Abstract
Various Op artists have used simple geometrical patterns to create the illusion of motion in their artwork. One explanation for the observed illusion involves retinal shifts caused by small involuntary eye movements that observers make while they try to maintain fixation. Earlier studies have suggested a prominent role of the most conspicuous of these eye movements, small rapid position shifts called microsaccades. Here, we present data that could expand this view with a different interpretation. In three experiments, we recorded participants' eye movements while they tried to maintain visual fixation when being presented with variants of Bridget Riley's Fall, which were manipulated such as to vary the strength of induced motion. In the first two experiments, we investigated the properties of microsaccades for a set of stimuli with known motion strengths. In agreement with earlier observations, microsaccade rates were unaffected by the stimulus pattern and, consequently, the strength of induced motion illusion. In the third experiment, we varied the stimulus pattern across a larger range of parameters and asked participants to rate the perceived motion illusion. The results revealed that motion illusions in patterns resembling Riley's Fall are perceived even in the absence of microsaccades, and that the reported strength of the illusion decreased with the number of microsaccades in the trial. Together, the three experiments suggest that other sources of retinal image instability than microsaccades, such as slow oculomotor drift, should be considered as possible factors contributing to the illusion.
Highlights
Visual illusions have received considerable interest, one reason being that they may provide an important source of information about the working of the human visual system
Our psychophysical study (Zanker et al 2010), in which we found no effect of the fixation stimulus on the strength of the illusion, did not control for eye movements
Retinal shifts caused by eye movements made during visual fixation of the pattern, such as microsaccades and oculomotor drift, might be responsible for this motion percept
Summary
Visual illusions have received considerable interest, one reason being that they may provide an important source of information about the working of the human visual system. One type of illusion that has been studied extensively is illusory motion, such as instances in which a static image results in a strong motion percept, no physical motion is present. These illusions were used by artists as a way to convey motion in images composed of simple black and white patterns. With possibly different underlying mechanisms or combinations of underlying mechanisms, exist, such as the Enigma illusion, consisting of radial lines with superimposed rings (Leviant 1982; Troncoso et al 2008). Works of arts like Riley’s Fall and the Enigma illusion have received considerable interest from vision scientist debating the origin of the perceived motion illusions in static patterns. It has been suggested that small eye movements during visual fixation, known as fixational eye movements, play an important role in the illusion (Troncoso et al 2008; Zanker and Walker 2004)
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