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Davis Barch, Tribhawan Kumar, Donald A. Glaser; Modeling the illusory motion of Enigma with an excitable neuronal array. Journal of Vision 2003;3(12):72. doi: https://doi.org/10.1167/3.12.72.
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The Enigma illusion, as designed by I. Leviant  is a static image, featuring multiple closely spaced lines or bars, radiating outward from a central point. These features give rise to an illusory percept of motion. We demonstrate here that the neural activity which may give rise to this effect can be represented by an Excitable Neuronal Array model of the visual cortex.
The Excitable Neuronal Array (ENA) is a two-dimensional array of connected neural elements, based on the architecture of the primary visual cortex. Stimuli applied to such an array give rise to traveling waves of activity, propagating away from the stimulus. Within a broad range of ENA parameters, a static source of activity yields activity waves that die away quickly, while those arising from a moving stimulus yield long-lived activity waves, of a higher amplitude and characteristic shape, which effectively track the moving stimulus. It has been demonstrated [1, 2, 3] that the properties of such waves can be used to detect and ch aracterize the motion of the stimulus, and to model certain features of the psychophysics of motion. These activity waves can serve as a motion signal used as input to higher levels of the visual system which are responsible for the actual perception of t he motion stimulus.
When Enigma-like stimuli are applied to the ENA, the presence of the closely spaced line stimuli causes periodic generation of activity waves which travel between the static stimulus lines radially outward from the center of the Eni gm a. These radial waves cease when the spacing between adjacent radial lines is increased, both psychophysically , and in the ENA. These spontaneous waves of activity mimic those normally associated with moving stimuli in the ENA, and may therefor e be responsible for the illusory motion percept.
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