Look into my eyes - What can eye-based measures tell us about the relationship between physical activity and cognitive performance?

Introduction: There is a growing interest in understanding the neurobiological mechanisms that can explain the positive effects of physical activity and fitness on measures of cognitive performance. As eye-based measures (e.g., eye movement measures such as saccades, pupillary measures such as pupil dilation, and vascular measures such as retinal vessel diameter) are deemed to be proxies for specific neurobiological mechanisms, several studies employed these measures to better understand the neurobiological mechanisms that drive the positive associations of physical activity and fitness with measures of cognitive performance. However, there is currently no systematic review that provides a comprehensive overview of the studies that employed such eye-based measures in the field of exercise-cognition science. Thus, this systematic review aims to address this gap in the literature. Methods: Five electronic databases were searched to identify eligible studies on October 23rd, 2022. Two researchers independently extracted data and assessed the risk of bias using a modified version of the Tool for the assEssment of Study qualiTy and reporting in EXercise (TESTEX scale – for interventional studies) and the critical appraisal tool of the Joanna Briggs Institute (for cross-sectional studies). Results: Our systematic review (N = 35 studies) provided the following main findings: (i) there is insufficient evidence available to make solid conclusions concerning gaze-fixation-based measures, (ii) the evidence that pupillometric measures, as a proxy of the noradrenergic system, can explain the positive effect of acute exercise and cardiorespiratory fitness on cognitive performance is mixed, (iii) physical training- or fitness-related changes of the cerebrovascular system (i.e., operationalized via changes in retinal vasculature) are, in general, positively associated with cognitive performance improvements, (iv) acute and chronic exercise have a positive effect on an oculomotor based measures of executive function (i.e., operationalized via antisaccade tasks), and (v) the positive association between cardiorespiratory fitness and cognitive performance is partly mediated by the dopaminergic system operationalized via the spontaneous eye-blink rate. Conclusion: Our systematic review provides evidence that eye-based measures can provide valuable insights into the neurobiological mechanisms that possibly drive the positive associations between physical activity and fitness with measures of cognitive performance. However, due to the limited number of studies that utilized specific methods to obtain distinct eye-based measures (e.g., pupillometry, retinal vessel analysis, spontaneous eye blink rate) or that investigated a possible dose-response relationship, further research is necessary before more nuanced conclusions can be drawn. Given that eye-based measures can be economically and non-invasively used, we hope that the present systematic review fosters the future application of eye-based measures in the field of exercise-cognition science.