A Model That Relates Canal-Ocular to Otolith-Ocular Responses

Abstract

Rotation about the vertical stimulates primarily the horizontal semicircular canals and produces compensatory horizontal nystagmus whose slow component velocity response during constant velocity can be approximated as having a simple exponential decay time constant, Tvor. Constant velocity yaw rotation about a horizontal axis stimulates both the horizontal canals and the otolith organs, producing two additional nystagmus components thought to be of otolithic origin: a steady component called bias and a periodic component known as modulation. We tested a group of 7 human subjects using rotation about each of these axes. We found a strong, negative correlation (r = 0.956) between these individuals' dominant time constants, Tvor, and the magnitude of their modulation components. Canal and otolith signals originate from different parts of the same endorgan and travel separately to the vestibular nucleus. The reflexive eye movements in response to these inputs are thought to be the result of additional processing by the central vestibular and ocular motor systems. Thus, the source of these strongly correlated otolith-ocular and canal-ocular reflex components could logically be due to common factors affecting peripheral transduction or to a subsequent common central processing step. Using anatomical measurements of human vestibular end organs, biophysical endorgan models, and models of central vestibular processes, we examined the alternatives to determine which was the most likely. Correlations were not found between Tvor and the anatomical data or between the Tvor and the biophysical model elements. However, modifications of the velocity storage integrator of Raphan to incorporate either a highpass filter (HPF) or a lowpass filter input for otolith modulation signals allowed for the desired strong negative correlation between Tvor and modulation. We argue in favor of the HPF configuration because it better explains the tendency for the modulation component to increase in amplitude as the modulation frequency is increased. In the above mentioned representation of the central processing of vestibular afferent inputs, we conclude that the modulation component input to the vestibuloocular reflex is within the indirect pathway, but it is "down stream" from the velocity storage integrator.