Attentional shifts bias microsaccade direction but do not cause new microsaccades

Liu, Baiwei; Alexopoulou, Zampeta-Sofia; van Ede, Freek · 2024 · DOAJ

DOI: 10.1038/s44271-024-00149-7

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Summary

This study investigates the mechanistic relationship between covert shifts of spatial attention and microsaccades, specifically addressing whether attention shifts trigger new microsaccades or merely bias the direction of ongoing ones. While previous research established that microsaccade direction correlates with attentional deployment, it remained unclear if this correlation resulted from the generation of new eye movements or a directional biasing of existing fixational movements. The authors aimed to disambiguate these scenarios to clarify whether the link between attention and microsaccades is deterministic or probabilistic. To test this, the researchers employed an internal selective-attention task where participants maintained two visual items in working memory. During the retention interval, a central fixation dot changed color. In half of the trials, the color matched one of the memorized items, serving as an attention-directing retrocue that prompted participants to shift attention to that specific memory location. In the other half, the color did not match either item, serving as a neutral cue that did not invite an attention shift. Crucially, the sensory input of the cues was identical across conditions, isolating the effect of voluntary attention shifts. Eye movements were recorded at 1000 Hz, and saccades were detected using a velocity-based method. The study compared overall microsaccade rates and directional biases between attention-directing and neutral cue trials. The results confirmed that participants effectively used the attention-directing cues, showing improved accuracy and faster reaction times compared to neutral trials. Consistent with prior literature, attention-directing cues produced a robust spatial bias in microsaccade direction, with significantly more saccades occurring toward the attended memory location than away from it. However, contrary to the hypothesis that attention shifts trigger new microsaccades, there was no significant increase in the overall microsaccade rate following attention-directing cues compared to neutral cues. Bayesian analysis provided strong evidence for the null hypothesis regarding rate increases. Furthermore, this finding held true regardless of the baseline saccade rate preceding the cue, ruling out ceiling effects. The directional modulation was confined to the microsaccade range (below 1 degree visual angle). These findings indicate that shifting spatial attention biases the direction of ongoing microsaccades without altering the probability of their occurrence. This dissociation between direction and rate suggests that the link between covert attention and microsaccades is probabilistic rather than obligatory; attention can shift without generating a new eye movement. The results imply that upstream oculomotor circuitry, such as the superior colliculus, may modulate the balance of neuronal activity to bias direction rather than adding new activity to trigger new saccades. This clarifies the functional role of microsaccades in attentional selection and guides future neurophysiological investigations into the shared brain mechanisms of eye movement control and covert attention.

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• attention selective divided