Time-dependent influence of sensorimotor set on automatic responses in perturbed stance

These experiments tested the hypothesis that the ability to change sensorim otor set quickly for automatic responses depends on the time interval betwe en successive surface perturbations. Sensorimotor set refers to the influen ce of prior experience or context on the state of the sensorimotor system. Sensorimotor set for postural responses was influenced by first giving subj ects a block of identical backward translations of the support surface, cau sing forward sway and automatic gastrocnemius responses. The ability to cha nge set quickly was inferred by measuring the suppression of the stretched antagonist gastrocnemius responses to toes-up rotations causing backward sw ay, following the translations. Responses were examined under short (10-14 s) and long (19-24 s) inter-trial intervals in young healthy subjects. The results showed that subjects in the long-interval group changed set immedia tely by suppressing gastrocnemius to 51% of translation responses within th e first rotation and continued to suppress them over succeeding rotations. In contrast, subjects in the short-interval group did not change set immedi ately, but required two or mon rotations to suppress gastrocnemius response s. By the last rotation, the short-interval group suppressed gastrocnemius responses to 33%, similar to the long-interval group of 29%. Associated sur face plantarflexor torque resulting from these responses showed similar res ults. When rotation and translation perturbations alternated, however, the short-interval group was not able to suppress gastrocnemius responses to ro tations as much as the long-interval group, although they did suppress more than in the first rotation trial after a series of translations. Set for a utomatic responses appears to linger, from one trial to the next. Specifica lly, sensorimotor set is more difficult to change when surface perturbation s are given in close succession, making it appear as if set has become prog ressively stronger. A strong set does not mean that responses become larger over consecutive trials. Rather, it is inferred by the extent of difficult y in changing a response when it is appropriate to do so. These results sug gest that the ability to change sensorimotor set quickly is sensitive to wh ether the change is required after a long or a short series of a prior diff erent response, which in turn depends on the time interval between successi ve trials. Different rate of gastrocnemius suppression to toes-up rotation of the support surface have been reported in previous studies. This may be partially explained by different inter-trial time intervals demonstrated in this study.