A single mechanism for the timing of spontaneous and evoked saccades

The study of saccadic latency-the reaction time between presenting a visual stimulus and initiating an eye movement to look at it--has led to a better understanding of decision mechanisms in general, through the development of quantitative models such as LATER. But outside the laboratory, evoked saccades of this kind are rare. Most saccades are made spontaneously while viewing static scenes. Can their initiation be explained by the same decision mechanism? We suggest that in a series of spontaneous saccades, each can be considered to be evoked by the new retinal image generated by its predecessor, so that the intersaccadic interval (ISI) can be regarded as equivalent to latency. We measured ISIs in subjects spontaneously viewing static scenes, and found their distributions to be qualitatively similar to those of evoked saccades, differing quantitatively in just two respects: (1) the main part of the distribution is slower; and (2) there is an increased number of very early responses. By using novel saccadic tasks we show that (1) can be accounted for by lateral inhibition between multiple, suddenly presented image elements, and (2) by the fact that the stimulus is necessarily extremely predictable. Adding these two factors to an evoked saccadic task produced latency distributions indistinguishable from those of spontaneous ones. This suggests that the mechanisms generating evoked and spontaneous movements may be less functionally distinct than is commonly assumed. Both clinically and scientifically, a common, unified framework for explaining both spontaneous and evoked movements is an exciting prospect.