Microsaccade biases can reflect task-specific spatial memorization strategies

(2019) and demonstrate that this assumption may not hold. The gaze bias instead reveals that participants might deploy an alternative spatial-based strategy, memorizing the endpoint locations of the bars, thereby allowing them to solve the task without memorizing orientations. Although we do not call into question the conclusion that internal attention is inherently spatially organized, our results do imply that directional microsaccade biases might also reflect attention directed at taskrelevant, location-specific stimulus properties, rather than reflecting internal attention directed at memorized orientations.Studies on internal attention using gaze have primarily used rectangular oriented bars (van Ede et al., 2019(van Ede et al., , 2020;;Liu et al., 2022;Draschkow et al., 2022). Bar stimuli are usually rotated around their centre of mass, which means that any orientation maps onto unique endpoints. Therefore, participants could memorize these endpoints instead of the orientation information per se. This strategy could serve as a form of cognitive offloading; akin to keeping a finger pointed at the former stimulus location, maintaining a microsaccade bias during the delay allows to 'remember' a location with minimal mental effort. We tested if participants' microsaccade directions depended on the endpoint/orientation of the stimuli by grouping trials based on the location of the cued item (left versus right) and the average orientation of the bars (45° (orientation range: 20° to 70°) versus 135° (110° to 160°), relative to horizontal midline). By doing so, we divided the original dataset into four new conditions of interest. In each condition, the endpoints of the bars occupied unique and distinguishable positions on the screen (see Figure 1ABDE). For each of these conditions we detected and calculated the average direction of microsaccades occurring 400 ms to 100 ms after the cue. Visual inspection suggested that microsaccade direction coincided with the upper endpoint of the bar in all conditions. Statistical tests confirmed that microsaccades were biased away from the left and right bar centres in all conditions (Left 45° = 3.14*, 95% CI [1.39, 2.45], Right 135° = 0*, 95% CI [0.39, 1.01], Left 135° = 3.14*, 95% CI [2.11,2.93], Right 45° = 0*, 95% CI [0.074, 0.63], units in radians, Figure 1C). Moreover, microsaccade direction did not significantly differ from the direction of the bar endpoints located in the upper visual field in 3 out of 4 conditions 1C). Taken together, these results show that attention, as indexed through microsaccade biases, was specifically directed to the former location of the bar's "upper" endpoint, rather than its centre of mass.(If participants indeed rely on memorized locations (i.e., bar endpoints) for their final report, errors in their memory should translate into corresponding errors in their gaze. For example, if participants memorized the upper endpoint of a bar oriented 45° on the right (Figure 1E, cyan bar), we expect their microsaccades to point towards the bar's endpoint at 15° (Figure 1E, dotted line). If microsaccades instead point towards a more clockwise direction (e.g., 10°), this could indicate that they erroneously memorized a different location, consistent with a more clockwise-oriented bar (oriented 30°). If participants indeed report orientation based on a memorized location -rather than orientation, we would expect their report to be biased clockwise relative to the true location (by 15°). This hypothesis predicts specific correlations for each condition due to the vertical symmetry of the stimulus display. To the left of fixation, larger clockwise microsaccade errors (saccade angle -endpoint angle) should result in larger clockwise report errors (reported angle -true angle) and thus a positive correlation. Conversely, to the right of fixation, larger clockwise microsaccade errors should lead to larger counterclockwise report errors and therefore a negative correlation. We tested this by performing within-subject trial-by-trial Pearson correlations between the report error and the microsaccade error. Group-level t-tests on the correlation coefficients indicated positive relationships in the Left 45° (rho = 0.025) and Left 135° (rho = 0.046) conditions and negative relationships in the Right 45° (rho = -0.052) and Right 135° (rho = -0.013, Figure 1F). When averaging these biases across all four conditions (sign-flipping the "Right" conditions), we found significantly positive correlations between gaze bias and report error across subjects. This highly specific pattern of results directly supports the hypothesis that participants infer the stimulus orientation from memorized locations. While we cannot completely exclude the possibility that participants also encoded orientation, the bias towards upper endpoints and the correlation between gaze and report errors, demonstrate that the spatial strategy was likely a significant driver of the observed gaze patterns.Our results have an important implication for the interpretation of microsaccade biases, namely that these biases can reflect a stimulus specific spatial strategy that does not involve memorization of orientation per se. Despite these new implications, we do not call into question the main theoretical conclusions of van Ede et al. ( 2019) that internal attention is inherently spatially organized. Since then, similar microsaccade biases were observed using stimuli without a clear endpoint (Ester et al. 2023;Liu et al., 2024;Arora et al., 2024). In fact, we recently conducted a retro-cue paradigm like that of van Ede et al. ( 2019) but using oriented Gabors instead of bars (Arora et al., 2024) and found no evidence for an endpoint bias. Moreover, in their Experiment 4, van Ede et al. (2019) showed that the horizontal microsaccade bias also emerges when participants need to report the colour of a bar that was cued via its orientation (0° vs 90°). In this case, maintaining attention (or gaze) at the bar endpoint is not a useful strategy for the task of reporting the colour. In sum, there is ample support for the original conclusions of van Ede et al. ( 2019) that visual working memory is inherently spatially organized.To conclude, we call for caution in interpreting microsaccade biases, similar to those reported in the article of van Ede et al. (2019), as unequivocally reflecting shifts of internal attention. Particular stimuli and task designs may allow for alternative (e.g., spatial) memorization strategies and can evoke stimulus-specific gaze patterns (Hafed & Ignashchenkova, 2013;Willet & Mayo, 2023). Furthermore, and theoretically of greater interest, our results show that observers can approach even the simplest working memory recall tasks in a variety of ways. When set-up and stimuli allow for, observers may deploy unexpected but resource-efficient strategies, highlighting the richness of visuospatial working memory use.