PURPOSE: In a horizontal pattern consisting of zigzagging strips of phase-shifted abutting line gratings, strong diagonal and lateral illusory motion is perceived when the pattern is moved up or down at moderate speed. We wished to study how different configurations affect the saliency of apparent motion and find out the how the illusory motion is induced.
METHODS: We performed psychophysical experiments on nine subjects to test the salience of apparent motion when changing the spacing, length of the inducing lines, width of zigzags as well as their luminance contrast.
RESULTS: The apparent motion is strongly influenced by the spacing, length of the inducing lines, width of zigzags as well as the luminance contrast. With isoluminant color defining the pattern and background, apparent motion is largely abolished. No motion is seen when the zigzagging grating strips are separated by a gap, when the grating lines are interleaved or when the grating strips are delineated at the interface by a continuous line.
CONCLUSIONS: Two types of motion were distinguished according to their directions, (i) motion along the diagonal interface between the zigzagging grating strips and (ii) collinear motion in the direction of the horizontal grating lines. We hypothesized, based on the equiluminant findings that the apparent motion could be dominantly processed by the magnocellular pathway, which encodes motion signals while being largely insensitive to color. The endpoints of grating lines is crucial for the existence of the illusory motion. The neural mechanisms underlying this illusion require further investigation.