CONTROL OF FIXATION DURATION IN A SIMPLE SEARCH TASK

To obtain insight into the control of fixation duration during visual search, we had 4 subjects perform simple search tasks in which we syst ematically vaned the discriminability of the target. The experiment wa s carried out under two conditions. Under the first condition (blocked ), the discriminability of the target was kept constant during a sessi on. Under the second condition (mixed), the discriminability of the ta rget varied per trial. Under the blocked condition, fixation duration increased with decreasing discriminability. For 2 subjects, we found m uch shorter fixation durations in difficult trials with the mixed cond ition than in difficult trials with the blocked condition. Overall, th e subjects fixated the target, continued to search, and then went back to the target in 5%-55% of the correct trials. In these trials, the r esult of the analysis of the foveal target was not used for preparing the next saccade. The results support a preprogramming model of the co ntrol of fixation duration. In a simple search task, control of fixati on duration appears to be indirect.