Tracking the mind’s eye

In popular culture, it is common to speak of the "mind's eye” to refer to a metaphorical eye that looks at images in the mind, e.g., imagination or visual memory. But what if this metaphorical eye was, in fact, traceable through the behaviour of our physical eyes, and therefore observable by researchers? In this PhD thesis, I will show how the mind's eye is indeed connected to our physical eyes through small eye movements named microsaccades. I will show how this can be used to 'read out' internal visual processing in the mind and how it is relevant to factor in such small eye movements when studying the neural bases of these internal visual processes. My thesis can be logically grouped into two parts. Part 1 (Chapters 2–4) focuses on visual working memory, a fundamental cognitive function that is at the heart of this thesis. I will use observable microsaccades as a tool to track core processes in the mind's eye, focusing on the selective prioritisation and transformation of internal visual information held within the spatial layout of visual working memory. Through this approach, I will show the continuous, adaptive nature of visual working memory in dynamic scenarios. Part 2 (Chapters 5–8) addresses the logical question that emerges from Part 1: given that spatial biases in microsaccades are a robust signal correlated with the aforementioned cognitive factors, how do microsaccades relate to human neuroscience signals that have classically been attributed to these top-down cognitive modulations? I demonstrate that while microsaccade direction reliably covaries with these neural modulations, the relationship is not obligatory. Finally, I also show that attention biases the direction—but not the occurrence—of ongoing microsaccades, offering a mechanistic explanation for their probabilistic link. Together, this thesis positions microsaccades as a sensitive and temporally precise index of internal cognitive processes and underscores the importance of eye movement behaviour in understanding the neural basis of visual attention and working memory.