Differential effects of target probability on saccade latencies in gap and warning tasks.

Saccade latencies are significantly reduced by extinguishing a foveal fixation stimulus before the appearance of a saccade target. It has been shown recently that this "fixation offset effect" (FOE) can be modulated by varying target probability. Cortico-collicular top-down effects have been assumed to mediate this strategic FOE modulation. Here, we have investigated strategic FOE modulation in 14 healthy human subjects performing gap and warning tasks. In the former task, the central fixation point was extinguished 200 ms before target onset. In the latter task, the central fixation point changed its colour 200 ms before target onset, but remained illuminated until the target appeared. Target probability was varied block-wise between 25 and 75%. In both tasks, mean latencies decreased with increasing target probability. However, in contrast with what can be expected from preceding studies, we found no differential modulation of mean latencies by target probability between tasks. Instead, we observed differential probability-dependent changes in latency distributions. In the gap task, discrete changes of saccade latencies were found, with a probability-dependent change in frequency of express and regular latencies. By contrast, in the warning task a shift of the entire latency distribution towards longer latencies with low target probability was found. We conclude that strategic modulation of saccade latencies by target probability may be mediated by two distinct neural mechanisms. Selection of either mechanism seems to depend critically on activation of the fixation system.