Illusory motion in Enigma: A psychophysical investigation

Abstract

Experiments to determine the optimum conditions for perceiving illusory motion in Engima-like patterns have also demonstrated that the illusory motion is not the result of unintended motion of the image on the retina due to microsaccades or accommodative changes in the lens of the eye but instead has a cortical origin. The perceived illusory activity is believed to be a consequence of neural signals emanating from high-contrast bars and edges in the image that emit randomly fluctuating signals, as expected from spiking cortical neurons. These fluctuations may induce illusory motion in the channels by a mechanism similar to that responsible for the Omega effect, in which sequences of random patterns of black dots presented in an annular channel produce the perception of illusory rotation of these dots within the annulus.cortical noise ͉ Omega effect ͉ visual illusion T here are many examples of static pictures that produce the illusion of movement. One of these, the Enigma, was painted by I. Leviant in 1981 (1). Viewers of that painting report seeing fluid-like forms in the painting, although the painting itself is completely static. The static image consists of radial black and white lines with three concentric circular annular channels superposed on the radial grating.A gray-scale image based upon the Enigma is shown in Fig. 1. Most observers report seeing some fluid-like activity in the circular channels moving randomly clockwise or counterclockwise within the channels. The perceived direction of motion changes from time to time with no discernible regularity. Leviant (1) described the steps that led him to execute the Enigma and provided some simple rules that he developed to enhance the ''illusion.'' A very similar work of his is also on display at the Exploratorium in San Francisco as the ''Traffic Illusion,'' so-named, ''because moving forms seem to follow avenues'' (1).Zeki et al. (2) investigated the illusion using positron emission tomography and found a strong correlation between seeing the moving forms and an increase in the activity of cortical area V5, which is thought to be involved in the perception of motion. R. L. Gregory (refs. 3 and 4) attributed the perception of moving forms to accommodative changes in the lens of the eye or to microsaccades. He based these suggestions upon his finding that no motion is seen when the radial black and white lines in the MacKay illusion (refs. 5-7) are viewed through a 2-mm circular aperture. Zeki tested these suggestions for the Enigma image by demonstrating that the moving forms could still be seen when viewed through a 2-mm pupil, or when he treated his eyes with atropine to diminish or abolish the accommodative power of his lens. He therefore suggested a cortical mechanism for perceiving the moving forms (refs. 8 and 9). C. Fermuller et al. (10) presented a computational explanation of the moving forms and suggested that ''eye movements and͞or accommodation changes cause weak retinal motion signals, which are interpreted by higher-level processes in a ...

Microsaccades drive illusory motion in theEnigmaillusion

“…Thus, our findings are compatible with a causal relationship between microsaccade rates and the perception of illusory motion. Whereas motion-sensitive neurons in the extrastriate dorsal areas surrounding V5 (the human equivalent of primate area MT) (12) may be responsible for the perception of all motion percepts, including illusory motion in Enigma, our results fail to support the hypothesis that the Enigma effect is purely cortical in origin (3,9,(12)(13)(14)21).…”

“…To avoid adaptation across trials, the whole image was randomly rotated toward one of eight chosen points around the compass (45°, 90°, 135°, 180°, 225°, 270°, 315°) from one trial to the next. This rotation did not affect the strength of the illusion (13).…”

“…Subjects pressed a key, and a simplified version of the Enigma illusion (Fig. 1B), optimized to strengthen the illusory motion (3,13), appeared on the screen.…”

The neural basis of the Enigma illusion: A transcranial magnetic stimulation study

“…Their model proposes that eye movements are necessary to trigger the illusory rotation in the Enigma figure but that high-level cortical processes are mainly responsible for the illusion. Recently, Hamburger (2007) and Kumar and Glaser (2006) claimed that the illusory motion in the Enigma figure is not the result of motion of the image on the retina due to microsaccades but rather has a strictly cortical origin. provided data supporting the cortical role of this illusory phenomenon without excluding the possibility that FEMs were involved in the generation of the illusion.…”

Steady‐state VEP responses to uncomfortable stimuli

“…Spatially periodic stimuli are encountered in everyday environments in the form of stripes, text (Wilkins et al, 2007;Jainta et al, 2010) and works of op-art (e.g. Wade, 2003;Kumar & Glaser, 2006;Troncoso et al, 2008;Zanker et al, 2010).…”