Mental states inside out: subtle eye movements reveal the 'spotlight' within our inner mind

With the emergence and advancement of modern-day brain imaging, there has been a tremendous increase in our knowledge of (and interest in) the biological mechanisms that give rise to mental states – broadly construed as ‘the mind’. Our study reveals that what happens inside our mind does not always stay in the mind – i.e. that our inner mental states are, at least under some circumstances, also reflected in peripheral measurements of overt behaviour.
Mental states inside out: subtle eye movements reveal the 'spotlight' within our inner mind

In our study, published in Nature Human Behaviour, human volunteers were presented with multiple visual objects that they were required to keep in memory. After a brief memory period (in the order of seconds), participants received a cue to select the memorised visual object that was to be reported subsequently. Our data reveal that such internal focusing (putting the ‘spotlight’ on a representation in the mind; link) does not stay undetected in peripheral measurements of gaze. Instead, internal focusing is paired with an increase in tiny eye movements (called micro-saccades; link) in the direction of the memorised location of the focused memory representation (though there was nothing to look at on the screen). In other words, what gets focused in the mind’s eye can be ‘read-out’ from the actual eye. Importantly, because the neural architecture that supports such eye movements is well characterised (link), this also informs on the neural substrates that are engaged when we focus our mind’s eye.

The identification of this remarkable phenomenon was not planned, but serendipitous. As if often the case in laboratory experiments of mental states, eye-tracking was initially included solely for control purposes. Having stumbled across this phenomenon, however, we soon came to realise the exciting nature of it, and embarked on a series of targeted experiments specifically aimed at demonstrating and understanding the nature of this phenomenon. These are the experiments that we report on in our manuscript. 

Though our discovery was neither planned nor anticipated, it builds on a long line of research that had already linked the brain’s oculomotor system (that governs where we look) to the allocation of covert attention in the context of perception (link), as well as on other research that had already linked larger eye movements to memory retrieval processes (link).

Looking ahead, our study promotes the utility of carefully measuring peripheral responses to capture ‘fingerprints’ of mental processes that need not be confined to the inner space between our ears (see e.g. link for another example of this general idea from my own prior research). Indeed, while brain imaging continues to make magnificent advances in the study of mental processes, its often exclusive focus on the brain may sometimes lead us to forget that the brain is not an isolated organ, but rather an organ that is part of a larger eco-system; the body. From this body, I expect, a wide variety of highly informative behaviours can be extracted that will provide additional, complementary windows into the nature of our ‘inner’ mind.

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