Control of fixation duration in a simple search task

TLDR: A preprogramming model of the control of fixation duration during visual search appears to be indirect in a simple search task and is supported by the results of an experiment carried out under two conditions.

Abstract: Toobtain insight into the control of fixation duration during visual search, we had 4 subjects perform simple search tasks in which we systematically varied the discriminability of the target. The experiment was carried out under two conditions. Under the first condition (blocked), the discriminability of the target was kept constant during a session. Under the second condition (mixed), the discriminability of the target varied per trial. Under the blocked condition, fixation duration increased with decreasing dis­ criminability. For 2 subjects, we found much shorter fixation durations in difficult trials with the mixed condition than in difficult trials with the blocked condition. Overall, the subjects fixated the target, continued to search, and then went back to the target in M6-55% of the correct trials. In these trials, the result of the analysis of the foveal target was not used for preparing the next saccade. The results sup­ port a preprogramming model of the control of fixation duration. In a simple search task, control of fix­ ation duration appears to be indirect. In daily life, the oculomotor system and the visual sys­ tem work in close cooperation. On the one hand, eye po­ sition determines the part of the environment that is ac­ cessible to visual perception. On the other hand, visually perceived information is essential for making goal-directed eye movements. Extensive visual search and reading are good examples of this cooperation. In both tasks, a se­ quence of eye movements is required to gather visual in­ formation from a display that exceeds the area covered by a single glance. During periods of fixation (intersac­ cadic intervals), at least three processes relating to vision may occur. These processes are samplings of the visual field, analysis of the foveal part of the visual field, and planning ofthe next eye movement (Viviani, 1990). These three processes take time. Analysis of the foveal target takes at least 100 to 150 msec (Eriksen & Eriksen, 1971) and eye-movement programming takes about 150 to 200 msec (Becker & Jurgens, 1979). These two processes are assumed to act in parallel, but not much is known about the amount ofoverlap (Viviani, 1990). In this visual­ search study, we were interested in the relationship be­ tween the analysis of the foveal target and the control of fixation duration. In other words: Is the result ofthe analy­ sis ofthe foveal target used in the planning ofthe next eye movement? Two models have been proposed. The first is the process-monitoring model (Rayner, 1978), in which the analysis of the foveal target is monitored by the mecha­ nism that controls the fixation duration. The planning of the saccade starts after the analysis of the foveal target has been completed. Analysis ofthe foveal target and plan