Pupil dilation scales with movement distance of real but not of imagined reaching movements

Pupillary responses have now been reliably identified for cognitive and motor tasks, but less is known about their relation to mentally simulating movements (known as motor imagery). Previous work found pupil dilations during the performance of simple finger movements, where peak pupillary dilation scaled with the complexity of the finger movement and force required. Recently, pupillary dilations were reported during imagery of grasping and piano playing. Here we examined whether pupillary responses are sensitive to the dynamics of the underlying motor task for both executed and imagined reach movements. Participants reached or imagined reaching to one of three targets placed at different distances from a start position. Both executed and imagined movement times scaled with target distance, and they were highly correlated, confirming previous work and suggesting that participants did imagine the respective movement. Increased pupillary dilation was evident during motor execution compared to rest, with stronger dilations for larger movements. Pupil dilations also occurred during motor imagery, however they were generally weaker than during motor execution and they were not influenced by imagined movement distance. Instead, dilations during motor imagery resembled pupil responses obtained during a non-motor imagery task (imagining a previously viewed painting). Our results demonstrate that pupillary responses can reliably capture the dynamics of an executed goal-directed reaching movement, but suggest that pupillary responses during imagined reaching movements reflect general cognitive rather than motor-specific components of the motor imagery process. New and noteworthy Pupil size is influenced by the performance of cognitive tasks. Here we demonstrate that pupil size increases during execution and mental simulation of goal-directed reaching movements compared to rest. Pupil dilations scale with movement amplitude only during motor execution, whereas they are similar during motor imagery and a non-motor imagery, cognitive task.