Speed tuning properties of mirror symmetry detection mechanisms

Abstract

Motion-direction does not contribute to symmetry perception, but limiting the lifetime of pattern elements improves symmetry detection1. Here we examine whether symmetry mechanisms are tuned to motion speed and if there are speed-selective symmetry channels. Stimuli were dynamic dot patterns containing varying amounts of symmetry. We measured symmetry detection thresholds for stimuli in which symmetric and noise elements moved at the same or different speeds and compared them to those obtained with static and limited-lifetime static patterns. In a second experiment, we measured percentage correct in two conditions: segregated – symmetric and noise elements drifted at different speeds, and non-segregated – symmetric elements drifted at two different speeds, as did the noise elements. We found that (i) performance was worse when symmetric and noise elements had the same speed and improved gradually as the speed difference increased, (ii) thresholds were comparable for all limited-lifetime static conditions, (iii) performance was improved in segregated compared to non-segregated conditions, (iv) modelling suggests that there are speed-selective channels that combine their outputs using probability summation. In conclusion, symmetry mechanisms are tuned to speed, and there are speed-selective symmetry channels, (v) better performance for moving compared to static patterns.