UP-DOWN ASYMMETRY IN VERTICAL INDUCED MOTION

Induced motion (IM) is the illusory movement of an object in the direc tion opposite to the real motion of adjacent detail. One theory of IM suggests that it results, in part, from suppression of optokinetic nys tagmus (OKN) by fixational (smooth-pursuit) effort. In several studies an asymmetry in human vertical OKN has been reported, with upward opt okinetic stimulation eliciting higher OKN gain than downward motion. T his provides a test of the nystagmus-suppression theory of IM. If supp ression of OKN contributes significantly to IM, upward inducing stimul i should result in a greater magnitude of the illusion than should dow nward stimulus motion. Additionally, the asymmetry of vertical OKN sho uld become more pronounced at higher stimulus velocities. Therefore, t he asymmetry of vertical IM should be greater at higher inducing-stimu lus velocities. Twelve subjects viewed a large, random-dot stimulus, w hich moved either upward or downward at a velocity of 10, 40, or 70 de g s-1. Subjects fixated a horizontally moving laser spot and adjusted a rod to match the apparent slope of the motion path of the spot. IM m agnitude was derived from these measures. Mean IM velocity was signifi cantly higher with upward than with downward stimulation, and the diff erence was maximal at velocities of 40 and 70 deg s-1. The results are discussed within the context of the nystagmus-suppression theory and other theories of IM.