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J.M. Zanker; What Can We Learn From Motion Illusions Seen in Op Art Paintings? . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4320.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Vivid dynamic illusions can be elicited when observing certain simple and static black and white patterns, such as used in Op Art. As a typical example, the fine grating of wiggly lines of Bridget Riley’s ‘Fall’ generates a strong motion illusion. The neural mechanisms underlying these effects are still a matter of a lively debate, which repeatedly touched upon the role of gaze stability. Based on recordings of small involuntary eye movements carried out during observation of Riley’s ‘Fall’, the motion signal distributions generated by image displacements were studied with a computational approach. Methods: The consequences of shifting the retinal image of synthetic wave gratings ('riloids') and a variety of control patterns were analysed by means of a two-dimensional array of motion detectors (2DMD model), which generates maps representing the spatial distribution of motion signals generated by such stimuli. Results: Motion signal maps resulting from a two-frame sequence reflecting a saccadic pattern displacement are characterised by extended patches of the response in which local direction is homogeneous. However, the local direction differs considerably between neighbouring patches and usually does not correspond to the direction of pattern displacement. This appearance can be expected from the geometry of the curved gratings as an instance of the so-called 'aperture problem'. The patchy structure of the motion detector response, which is found for displacement of riloids but not for control patterns, resembles the motion illusion which is not perceived as coherent shift of the whole pattern but as a wobbling and jazzing of ill-defined regions. Conclusions: Although other explanations cannot be excluded, these findings support the view that the puzzle of Op Art motion illusions could be explained in terms of small involuntary eye movement leading to image shifts that are picked up by well-known motion detectors in the early visual system. One consequence of this explanation is related to the question why similar illusions are not perceived with other patterns. The simulation of control conditions suggests that it is the unavailability of any coherent large-field motion signal for riloids that prevents the compensation of signal components which are due to the small image shifts, and thus leads to the illusion.
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