Pupil Diameter Tracked During Motor Adaptation in Humans

Pupil diameter, under constant illumination, is known to reflect individuals’ internal states, such as surprise about observation and environmental uncertainty. Despite the growing use of pupillometry in cognitive learning studies as an additional measure for examining internal states, few studies have used pupillometry in human motor learning studies. Here we provide the first detailed characterization of pupil diameter changes in a short-term reach adaptation paradigm. We measured pupil changes in 91 human participants while they adapted to abrupt, gradual, or switching force field conditions. Sudden increases in movement error caused by the introduction/reversal of the force field resulted in strong phasic pupil dilation during movement accompanied by a transient increase in tonic pre-movement baseline pupil diameter in subsequent trials. In contrast, clear changes in pupil responses were absent when the force field was gradually introduced, indicating that error drove the changes in pupil responses. Nevertheless, we found an association between baseline pupil diameter and awareness of the gradually-introduced perturbation assessed post-experimentally. In all experiments, we also found a strong co-occurrence of larger baseline pupil diameter and slower reaction and movement time after each set break. Interestingly, error-induced pupil responses gradually became insensitive after experiencing multiple reversals. Collectively, these results suggest that tonic baseline pupil diameter reflects one’s belief about environmental uncertainty, whereas phasic pupil dilation during movement reflects surprise about a sensory outcome (i.e., movement error), and both effects are modulated by novelty. Our results provide a new approach for non-verbally assessing participants’ internal states during motor learning.