Vestibular influences on human postural control in combinations of pitch and roll planes reveal differences in spatiotemporal processing

The present study examined the influence of bilateral peripheral vestibular loss (BVL) in humans on postural responses to multidirectional surface rot ations in the pitch and roll planes. Specifically, we examined the effects of vestibular loss on the directional sensitivity, timing, and amplitude of early stretch, balance correcting, and stabilizing reactions in postural l eg and trunk muscles as well as changes in ankle torque and trunk angular v elocity following multidirectional rotational perturbations of the support surface. Fourteen normal healthy adults and five BVL patients stood on a du al axis rotating platform which rotated 7.5 degrees at 50 degrees /s throug h eight different directions of pitch and roll combinations separated by 45 degrees. Directions were randomized within a series of 44 perturbation tri als which were presented first with eyes open, followed by a second series of trials with eyes closed. Vestibular loss did not influence the range of activation or direction of maximum sensitivity for balance correcting respo nses (120-220 ms). Response onsets at approximately 120 ms were normal in t ibialis anterior (TA), soleus (SOL), paraspinals (PARAS), or quadriceps mus cles. Only SOL muscle activity demonstrated a 38- to 45-ms delay for combin ations of forward (toe-down) and roll perturbations in BVL patients. The am plitude of balance correcting responses in leg muscles between 120 and 220 ms was, with one exception, severely reduced in BVL patients for eyes open and eyes closed conditions. SOL responses were decreased bilaterally for to e-up and toe-down perturbations, but more significantly reduced in the down hill (load-bearing) leg for combined roll and pitch perturbations. TA was s ignificantly reduced bilaterally for toe-up perturbations, and in the downh ill leg for backward roll perturbations. Forward perturbations, however, el icited significantly larger TA activity in BVL between 120 and 220 ms compa red to normals, which would act to further destabilize the body. As a resul t of these changes in response amplitudes, BVL patients had reduced balance correcting ankle torque between 160 and 260 ms and increased torque betwee n 280 and 380 ms compared to normals. There were no differences in the orie ntation of the resultant ankle torque vectors between BVL and normals, both of which were oriented primarily along the pitch plane. For combinations o f backward (toe-up) and roll perturbations BVL patients had larger balance correcting and stabilizing reactions (between 350 and 700 ms) in PARAS than normals and these corresponded to excessive trunk pitch and roll velocitie s. During roll perturbations, trunk velocities in BVL subjects after 200 ms were directed along directions different from those of normals. Furthermor e, roll instabilities appeared later than those of pitch particularly for b ackward roll perturbations. The results of the study show that combinations of roll and pitch surface rotations yield important spatiotemporal informa tion, especially with respect to trunk response strategies changed by BVL w hich are not revealed by pitch plane perturbations alone. Our results indic ate that vestibular influences are earlier for the pitch plane and are dire cted to leg muscles, whereas roll control is later and focused on trunk mus cles.