Spatial orientation of the angular vestibulo-ocular reflex (aVOR) after semicircular canal plugging and canal nerve section

Abstract

We investigated spatial responses of the aVOR to small and large accelerations in six canal-plugged and lateral canal nerve-sectioned monkeys. The aim was to determine whether there was spatial adaptation after partial and complete loss of all inputs in a canal plane. Impulses of torques generated head thrusts of ≈3,000°/s2. Smaller accelerations of ≈300°/s2 initiated the steps of velocity (60°/s). Animals were rotated about a spatial vertical axis while upright (0°) or statically tilted fore-aft up to ±90°. Temporal aVOR yaw and roll gains were computed at every head orientation and were fit with a sinusoid to obtain the spatial gains and phases. Spatial gains peaked at ≈0° for yaw and ≈90° for roll in normal animals. After bilateral lateral canal nerve section, the spatial yaw and roll gains peaked when animals were tilted back ≈50°, to bring the intact vertical canals in the plane of rotation. Yaw and roll gains were identical in the lateral canal nerve-sectioned monkeys tested with both low- and high-acceleration stimuli. The responses were close to normal for high-acceleration thrusts in canal-plugged animals, but were significantly reduced when these animals were given step stimuli. Thus, high accelerations adequately activated the plugged canals, whereas yaw and roll spatial aVOR gains were produced only by the intact vertical canals after total loss of lateral canal input. We conclude that there is no spatial adaptation of the aVOR even after complete loss of specific semicircular canal input.

Tuning of gravity-dependent and gravity-independent vertical angular VOR gain changes by frequency of adaptation

“…The model predicts that the aVOR gain in upright or in statically tilted forward or backward positions would be the same after gain adaptation in the side-down position. Indeed, this was observed experimentally for horizontal and vertical aVOR regardless of whether the aVOR gain was increased or decreased (Yakushin et al 2003a(Yakushin et al , 2003c(Yakushin et al , 2005b.…”

“…1E). Previous experiments on yaw and pitch aVOR gain adaptation indicated that the spatial phase ␤, or the head orientation in which maximal gain changes were observed, is close to the position of adaptation (Yakushin et al 2003b(Yakushin et al , 2003c(Yakushin et al , 2005a(Yakushin et al , 2005b. Because a major purpose of this study was to determine the amount of gravitydependent and -independent gain changes as a function of frequency, it was assumed that the peak gain changes occurred in the position of adaptation in this study.…”

“…In one experiment aVOR gain was also decreased at 1 Hz. Previous studies had demonstrated that retention of gravity-dependent gain changes could be observed for several days (Schubert et al 2008;Yakushin et al 2003aYakushin et al , 2003b; therefore, at least a week was allowed before the next adaptation session.…”

“…The dependence of the aVOR gain changes on gravity (Baker et al 1987;Tan et al 1992;Tiliket et al 1993;Yakushin et al 2000a) is contextual learning, which has been investigated in more detail than any other context and has broad implication regarding global and localized learning (Schubert et al 2008;Yakushin et al 2003aYakushin et al , 2003bYakushin et al , 2003cYakushin et al , 2005aYakushin et al , 2005bYakushin et al , 2009. Specifically, when changes in the horizontal/yaw or vertical/pitch aVOR are induced in a particular head orientation, for example, in the left-side-down head position, the changes in gain will be maximal in the position of adaptation and minimal in the alternate side-down position.…”

“…The work of myself and my colleagues (Yakushin et al 2003a(Yakushin et al , 2003b(Yakushin et al , 2005a(Yakushin et al , 2005b on gravity-dependent adaptation has been focused on understanding the spatial aspects of the adaption and was studied at a single frequency of adaptation (0.2 or 0.5 Hz). The dependency of gravity-dependent adaptation induced by the single frequency of adaptation on the postadaptive gain as a function of frequency is not known.…”