The Neural Control of Visually Guided Eye Movements
D. Temporal Factors in Visually Guided Saccadic Eye Movements
          1. The conditions under which express saccades can and cannot be generated.

c. Can one obtain express saccades when more than one target appears that requires a decision to be made as to where to look next?

Three conditions were run in randomized sequences to answer this question. The first is the standard condition already described, in which on each trial a single target is presented randomly at one of four locations with a gap of 100ms. This procedure for the monkey shown produced many express saccades as demonstrated in the top panel of Figure 32.

The second condition used the simplest multiple target task we could devise. The monkey was shown two identical targets after the termination of the fixation spot and was rewarded for making a saccade to either target. As it turned out, the frequency with which the left and right targets were chosen with saccadic eye movements was nearly the same in this situation (47% and 53%). One presumes that on each trial the monkey had to make a decision as to which target to look at, simple as that decision may have been. As shown in the middle panel of Figure 32 the monkey made no express saccades under these conditions.

For the third procedure monkeys were trained on an oddity task: four targets appeared one of which was different from the others. The animal was rewarded only when a saccadic eye movement was made to the odd stimulus which on each trial appeared randomly at one of the four locations . The bottom panel in Figure 32 shows the data: the monkey made no express saccades. The mean latency of the regular saccades for this condition is significantly longer than that obtained for the other two conditions.

Thus it appears that express saccades arise only when a single target appears in the visual scene at any given moment in time.