Gap-overlap effects on latencies of saccades, vergence and combined vergence-saccades in humans

We examined the effect of gap-overlap stimuli on the distribution of latencies for pure saccades, pure vergence and combined saccades and vergence in three normal subjects. With the gap stimulus, a distinct peak of "express saccades" occurred, both with and without associated vergence, but a distinct "express vergence" response was not identified. Nevertheless, with the gap stimulus there was a decrease in vergence latencies (17 msec), but less so than for saccades (41 msec). In the combined paradigm the gap effects on saccades and vergence resembled those for each component made alone. In addition, the latencies of the saccade and vergence components were linearly correlated with an average slope of 0.5. To explain these results we suggest that there is common signal processing at an early stage of saccade and vergence initiation, which is followed by activity that builds in separate trigger mechanisms that can be influenced by the conditions of fixation.