Occurrence of human express saccades depends on stimulus uncertainty and stimulus sequence.

Saccadic latencies measured in response to a step-wise displacement of the target may be substantially reduced if a gap separates the offset of the initial fixation point and the onset of the peripheral target. According to Fischer and Ramsperger (1984) this paradigm provokes a bimodal latency distribution which consists of a peak of very fast saccadic responses (express saccades) at about 110 ms and another peak arising from somewhat slower saccades (regular saccades). Using again the gap paradigm, we investigated the effect of an additional go/no-go (i.e. target trial/catch trial) decision on saccadic latencies. The experiments yielded the following results: (i) the distribution between the peaks of express and regular saccades strongly depends on the proportion of catch trials introduced into the trial sequence, which suggests the existence of different modes of operation of the decision processes for express and regular saccades. (ii) The catch trial effect on saccadic latency proved to be a local phenomenon in time: saccades which follow catch trials tend to be slower than those following target trials.