Adjustment of fixation duration in visual search.

To investigate whether fixation durations are adjusted to the duration of a foveal analysis task, we designed a search task in which each stimulus element yielded information about the position of the target. We asked subjects to look for the target by making eye movements in the direction indicated by each stimulus element. We explicitly asked the subjects to make the eye movements in the correct direction, but they did not always do this. They made only 65-80% of the eye movements in directions indicated by the stimulus elements. From these results we conclude that fixation durations are not solely determined by the immediate visual stimulus and that subjects encounter difficulties when trying to increase fixation durations to values that would enable them to direct saccades accurately. In a second experiment we shortened the presentation time in order to provide an incentive for the subjects to speed up search. Shortening the presentation time did not affect fixation duration. Therefore, we suggest that fixation duration is controlled by a mechanism that uses estimations of the foveal analysis time of previous fixated stimulus elements.