Neural basis for a powerful static motion illusion

Abstract

Most people see movement in Figure 1, although the image is static. Motion is seen from black --> blue --> white --> yellow --> black. Many hypotheses for the illusory motion have been proposed, although none have been tested physiologically. We found that the illusion works well even if it is achromatic: yellow is replaced with light gray, and blue is replaced with dark gray. We show that the critical feature for inducing illusory motion is the luminance relationship of the static elements. Illusory motion is seen from black --> dark gray --> white --> light gray --> black. In psychophysical experiments, we found that all four pairs of adjacent elements when presented alone each produced illusory motion consistent with the original illusion, a result not expected from any current models. We also show that direction-selective neurons in macaque visual cortex gave directional responses to the same static element pairs, also in a direction consistent with the illusory motion. This is the first demonstration of directional responses by single neurons to static displays and supports a model in which low-level, first-order motion detectors interpret contrast-dependent differences in response timing as motion. We demonstrate that this illusion is a static version of four-stroke apparent motion.

Figure 1.

Rotating Snakes, by A. Kitaoka. A static motion illusion in color (top) and grayscale (bottom) is shown.

Figure 2.

Human observers indicated that all element pairs in the static motion illusion contribute to the illusory motion perception. A, A single trial of the blue/white stimulus. B, A single trial of the luminance version of the blue/white stimulus, in which the blue was replaced with dark gray. C, Results for colored element pairs. The dashed lines indicate the 95% confidence limits for random choices, for any individual subject. D, Results for grayscale element pairs. E, Results averaged over all subjects; mean ± SD.

Figure 3.

Both V1 and MT cells show longer latency responses to lower-contrast stimuli. A, Average responses of a V1 cell in an alert macaque to at least 300 presentations of each of the four different grayscale bars, as indicated, flashed on an intermediate gray background for 27 ms in the receptive field of the cell. B, Average responses of an MT cell in an alert macaque to at least 300 presentations of each of the four bars, as indicated, flashed for 27 ms in the receptive field.

Figure 4.

Direction-selective cells responded more strongly to static presentations of adjacent element pairs when the motion percept of a given element pair was congruent with the direction preference of the cell. A, Checked bars, Histograms of C.I.s of 20 weakly direction-selective V1 cells (D.I.s < 0.3) for the two same-contrast pairs averaged together (top; median, +0.005), for the two opposite-contrast pairs averaged together (middle; median, -0.0015), and for all four contrast pairs averaged together (bottom; median, -0.0037). Black bars, Histograms of C.I.s for 19 strongly direction-selective V1 cells (D.I.s >0.3) for the two same-contrast pairs averaged together (top; median, +0.06), for the two opposite-contrast pairs averaged together (middle; median, +0.03), and for all four contrast pairs averaged together (bottom; median, +0.06). B, Histograms of congruency indices (see Materials and Methods) for 20 MT cells for the two same-contrast pairs averaged together (top; median, +0.13), for the two opposite-contrast pairs averaged together (middle; median, +0.09), and for all four contrast pairs averaged together (bottom; median, +0.09).