Unlocking the Brain Through the Eyes: The Power of Pupillometry in fMRI Research

Pupillometry– the measurement of pupil size and responses – is a tool that has long been available but remains surprisingly underused in fMRI research. In his presentation at this year’s fMRI meeting, Dr. Jakub Wojciechowski discussed how pupillometry works, why the field is interested in it, and what recent literature shows about its use as an indicator of arousal, cognitive effort, and emotional processing.

Jakub provided an overview of existing studies and the underlying biology, as well as reflections on how the method could be more effectively used in future work.

Why pupillometry matters in fMRI

Pupil size is regulated by the dynamic interplay between the sympathetic and parasympathetic nervous systems, meaning that the pupil reacts not only to changes in light, but also to cognitive demands, emotional states, and anticipatory expectations. Consequently, pupillometry has been employed in behavioral and fMRI research to measure arousal through pupil dilation, track learning processes over time, assess cognitive effort, and monitor responses to rewards or threats.

What the literature shows – examples from the field

Throughout his talk, Jakub presented several studies from multiple research groups to illustrate the breadth of applications. Examples included:

-           Reward studies, showing that monetary rewards often produce stronger pupil dilation than social or neutral stimuli.

-           Classical conditioning and fear studies, where the pupil increases during anticipation and follows the learning curve as threats are introduced and later removed.

-           Working memory tasks, where pupil size grows with increasing cognitive demands.

Why should pupillometry be considered more often?

Jakub argued that pupillometry is a “low threshold” addition in many experiments. It provides insight at multiple levels while being non-invasive, affordable and easy to combine with other data types. He noted, however, that pupillometry remains unevenly represented in conference presentations and publications, suggesting that its potential is far from fully utilized.

Jakub’s team was among the first to use the upgraded VisualSystem HD (VSHD) by NordicNeuroLab. This system, equipped with dual cameras, made it easier to monitor and calibrate pupil position in real time.

The future of pupillometry in fMRI

In conclusion, pupillometry provides a valuable, yet often overlooked, “window” into the brain. By capturing subtle changes in pupil size, researchers can gain insight into arousal, cognitive effort, emotional processing, and learning dynamics – all without adding significant burden or invasiveness to an experiment. Jakub’s overview highlights how pupillometry complements behavioral and fMRI measures, offering multidimensional perspective on the processes underlying human thought and behavior.

Despite its accessibility and potential, pupillometry remains underrepresented in neuroimaging research, partly due to equipment limitations and the prevailing focus on more traditional readouts. As more laboratories adopt eye-tracking technology and recognize the richness of the data it provides, pupillometry could become a standard component of cognitive neuroscience studies. Ultimately, integrating pupil measurements with behavioral, neural, and physiological data promises a fuller and more nuanced understanding of how the brain responds to both internal and external demands, paving the way for innovative research across domains from reward processing to emotional regulation and cognitive control.

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Watch the full presentation:

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