Pupil diameter and SCM muscle activity serve as reliable indicators of surprise and adaptation in dynamic locomotor tasks, highlighting coordinated autonomic and neuromuscular responses to uncertainty during split-belt treadmill walking with probabilistic perturbations.
Key Findings
Results
The initial 5 strides of the perturbation trial elicited significant increases in pupil diameter, reflecting surprise and novelty responses.
Twenty-four healthy young adults participated in the study
Participants walked on a split-belt treadmill with probabilistic perturbations triggered at 80% of right heel strikes by rapid belt decelerations
Pupil diameter increase during initial 5 strides was statistically significant (p < 2.2e-16)
Pupil diameter reached maximal values during strides 16-50 before returning to Baseline equivalent values for perturbation strides beyond 100
Results
The initial 5 strides of the perturbation trial elicited significant increases in sternocleidomastoid (SCM) muscle activation, indicative of startle responses.
SCM activation increase during initial 5 strides was statistically significant (p = 9.444e-06)
SCM activation showed sustained increases relative to Baseline values for perturbed strides for the entirety of the 15-minute Perturbation trial
Unlike pupil diameter, SCM activation did not return to baseline-equivalent values during the perturbation trial
SCM activity was used as an indicator of startle responses to locomotor uncertainty
Results
Pupil diameter and SCM activity exhibited distinct temporal adaptation patterns in response to probabilistic perturbations.
Pupil diameter showed a non-linear pattern, peaking at strides 16-50 and returning to baseline-equivalent values after stride 100
SCM activation remained elevated above baseline for the entirety of the 15-minute perturbation trial
The two measures showed different temporal profiles, suggesting they capture distinct aspects of the neurophysiological response to uncertainty
These distinct temporal patterns were modulated by perturbation status
Results
Generalized Additive Mixed Models (GAMMs) revealed non-linear relationships between perturbations, pupil diameter, and SCM activity.
GAMMs were used to model the non-linear temporal dynamics of both neurophysiological measures
The models revealed non-linear relationships between perturbation occurrence and both pupil diameter and SCM activity
Distinct temporal patterns were identified that were modulated by perturbation status
This analytical approach captured the dynamic adaptation processes not addressable by linear models
Background
The study was designed to investigate whether pupil diameter and SCM activity reflect neurophysiological responses to uncertainty during locomotor adaptation based on Bayesian inference principles.
The brain is thought to rely upon Bayesian inference to integrate sensory input with prior beliefs, updating its environmental model through prediction errors
Pupil diameter was used as a proxy for cognitive arousal
SCM activity was used as indicative of startle responses
Perturbations occurred probabilistically at 80% of right heel strikes, creating an uncertain but learnable environment
Roberts B, Seidler R. (2026). Neurophysiological metrics of surprise during locomotor uncertainty.. Journal of neuroscience methods. https://doi.org/10.1016/j.jneumeth.2026.110722