Quantifying and modeling the strength of motion illusions perceived in static patterns

Abstract

The origin of motion illusions in simple black and white patterns such those as used by Op artists has been at the center of a lively scientific debate, relating motion processing mechanisms to involuntary eye movements that generate characteristic motion patterns. To overcome the limitations of using subjective ratings as a measure of illusory effects, we developed a new method to quantify the strength of the illusion for synthetic 'riloids' that were inspired by Bridget Riley's 'Fall'. In a 2AFC paradigm, test stimuli were compared to a reference set of patterns that elicit illusory motion of variable strength. We found that pattern parameters influencing the distribution of local orientation in the riloids (the amplitude and the spatial period of the line undulation) systematically affect illusion strength, whereas other parameters, such as the spatial period of the lines themselves, the duration of the stimulus, or fixation conditions, have little effect. These behavioral data are compared in computer simulations to the predicted activity generated by motion detector networks for displacements of the riloids that reflect small eye movements. The match between predicted illusion strength and experimental data support an explanation of the motion illusion in terms of retinal image shifts.

Looking at Op Art: Gaze Stability and Motion Illusions

“…The main finding of the study was that the strength of the motion illusion grows with the amplitude ( A ) and the spatial period (μ) of the wave-like undulations of the line gratings, but was unaffected by the spatial wavelength of the gratings (λ). In addition, we observed that the strength of the motion illusion was not influenced by the presence or absence of a fixation target (Zanker et al 2010). This latter finding provides an interesting opportunity to determine how the motion illusion relates to certain aspects of fixational eye movements.…”

“…In addition to using a set of stimuli with the same three contrast mixtures as in Experiment 1, a second set was created in which the overall contrast of the pattern was strongly reduced (see Figure 5b) to test for the effects of the contrast of the pattern, which has been shown to strongly affect the motion illusion (Zanker et al 2010). By adding these low-contrast stimuli, the number of trials doubled to 72 (12 repetition of each of 6 contrast conditions).…”

“…To test this explanation of illusory motion in Fall , we investigated the relation between the illusion strength and properties of microsaccades occurring during the inspection of the image in Experiments 1 and 2. Stimuli with known illusion strengths from a previous study (from Zanker et al 2010) were used, and microsaccade properties were examined across the different images. No clear differences were found in the patterns of microsaccades for the different stimuli reflecting different motion illusion strengths, suggesting that rates, amplitudes and directions of microsaccades are not associated with the strength of the motion illusion.…”

“…Most studies report frequencies in human observers under standard conditions in the order of one microsaccade per second (eg, Engbert and Kliegl 2003b). As a consequence, the duration of the interval between microsaccades can be relatively long, and it is rather possible that no microsaccades occur during brief stimulus presentations, while motion illusions have been found for stimuli presented as briefly as 250ms (Zanker et al 2010). Given the relatively low frequency of microsaccades, it therefore needs to be assumed that their effects on the motion system are reasonably long-lasting if they were the full explanation of the motion illusion.…”

“…In psychophysical experiments (Zanker et al 2010), it was shown that variations of the amplitude ( A ) and the spatial period of phase modulation (μ) affected the strength of the illusion, whereas the spatial wavelength (λ) had little effect.…”