Microsaccades drive illusory motion in theEnigmaillusion

Abstract

Visual images consisting of repetitive patterns can elicit striking illusory motion percepts. For almost 200 years, artists, psychologists, and neuroscientists have debated whether this type of illusion originates in the eye or in the brain. For more than a decade, the controversy has centered on the powerful illusory motion perceived in the painting Enigma, created by op-artist Isia Leviant. However, no previous study has directly correlated the Enigma illusion to any specific physiological mechanism, and so the debate rages on. Here, we show that microsaccades, a type of miniature eye movement produced during visual fixation, can drive illusory motion in Enigma. We asked subjects to indicate when illusory motion sped up or slowed down during the observation of Enigma while we simultaneously recorded their eye movements with high precision. Before ''faster'' motion periods, the rate of microsaccades increased. Before ''slower/no'' motion periods, the rate of microsaccades decreased. These results reveal a direct link between microsaccade production and the perception of illusory motion in Enigma and rule out the hypothesis that the origin of the illusion is purely cortical.bistable perception ͉ fixational eye movements ͉ miniature eye movements ͉ subjective motion ͉ visual fixation

Microsaccades and Blinks Trigger Illusory Rotation in the “Rotating Snakes” Illusion

“…Figure 3A plots the distribution of microsaccade magnitudes for the subject population [up to 3°(Otero-Millan et al, 2008)]. Of these, we studied further only microsaccades smaller than 2°(n ϭ 23,610) (Troncoso et al, 2008b). Equivalent results were obtained for 1-or 3-degrees-magnitude thresholds (data not shown) (Troncoso et al, 2008b).…”

“…Of these, we studied further only microsaccades smaller than 2°(n ϭ 23,610) (Troncoso et al, 2008b). Equivalent results were obtained for 1-or 3-degrees-magnitude thresholds (data not shown) (Troncoso et al, 2008b). Figure 3B shows the peak velocity-magnitude relationship for all analyzed microsaccades.…”

“…As with other bistable stimuli paradigms (Spillmann and Kurtenbach, 1992;van Ee, 2005, 2006a,b;MartinezConde et al, 2006;Troncoso et al, 2008b), subjects reported that their perception oscillated between two alternating states (stillness vs rotation). We found a clear positive relationship between increased microsaccade rates and the perception of snake rotation.…”

“…We moreover removed those portions of the data corresponding to very fast decreases and increases in pupil area (Ͼ20 units per sample) plus the 200 ms before and after. Such periods are probably due to partial blinks, where the pupil is never fully occluded (thus failing to be identified as a blink by the EyeLink 1000 software) (Troncoso et al, 2008b).…”

“…To do this, we discarded latency values Ͼ1500 ms [1% of all reported transitions; such latencies were considered too long to be meaningful (Einhäuser et al, 2008;Troncoso et al, 2008b)]. Table 1 summarizes the corresponding parameters.…”