The frequency with which express saccades are generated under a
variety of conditions in rhesus monkeys was examined. Increasing the
gap time between fixation spot termination and target onset increased
express saccade frequency but was progressively less effective in doing
so as the number of target positions in the sample was increased.
Express saccades were rarely produced when two targets were presented
simultaneously and the choice of either of which was rewarded; a
temporal asynchrony of only 17 ms between the targets reinstated
express saccade generation. Express saccades continued to be generated
when the vergence or pursuit systems was coactivated with the saccadic
system.