Initiation of smooth pursuit in humans. Dependence on target saliency.

We examined the influence of target saliency on the initiation of smooth pursuit. The eye movements of five human subjects were recorded with the scleral search-coil technique. A video-projection system was used to create a pursuit target, consisting of a cluster of 14 red or green dots (0.5 degrees squares) extending randomly over a 3 degrees x3 degrees region, and a surrounding background, consisting of stationary, random dots of the same size and density extending over an area 70 degrees x 40 degrees. When the dots in the background and the target were of the same color, the target was indistinguishable from the background until it started to move. On the other hand, when the colors were different, the target was salient, even when stationary. We measured the changes in eye position over the 70-ms interval starting 70 ms after the onset of target motion (initial tracking response). When the target moved toward the fovea (centripetal motion), the initial tracking responses developed earlier when the dots in the target and background were of different color than when the two sets of dots were of the same color. However, in order to see this effect of target salience, it was critical that the colors be different before the onset of motion, but not afterwards. When the target moved away from the fovea (centrifugal motion), the initial tracking responses were independent of whether the colors of the target and the background were the same or different. Our data indicate that the initiation of tracking responses is very sensitive to the saliency of the target before the onset of target motion when that motion is toward the fovea.