Startle Stimuli Reduce the Internal Model Control in Discrete Movements

Abstract

Preplanned motor programs have been shown to release early from subcortical areas by administering a startling acoustic stimulus (SAS). The kinematics and dynamics of the released movement remain consistent with the intended movement. Our question is whether this response might also contain a recently learned internal model, which draws on experience to predict and compensate for expected perturbations in a feedforward manner. The unexpected removal of the perturbation leads to sporadic errors in a movement, termed after-effects. We tested the effects of startle on learning by introducing SAS during catch trials (i.e. removal of perturbation) in typical adaptation paradigm which used robotic forces (or visual distortions). We found that, in all subjects that exhibited a startle response complete with an early release of movement and neck muscle activity, the startle stimulus (1) reduced performance of the recently learned task (2) reduced after-effect magnitudes, suggesting that startle disrupts the release of a recently learned internal model. Because startle reduced but did not eliminate the recall of learned control, we suggest that multiple neural centers (cortical and subcortical) are involved in such learning and adaptation, which can impact training areas such as piloting, teleoperation, sports, and rehabilitation.