Abstract
Electrical vestibular neurostimulation may be a viable tool for modulating vestibular afferent input to restore vestibular function following injury or disease. To do this, such stimulators must provide afferent input that can be readily interpreted by the central nervous system to accurately represent head motion to drive reflexive behavior. Since vestibular afferents have different galvanic sensitivity, and different natural sensitivities to head rotational velocity and acceleration, and electrical stimulation produces aphysiological synchronous activation of multiple afferents, it is difficult to assign a priori an appropriate transformation between head velocity and acceleration and the properties of the electrical stimulus used to drive vestibular reflex function, i.e., biphasic pulse rate or pulse current amplitude. In order to empirically explore the nature of the transformation between vestibular prosthetic stimulation and vestibular reflex behavior, in Rhesus macaque monkeys we parametrically varied the pulse rate and current amplitude of constant rate and current amplitude pulse trains, and the modulation frequency of sinusoidally modulated pulse trains that were pulse frequency modulated (FM) or current amplitude modulated (AM). In addition, we examined the effects of differential eye position and head position on the observed eye movement responses. We conclude that there is a strong and idiosyncratic, from canal to canal, effect of modulation frequency on the observed eye velocities that are elicited by stimulation. In addition, there is a strong effect of initial eye position and initial head position on the observed responses. These are superimposed on the relationships between pulse frequency or current amplitude and eye velocity that have been shown previously.
Highlights
The semicircular canals transduce head rotation to modulate afferent inputs to the vestibular brainstem and cerebellum
For the determination of the velocity of eye movements elicited by constant pulse frequency and constant current amplitude stimulation initiated from primary eye position (Experiment 1), all animals and all canals were used
In this paper we examined the relationships between slow phase eye movement velocity and electrical stimulation parameters during eye movements elicited by biphasic pulse electrical stimulation with a unilateral vestibular neurostimulator in a range of different contexts
Summary
The semicircular canals transduce head rotation to modulate afferent inputs to the vestibular brainstem and cerebellum This transformation has been modeled as a simple torsion pendulum, which, over a range of frequencies, provides neural representations of head velocity and acceleration to drive a fully compensatory vestibular ocular reflex (VOR), among other behaviors. The relative simplicity of this first approximation model masks an extremely complex set of central and peripheral neural elements and physiological processes which work in combination to perform the job of creating a reliable behavioral response from a range of inputs. These complex mechanisms have been largely elucidated in animal models, and much of this work was performed in rhesus monkeys, which have similar anatomy and behavior to humans. We vary the context of the electrical stimulation by changing the starting eye orbital position and head orientation during stimulation to examine the extent to which the gain of the VOR in monkeys is maintained in response to electrical stimulation in physiologically relevant situations
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