display did not elicit the expected illusory percept These findings confirm that a local bias towards perpendicularity at the intersection points between the dot trajectory and the tilted lines cause the illusion, and highlight that higher-level cortical processes are involved in interpreting and amplifying the biased local motion signals into a global illusion of trajectory perception. The slalom illusion has received little attention in the literature since its initial publication, but it does offer important insight into the neurocognitive mechanisms that allow observers to quickly construct a coherent trajectory percept In their original paper, the authors reported three main findings: more acute angles of intersection, slower dot speeds, and smaller distances between the lines all caused a larger illusory amplitude to the perceived trajectory. In the differential processing account[15,16], a perpendicularity bias observed with random dot kinematograms in the context of a single tilted line was successfully modelled on the psychophysical finding that object-relative motion components (perpendicular to the line) contribute more to the perceived dot speed than non-object-relative motion components (parallel to the line)[17]

Results

Discussion

Conclusion