THE GAP EFFECT AND EXPRESS SACCADES IN THE AUDITORY MODALITY

Latencies of eye movements to peripheral targets are reduced when ther e is a short delay (typically 200 ms) between the offset of a central visual fixation point and the target onset. This has been termed the g ap effect. In addition, some subjects, usually with practice, exhibit a separate population of very short latency saccades, called express s accades. Both these phenomena have been attributed to disengagement of visual attention when the fixation point is extinguished. A competing theory of the gap effect attributes it to disengagement of oculomotor fixation during the temporal gap. It is known that auditory targets a re effective in eliciting saccadic eye movements, and also that covert attention operates in the auditory modality. If the gap effect and ex press saccades are due to disengagement of spatial attention, both sho uld persist in the auditory modality. However, fixation of gaze is lar gely under visual control. If the gap effect results from disengagemen t of fixation, then at least a reduced effect should be seen in the au ditory modality. Human subjects performed the gap task and a control t ask in the dark, using auditory fixation points and saccadic targets, on five successive days. Despite this practice, ex press saccades were not observed. There was a reliable gap effect, but the reduction in s accadic latency was only 17 ms, compared with 32 ms for the same subje cts in the visual modality. This suggests that about half the gap effe ct is due to disengagement of visual fixation. The remainder was not d ue to non-specific warning effects and could be attributed to offset o f the auditory fixation stimulus.