ROLE OF VESTIBULAR INFORMATION IN INITIATION OF RAPID POSTURAL RESPONSES

Patients with bilateral vestibular loss have difficulty maintaining ba lance without stepping when standing in tandem, on compliant surfaces, across narrow beams, or on one foot, especially with eyes closed. Nor mal individuals (with no sensory impairment) maintain balance in these tasks by employing quick, active hip rotation (a ''hip strategy''). T he absence of a hip strategy in vestibular patients responding to tran slations of a short support surface has previously been taken as evide nce that the use of hip strategy requires an intact vestibular system. However, many tasks requiring hip strategy alter one or a combination of important system characteristics, such as initial state of the bod y (tandem stance), dynamics (compliant surfaces), or biomechanical lim its of stability (narrow beams). Therefore, the balance deficit in the se tasks may result from a failure to account for these support surfac e alterations when planning and executing sensorimotor responses. In t his study, we tested the hypothe sis that vestibular information is cr itical to trigger a hip strategy even on an unaltered support surface, which imposes no changes on the system characteristics. We recorded t he postural responses of vestibular patients and control subjects with eyes closed to rearward support surface translations of varying veloc ity, in erect stance on a firm, flat surface. Subjects were instructed to maintain balance without stepping, if possible. Faster translation velocities (25 cm/s or more) produced a consistent pattern of early h ip torque (first 400 ms) in control subjects (i.e., a hip strategy). M ost of the patients with bilateral vestibular loss responded to the sa me translation velocities with similar torques. Contrary to our hypoth esis, we conclude that vestibular function is not necessary to trigger a hip strategy. We postulate, therefore, that the balance deficit n v iously observed in vestibular patients during postural tasks that elic it a hip strategy may have been due to the sensorimotor consequences o f the system alterations imposed by the postural tasks used in those s tudies. Preliminary results from two younger patients who lost vestibu lar function as infants indicate that age, duration of vestibular loss , and/or the timing of the loss may also be factors that can influence the use of hip strategy as a rapid postural response.