Refixation control in free viewing: a specialized mechanism divulged by eye-movement-related brain activity

Abstract

In free viewing, the eyes return to previously visited locations rather frequently, even though the attentional and memory-related processes controlling eye-movement show a strong antirefixation bias. To overcome this bias, a special refixation triggering mechanism may have to be recruited. We probed the neural evidence for such a mechanism by combining eye tracking with EEG recording. A distinctive signal associated with refixation planning was observed in the EEG during the presaccadic interval: the presaccadic potential was reduced in amplitude before a refixation compared with normal fixations. The result offers direct evidence for a special refixation mechanism that operates in the saccade planning stage of eye movement control. Once the eyes have landed on the revisited location, acquisition of visual information proceeds indistinguishably from ordinary fixations. NEW & NOTEWORTHY A substantial proportion of eye fixations in human natural viewing behavior are revisits of recently visited locations, i.e., refixations. Our recently developed methods enabled us to study refixations in a free viewing visual search task, using combined eye movement and EEG recording. We identified in the EEG a distinctive refixation-related signal, signifying a control mechanism specific to refixations as opposed to ordinary eye fixations.

Keywords: EEG; attention; eye movement; memory; refixation.

Fig. 1.

Refixation requirements. Circles represent areas within a radius of 2° of visual angle around the fixation point. Numbers indicate locations of subsequent fixations. A–C illustrate possible refixation scenarios (see text for details). A: no refixation is counted. B: one Fixation-Refixation pair is counted (1–4). C: two Fixation-Refixation pairs are counted (1–3 and 1–5).

Fig. 2.

Eye movements entered into the presaccadic and postsaccadic analyses. Top: schema of 4 positions of eye movement events considered in this study. Saccade onset and offset indicate time points relative to which EEG was time locked in the presaccadic and postsaccadic analyses, respectively. Bottom: distributions of onsets of intervening (−1 and +1) saccades pooled across participants. Distributions are plotted after setting limits on eye movement characteristics and matching.

Fig. 3.

Eye movement results. The schema at the top illustrates 4 positions of eye movement events considered in this study. Saccade onset and offset indicate time points relative to which EEG was time locked in presaccadic and postsaccadic analyses, respectively. A: saccade duration (ms) for Actual Fixation (Fix Act), Actual Refixation (Refi Act), Mock Fixation (Fix Mock), and Mock Refixation (Refi Mock) conditions. B: saccade duration for Actual and Mock conditions (pooled across Fixation-Refixation conditions). C: saccade duration for Fixation and Refixation conditions (pooled across Actual-Mock conditions). D: Fixation duration (ms) for Actual Fixation, Actual Refixation, Mock Fixation, and Mock Refixation conditions. E: Fixation duration for Actual and Mock conditions (pooled across Fixation-Refixation conditions). Asterisks indicate significant differences between Actual and Mock (B and E) and Fixation and Refixation (C) conditions. Data points are means, and error bars are standard errors across 21 participants.

Fig. 4.

Presaccadic potentials for Actual (A and C) and Mock (B and E) conditions. A and B: grand-averaged presaccadic potential baseline-corrected at −300–280 ms from saccade onset for 8 ROIs. L, left; R, right; F, frontal; C, central; P, parietal; O, occipital. Map insets in PL panels show amplitude distribution at −200 ms for Fixation (top) and Refixation (bottom) conditions. C and E: mean amplitude in the interval −220–180 ms, where an interaction between Fixation-Refixation and ROI was found in Actual condition. Asterisks indicate significant differences at ROIs between Fixation and Refixation conditions. D: Fixation-Refixation amplitude difference in Actual and Mock conditions for ROIs where a significant effect in the Actual condition was found (C). Error bars indicate standard errors of the means across 16 participants.

Fig. 5.

Postsaccadic potentials for Actual (A and C) and Mock (B and D) conditions. A and C: grand-averaged postsaccadic potential baseline-corrected at 0–20 ms from saccade offset, for 8 ROIs. L, left; R, right; F, frontal; C, central; P, parietal; O, occipital. Map insets in OR panels show amplitude distribution at 270 ms for Fixation (bottom) and Refixation (top) conditions. B and D: mean amplitude in the interval 250–290 ms, where interactions between Fixation-Refixation and ROI were found. Asterisks indicate significant differences at ROIs between Fixation and Refixation conditions. Error bars indicate standard errors of the means across 14 participants.