Gaze Stability of Observers Watching Op Art Pictures

Abstract

It has been the matter of some debate why we can experience vivid dynamic illusions when looking at static pictures composed from simple black and white patterns. The impression of illusory motion is particularly strong when viewing some of the works of 'Op Artists, such as Bridget Riley's painting Fall. Explanations of the illusory motion have ranged from retinal to cortical mechanisms, and an important role has been attributed to eye movements. To assess the possible contribution of eye movements to the illusory-motion percept we studied the strength of the illusion under different viewing conditions, and analysed the gaze stability of observers viewing the Riley painting and control patterns that do not produce the illusion. Whereas the illusion was reduced, but not abolished, when watching the painting through a pinhole, which reduces the effects of accommodation, it was not perceived in flash afterimages, suggesting an important role for eye movements in generating the illusion for this image. Recordings of eye movements revealed an abundance of small involuntary saccades when looking at the Riley pattern, despite the fact that gaze was kept within the dedicated fixation region. The frequency and particular characteristics of these rapid eye movements can vary considerably between different observers, but, although there was a tendency for gaze stability to deteriorate while viewing a Riley painting, there was no significant difference in saccade frequency between the stimulus and control patterns. Theoretical considerations indicate that such small image displacements can generate patterns of motion signals in a motion-detector network, which may serve as a simple and sufficient, but not necessarily exclusive, explanation for the illusion. Why such image displacements lead to perceptual results with a group of Op Art and similar patterns, but remain invisible for other stimuli, is discussed.

Microsaccades drive illusory motion in theEnigmaillusion

“…Our results directly confirm the hypothesis proposed by Pritchard (15), Mon-Williams and Wann (17), and Zanker and colleagues (18)(19)(20) that small eye movements can drive the perception of illusory motion in Enigma and other static patterns. Thus, our findings are compatible with a causal relationship between microsaccade rates and the perception of illusory motion.…”

“…The third potential explanation of illusory motion in static repetitive patterns came from Pritchard (15), who proposed that the illusion might be due to changes in retinal stimulation produced by small eye movements. This proposal was supported by Evans and Marsden (16), Mon-Williams and Wann (17), and Zanker and colleagues (18)(19)(20), who investigated another op-art painting giving rise to illusory motion effects: Fall, by Bridget Riley (Fig. S1B).…”

“…Microsaccades may also be involved in the generation of illusory motion in other static patterns (39,55), such as the MacKay rays (2), Bridget Riley's ''Fall'' (18,19,20), or the Ouchi illusion (56-58) (Fig. S1).…”

A new look at Op art: towards a simple explanation of illusory motion

“…Usually, the experience of viewing this painting is described in terms of introspection, with labels like 'shimmer', 'flicker', or 'waves of movement'; or in a more specific way as a pattern of horizontal, not clearly delineated bands in which gratings seem to move in one coherent but random and often changing direction, and different directions within each band. To assess the conditions for seeing the dynamic illusions (Zanker et al 2003), observers were asked to rate the strength of the illusion perceived in a number of stimuli on a scale between 1 (completely static) and 5 (moving vividly). In an initial experiment the expectation was confirmed that the strength of the illusion varies with the modulation amplitude of the riloid (i.e.…”

“…It is clear from these data that the illusion virtually disappeared when the retinal image of the pattern was stabilised, while ratings close to maximum Fig. 1 Riloids -synthetic Op art patterns -can be generated according to a simple mathematical expression (see Zanker et al 2003;Zanker 2004) by sinusoidal modulation of intensity with a grating period, l, and a phase modulation of amplitude A and modulation period m that can vary along the vertical dimension. Resulting grey-level bitmaps of wiggly line patterns resemble Riley's 'Fall' (Riley 1999), and were therefore dubbed 'riloids'.…”