Role of Microsaccades in Visual Working Memory

How the human brain rehearses and maintains information in working memory is a fundamental question in cognitive neuroscience. Working memory, our ability to temporarily hold and manipulate information, is crucial for everyday tasks from remembering a phone number to following complex instructions. While the brain’s oculomotor system, which controls eye movements, is known to be involved in spatial attention and the rehearsal of spatial information, its role in rehearsing visual objects when their location is not explicitly relevant is less clear. The research paper “Microsaccades track location-based object rehearsal in visual working memory” by Eelke de Vries and Freek van Ede (2024) addresses this intriguing question by investigating whether the oculomotor system contributes to rehearsing visual objects through their associated locations, even when those locations are incidental to the task.

Eye Tracking and Working Memory Study

This study highlights the critical importance of eye tracking, specifically the analysis of microsaccades, in unraveling the subtle mechanisms of working memory. Microsaccades are tiny, involuntary eye movements that occur even when we try to hold our gaze steady. Traditionally, eye tracking has been used to study overt attention – where a person is looking. However, de Vries and van Ede demonstrate how microsaccades can serve as a “direct time-resolved output from the oculomotor system,” offering a unique window into covert cognitive processes that are not directly observable.

In their experiments, participants were asked to remember the orientation and color of visual objects, but not their locations. By varying the spatial configuration of these objects at encoding (horizontal, diagonal, or vertical), the researchers could then observe whether microsaccades during the memory delay period were biased along the axis of the memorized content. This innovative approach allowed them to track the incidental use of locations for mnemonic rehearsal.

Microsaccade detection and quantification requires an eye tracker with a fast sampling rate and exceptionally high levels of precision. The authors used a desktop mounted SR Research EyeLink 1000 Plus, sampling at 1000 Hz.

Eye Tracking and Microsaccades Unravel the Mechanisms of Working Memory

Experiment 1 revealed that microsaccades continued to be biased along the axis of the memory content for several seconds into the working memory delay. This suggests that the oculomotor system was actively engaged in rehearsing the visual objects, even when their locations were not explicitly required for the task.

To further confirm this finding, Experiment 2 introduced control conditions where participants either passively viewed the same objects or made perceptual judgments without subsequent working memory demands. The results showed that the directional microsaccade bias was specific to memory demands, ruling out the possibility that the bias was merely a lingering effect from passive or attentive encoding. This demonstrates that eye tracking, through the detailed analysis of microsaccade directions, can effectively disentangle active working memory processes from other cognitive activities.

The authors conclude that microsaccades provide “unique evidence that the brain’s oculomotor system also participates in the rehearsal of visual objects in working memory, even when object location is never asked about.” This finding not only advances our understanding of working memory but also underscores the immense value of eye tracking as a tool in cognitive neuroscience. By precisely measuring and analyzing these subtle eye movements, researchers can gain crucial insights into the neural mechanisms underlying complex cognitive functions, ultimately leading to a more comprehensive understanding of how the human brain processes and retains information.

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