A study is reported how fixation durations are affected as observers voluntarily use information in the visual environment to direct saccades in search of a target. In experiments with direction-coded displays, search fixation durations for correct and incorrectly directed saccades were comparable during image-driven/bottom-up search. In contrast, for knowledge-driven search, the relationship depended on peripheral preview. With ample access to peripheral preview, search fixation durations preceding correct saccades were shorter than those preceding incorrectly directed saccades. However, they were longer when preview was constrained by a gaze-contingent moving window. Fixation control was modeled as sigmoid functions.

Subjects searched for a target among distractors which were arranged randomly or such that each distractor provided information about the relative position of a target. Trials were presented either in a blocked design (so that the subjects knew a priori the contextual information in the display) or in a mixed design. When the distractors provided information about target position, there were (i) shorter manual RTs, (ii) fewer fixations made in search of the target, (iii) longer mean fixation durations, (iv) shorter initial fixation durations, (v) shorter mean gaze shifts, (vi) a smaller area of fixation dispersion, and (vii) a greater percentage of optimally directed saccades. Except for gaze shifts, the results were uninfluenced by whether or not there was a blocked or a mixed presentation. The results of the study suggest that despite noise in the search mechanism, fixation durations were adjusted to process directly the currently fixated element(s).

Introduction: Asymmetries in processing by the healthy brain demonstrate regularities that facilitate the modeling of brain operations. The goal of the present study was to determine asymmetries in saccadic metrics during visual exploration, devoid of confounding clutter in the visual field.Methods: Twenty healthy adults searched for a small, low-contrast gaze-contingent target on a blank computer screen. The target was visible, only if eye fixation was within a 5 deg. by 5 deg. area of the target’s location.Results: Replicating previously-reported asymmetries, repeated measures contrast analyses indicated that up-directed saccades were executed earlier, were smaller in amplitude, and had greater probability than down-directed saccades. Given that saccade velocities are confounded by saccade amplitudes, it was also useful to investigate saccade kinematics of visual exploration, as a function of vertical saccade direction. Saccade kinematics were modeled for each participant, as a square root relationship between average saccade velocity (i.e., average velocity between launching and landing of a saccade) and corresponding saccade amplitude (Velocity = S*[Saccade Amplitude]^0.5). A comparison of the vertical scaling parameter (S) for up- and down-directed saccades showed that up-directed saccades tended to be slower than down-directed ones.Discussion: To motivate future research, an ecological theory of asymmetric pre-saccadic inhibition was presented to explain the collection of vertical saccadic regularities. For example, given that the theory proposes strong inhibition for the releasing of reflexive down-directed prosaccades (cued by an attracting peripheral target below eye fixation), and weak inhibition for the releasing of up-directed prosaccades (cued by an attracting peripheral target above eye fixation), a prediction for future studies is longer reaction times for vertical anti-saccade cues above eye fixation. Finally, the present study with healthy individuals demonstrates a rationale for further study of vertical saccades in psychiatric disorders, as bio-markers for brain pathology.