Temporospatial properties of the effects of bottom-up attention on smooth pursuit initiation in humans.

We examined the temporal and spatial properties of the effects of target saliency on the initiation of smooth pursuit eye movement in humans. Visual stimuli consisted of random dots projected on a large-field screen. During a fixation period, a cluster of dots (2 x 2 deg) was blinked (turned off for a short period) to make that region stand out from the remaining background and serve as a cue. After a delay (cue lead time), a cluster of dots (2 x 2 deg) started to move as a pursuit target. The target and cue were presented at either identical or different locations so that the subject could not predict the target location from the cue location. We examined the time course of the effect of the cue on pursuit initiation by changing the cue lead time. In half of the trials, the cue and target locations were identical, and in the other trials, they were not identical. There was a clear effect of the cue only for the trials where the cue and target locations were identical. The effect of the cue increased as cue lead time increased, peaked at approximately 160 ms, and then decreased. We also examined the spatial extent of the effect of the cue by varying the distance between the target and the cue. The largest effect was observed when the cue and the target were presented at the same location. Facilitating effects were observed when the target and the cue were presented in the same hemifield but not when presented in the opposite hemifield.