Relations Between Reflex and Perceptual Responses to Vestibular Stimulation

In the past, reflex responses to vestibular stimulation tended to be viewed as rather stereotyped examples of classical reflex function; for example in postural control, righting reflexes and auto-stabilization of the head and eyes (gaze) through vestibulo-ocular (VOR) and vestibulocolic reflexes. Indeed uncovering of the physical attributes of the vestibular sensory end-organs encouraged this view due to the strictly mechanical nature of their inertial transduction of movement stimuli. Thus accelerated head rotation generates mathematically predictable inertial forces acting on the endolymphatic fluid of the semicircular canals, causing minute but precisely defined patterns of relative fluid flow in the lumen of the canal. Such flow is instantly opposed by viscous forces, dependent explicitly on the relative velocity of fluid low. As a result, simple Newtonian mechanics predicts that at any given time the volumetric displacement of fluid (and hence also linear displacement of the cupula in the ampulla), rel ly reflects the instantaneous angular velocity (i.e. speed) of head rotation, at least for the generally short, sharp, patterns of most naturally occurring head movements. By and large the resulting mechano-neural transduction generates corresponding ""angular speedometer"" signals which project this message through peripheral vestibular nerve fibres into the brainstem (reviewed in Wilson and Melvill Jones, 1979), In short the predictable, stereotyped, sensory information derived from this source predisposes to an equally stereotyped concept of physiological response in the CNS and Its peripheral effector organs, particularly In the VOR whose ""elementary"" reflex arc is but a short (10-12 msec latency), disynaptic one.