Oculomotor behaviour in visual search for multiple targets

Neural correlates of task-related refixation behavior

Eye movement research has shown that attention shifts from the currently fixated location to the next before a saccade is executed. We investigated whether the cost of the attention shift depends on higher-order processing at the time of fixation, in particular on visual working memory load differences between fixations and refixations on task-relevant items. The attention shift is reflected in EEG activity in the saccade-related potential (SRP). In a free viewing task involving visual search and memorization of multiple targets amongst distractors, we compared the SRP in first fixations versus refixations on targets and distractors. The task-relevance of targets implies that more information will be loaded in memory (e.g. both identity and location) than for distractors (e.g. location only). First fixations will involve greater memory load than refixations, since first fixations involve loading of new items, while refixations involve rehearsal of previously visited items. The SRP in the interval preceding the saccade away from a target or distractor revealed that saccade preparation is affected by task-relevance and refixation behavior. For task-relevant items only, we found longer fixation duration and higher SRP amplitudes for first fixations than for refixations over the occipital region and the opposite effect over the frontal region. Our findings provide first neurophysiological evidence that working memory loading of task-relevant information at fixation affects saccade planning.

Refixation patterns reveal memory-encoding strategies in free viewing

We investigated visual working memory encoding across saccadic eye movements, focusing our analysis on refixation behavior. Over 10-s periods, participants performed a visual search for three, four, or five targets and remembered their orientations for a subsequent change-detection task. In 50% of the trials, one of the targets had its orientation changed. From the visual search period, we scored three types of refixations and applied measures for quantifying eye-fixation recurrence patterns. Repeated fixations on the same regions as well as repeated fixation patterns increased with memory load. Correct change detection was associated with more refixations on targets and less on distractors, with increased frequency of recurrence, and with longer intervals between refixations. The results are in accordance with the view that patterns of eye movement are an integral part of visual working memory representation.

Fixation duration surpasses pupil size as a measure of memory load in free viewing

Oculomotor behavior reveals, not only the acquisition of visual information at fixation, but also the accumulation of information in memory across subsequent fixations. Two candidate measures were considered as indicators of such dynamic visual memory load: fixation duration and pupil size. While recording these measures, we displayed an arrangement of 3, 4 or 5 targets among distractors. Both occurred in various orientations. Participants searched for targets and reported whether in a subsequent display one of them had changed orientation. We determined to what extent fixation duration and pupil size indicate dynamic memory load, as a function of the number of targets fixated during the search. We found that fixation duration reflects the number of targets, both when this number is within and above the limit of working memory capacity. Pupil size reflects the number of targets only when it exceeds the capacity limit. Moreover, the duration of fixations on successive targets but not on distractors increases whereas pupil size does not. The increase in fixation duration with number of targets both within and above working memory capacity suggests that in free viewing fixation duration is sensitive to actual memory load as well as to processing load, whereas pupil size is indicative of processing load only. Two alternative models relating visual attention and working memory are considered relevant to these results. We discuss the results as supportive of a model which involves a temporary buffer in the interaction of attention and working memory.

The dependence of visual scanning performance on saccade, fixation, and perceptual metrics

We sought to understand the basis of performance variability and perceptual learning in saccadic visual search. Four subjects searched for a target based on its shape in a linear array of densely packed, regularly spaced items, a configuration used to simplify the analysis of performance and to minimize search strategy variability. We measured the dependence of performance-search speed-on the oculomotor variables of fixation duration and saccade amplitude, both within and across experimental sessions. We also measured perceptual span, the area in visual space in which subjects could identify the target above chance, with a modified version of the task using a gaze-contingent display with transiently appearing targets. The principal finding of this study was that both within and across sessions, saccade metrics accounted for much more of the variability and improvement in performance than did fixation duration. Increases in search speed were due primarily to subjects processing information from a greater area of the visual field, rather than processing information from a fixed area more quickly, though there was a small but consistent decrease in fixation duration across sessions. The increase in performance derived from an increase in perceptual span and not merely from an increase in subjects' efficiency in 'tiling' the search array with regions of visibility.