Mechanisms of Vestibular Adaptation: Preventing Static Imbalance and Optimizing Dynamic Performance

A fundamental challenge to the brain is how to prevent intrusive movements when quiet is needed. A stable platform is also necessary to launch accurate movements. Accordingly, nature has designed motor control systems functioning in push-pull, around a steady level of balanced tonic activity, the set-point. To ensure the optimal level of balanced tonic activity the brain has evolved an adaptive capability — “set-point or bias adaptation” — as a core neurobiological mechanism to maintain states of physical quiet and increase the ability to detect and dynamically respond to small differences in changes in neural activity. This mechanism allows for an increase in the signal to noise ratio (to improve sensitivity), and an extension of the range of linear behavior over which the reflex can faithfully transduce head motion. Take the vestibulo-ocular reflex (VOR), for example, which generates eye movements that compensate for head movements. The semicircular canals, working in coplanar pairs, one in each labyrinth, are reciprocally excited and inhibited as they transduce head rotations. The otolith organs, also working in pairs in each labyrinth, are excited and inhibited as they transduce head translations due to linear acceleration, or static tilt of the head, due to a change in the pull of gravity. The relative changes in activity are relayed to the vestibular nuclei that operate around a set-point of stable balanced activity. When a pathological imbalance occurs, producing unwanted nystagmus, ocular misalignment or counterroll (torsion) without head movement, an adaptive mechanism restores the proper set-point and eliminates these pathological signs. Here we will discuss some mechanisms of set-point adaptation and their clinical implications (e.g., Bechterew’s phenomenon after sequential loss of function of each labyrinth).