Optokinetic backgrounds affect perceived velocity during ocular tracking

The perceived velocity of visually tracked moving objects may depend on interactions between reflexive and voluntary oculomotor mechanisms. To investigate this hypothesis, subjects were required to compare sequentially the velocity of a standard target with that of a test target moving at one of five velocities. The standard target was viewed against a plain field (no optokinetic stimulation), and the test target was viewed against (1) a plain field, (2) a stationary grating, (3) a grating drifting in the direction of target motion, or (4) a grating drifting against target motion. In one condition, subjects tracked the target; in the other, a stationary point was fixated. The tracking group overestimated velocity when backgrounds drifted against target motion, but underestimated velocity when gratings drifted with target motion. Subjects not required to track experienced no such misestimations. The results are discussed in relation to the interactive mechanisms of the two eye-movement systems.