Can pupillometry reveal perturbation detection in sensorimotor adaptation during grasping?

Humans adjust their motor actions to correct for errors both with and without being aware of doing so. Little is known, however, about what makes errors detectable for the actor, and how researchers can know when errors are detected. Here, we investigated pupillometry as an unobtrusive no-report marker of perturbation detection. We also replicate and extend prior work showing that motor adjustments may mask the very errors they correct for. N=48 participants grasped objects while a visuo-haptic size mismatch was applied either sinusoidally (with a continuously changing perturbation) or abruptly (with a constant perturbation that could more easily be adapted to). When mismatches started abruptly and thereafter stayed the same, participants adapted well but also showed decreasing discrimination performance and decreasing confidence in their responses. This was not the case for sinusoidally introduced perturbations. As hypothesized, parameters that characterize phasic and tonic pupil responses were predicted by stimulus parameters and differed depending on participants’ grasping and behavioral responses. In particular, phasic responses were smaller for stronger perturbations but larger in trials with correct and confident responses, while tonic responses were correlated positively with perturbation magnitude and response correctness, but negatively with response confidence. However, predicting response characteristics from pupil-dilation features using support-vector machine classifiers was not successful. This shows that while pupillometry may yet prove to be a useful no-report marker of perturbation and error detection, there are some challenges for trial-by-trial prediction.