Dissociation of spatial attention and saccade preparation.

The goal of this experiment was to determine whether the allocation of attention necessarily requires saccade preparation. To dissociate the focus of attention from the endpoint of a saccade, macaque monkeys were trained to perform visual search for a uniquely colored rectangle and shift gaze either toward or opposite this color singleton according to its orientation. A vertical singleton cued a prosaccade, a horizontal singleton, an antisaccade. Saccade preparation was probed by measuring the direction of saccades evoked by intracortical microstimulation of the frontal eye fields at variable times after presentation of the search array. Eye movements evoked on prosaccade trials deviated progressively toward the singleton that was also the endpoint of the correct eye movement. However, eye movements evoked on antisaccade trials never deviated toward the singleton but only progressively toward the location opposite the singleton. This occurred even though previous work showed that on antisaccade trials most neurons in frontal eye fields initially select the singleton while attention is allocated to distinguish its shape. Thus, sensorimotor structures can covertly orient attention without preparing a saccade.