Abstract
Abundant evidence has shown that saccades are affected by size illusions like the Müller-Lyer illusion, but the magnitude vary widely. Researchers have tested multiple factors (e.g. task, response delay, saccades latency, presentation time etc.) and consistently found that the magnitude of the illusion effect on saccades mainly relies the time of visual processing before saccades are initialized. However, these studies all asked subjects to initialize their saccades after the presentation of the illusion, allowing a potential planning for the saccades. Studies in visuomotor control have shown that online control is less influenced by illusion than planned movements are. This is possibly the cases in oculomotor control as well. To test this possibility, the current study investigated the Müller-Lyer illusion effect on saccade using delayed perturbation. On each trial, a line section with small fins were presented first and subjects were required to saccade from one vertex to the other as soon as the signal was given. On half of the trials, the orientations of the fins were reversed after the saccade signal was given and the delay from the presentation of saccade signal to the perturbation varied from 50ms to 150ms. Throughout this manipulation, the time gap from perturbation to the start of saccade varied relatively widely across the trials of the delayed perturbation condition. We observed that the illusion perturbation started to affect saccades when the gap was about 100ms and reached to the peak when the gap was about 200–250ms (the peak illusion effect of the delayed perturbation was compatible to ones observed in the unperturbed conditions). Moreover, illusion perturbation was compared with actual size perturbation in another experiment. Same time course of the perturbation effect were observed for both types of perturbation, indicating that illusory size information requires no further visual processing to the oculomotor system.
Acknowledgement: National Natural Science Foundation of China (NO. 31800949)