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

Postural responses to head displacements are triggered by the vestibular sy stem; responses to body displacements are triggered by the somatosensory sy stem. We examined the interaction of responses to combinations of head and support surface perturbations. Head displacements were always in the opposi te direction of body displacements. The time between head and support surfa ce perturbations was varied. We measured amplitude and latency of gastrocne mius 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 con trols. 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 pertur bations were presented closely in time (10-50 ms apart). These results illu strate complex integration of vestibular and somatosensory information, sug gesting that the vestibulospinal and somatosensory-spinal pathways are not two isolated systems independently driving motor neurons. Rather, these pat hways may influence one another at premotoneuronal levels where common circ uitry may be shared by both systems.