Postural responses to combinations of head and body displacements: vestibular-somatosensory interactions.

Postural responses to head displacements are triggered by the vestibular system; responses to body displacements are triggered by the somatosensory system. We examined the interaction of responses to combinations of head and support surface perturbations. Head displacements were always in the opposite direction of body displacements. The time between head and support surface perturbations was varied. We measured amplitude and latency of gastrocnemius and tibialis anterior EMGs for various head backward/body forward and head forward/body backward displacement combinations. These responses were compared to head-only or body-only displacement trials, which served as controls. Relative to controls, the latency of somatosensory-evoked responses to body displacement was longer and vestibular-evoked responses were absent or of low amplitude for combinations where head and support surface perturbations were presented closely in time (10-50 ms apart). These results illustrate complex integration of vestibular and somatosensory information, suggesting that the vestibulospinal and somatosensory-spinal pathways are not two isolated systems independently driving motor neurons. Rather, these pathways may influence one another at premotoneuronal levels where common circuitry may be shared by both systems.