Pupil size as a biomarker of cognitive (dys)functions: Toward a physiologically informed screening of mental states

The objective assessment of cognitive processes is of critical importance to understanding the mechanisms underlying various mental functions and dysfunctions. In recent years, the technological innovations related to the eye-tracking industry made the time right to organically integrate pupillometry in the assessment of cognitive profiles. Here, we review evidence showing that pupillometry offers a uniquely sensitive biomarker of the functioning of several distinct networks, cognitive functions, emotional states, and individual differences in their instantiation. We outline why and how pupillometry can be effectively exploited to enrich current research and behavioral paradigms, including those designed for clinical testing. By making the cases of anxiety disorders (both generalized and specific - e.g., generalized anxiety vs. math anxiety) and substance use disorders, we then exemplify how pupillometry can be leveraged to obtain clinically meaningful variables. We finally conclude by arguing that measuring pupil size has the potential to complement more traditional, but coarse assessment methods, to return a more graded, objective, and physiologically informed picture of cognitive functioning.