ATTENTION AND TARGET SELECTION FOR SMOOTH-PURSUIT EYE-MOVEMENTS

Two rhesus monkeys were trained to track a small moving target in the presence of a moving distracter. The target and distracter were distin guished by their color. Smooth pursuit eye movements were quantified i n terms of the latency of the eye movement and the open-loop eye accel eration profile. Smooth pursuit latencies for single targets were on t he order of 100 msec. When the target was paired with a distracter mov ing in the same direction as the target, pursuit latencies decreased t o roughly 85 msec. When the target was paired with a distracter moving in the opposite direction, pursuit latencies increased to roughly 150 msec. The motion of the distracter had no significant effect on the e ye acceleration profile. Experiments were performed to dissociate visu al search for the target from pursuit initiation by providing a spatia l cue rather than the color cue, These experiments showed that visual search necessarily preceded pursuit initiation only when the distracte r moved in the opposite direction relative to the target. In this case , visual search contributed about 25 msec to the overall latency of pu rsuit. Control experiments showed that the monkey need not attend to t he distracter in order for it to influence the latency of pursuit. A n etwork model was developed in which units that represent the motions o f the target and distracter compete against one another. Attention ser ves to bias the outcome of this competition toward the direction of th e selected target. The performance of this network exhibits a striking parallel to the effect of the distractor on smooth pursuit latency.