Superior Canal Dehiscence Syndrome: Relating Clinical Findings With Vestibular Neural Responses From a Guinea Pig Model.

Hypothesis: In superior canal dehiscence (SCD), fluid displacement of the endolymph activates type I vestibular hair cells in the crista of the affected canal and thus irregular superior canal (SC) neurons in Scarpa's ganglion, which provides the neurophysiological basis for the clinical presentation of SCD.

Background: Patients with SCD display sound- and vibration-induced vertigo/nystagmus and increased amplitudes of vestibular evoked myogenic potentials.

Methods: Extracellular recordings from n = 25 primary vestibular neurons of 16 female guinea pigs were analyzed.

Phase-locking of irregular guinea pig primary vestibular afferents to high frequency (>250 Hz) sound and vibration.

Phase-locking of cochlear neurons to sound has been of great value in understanding cochlear transduction. Phase-locking has also been reported previously in irregular vestibular afferents, but detailed information about it is sparse. We measured the phase-locking of guinea pig irregular otolithic neurons and canal neurons (after a semicircular canal dehiscence allowed them to respond) to both sound and vibration stimuli.

The response of guinea pig primary utricular and saccular irregular neurons to bone-conducted vibration (BCV) and air-conducted sound (ACS).

Unlabelled: This study sought to characterize the response of mammalian primary otolithic neurons to sound and vibration by measuring the resting discharge rates, thresholds for increases in firing rate and supra-threshold sensitivity functions of guinea pig single primary utricular and saccular afferents. Neurons with irregular resting discharge were activated in response to bone conducted vibration (BCV) and air conducted sound (ACS) for frequencies between 100 Hz and 3000 Hz. The location of neurons was verified by labelling with neurobiotin.

Cross-species comparison of anticipatory and stimulus-driven neck muscle activity well before saccadic gaze shifts in humans and nonhuman primates.

Recent studies have described a phenomenon wherein the onset of a peripheral visual stimulus elicits short-latency (<100 ms) stimulus-locked recruitment (SLR) of neck muscles in nonhuman primates (NHPs), well before any saccadic gaze shift. The SLR is thought to arise from visual responses within the intermediate layers of the superior colliculus (SCi), hence neck muscle recordings may reflect presaccadic activity within the SCi, even in humans. We obtained bilateral intramuscular recordings from splenius capitis (SPL, an ipsilateral head-turning muscle) from 28 human subjects performing leftward or rightward visually guided eye-head gaze shifts.

Loud preimpact tones reduce the cervical multifidus muscle response during rear-end collisions: a potential method for reducing whiplash injuries.

Background Context: Neck muscle responses after unexpected rear-end collisions consist of a stereotypical combination of postural and startle responses. Prior work using surface electromyography (EMG) has shown that the superficial neck muscle responses can be attenuated when a loud tone (105 dB) is presented 250 milliseconds before impact, but the accompanying response of the deeper multifidus muscles remains unknown. Quantifying this response in multifidus is important because this muscle attaches directly to the cervical facet capsule and can potentially increase the strain in the capsule during an impact and contribute to whiplash injury.

Dynamic and opposing adjustment of movement cancellation and generation in an oculomotor countermanding task.

Adaptive adjustments of strategies help optimize behavior in a dynamic and uncertain world. Previous studies in the countermanding (or stop-signal) paradigm have detailed how reaction times (RTs) change with trial sequence, demonstrating adaptive control of movement generation. Comparatively little is known about the adaptive control of movement cancellation in the countermanding task, mainly because movement cancellation implies the absence of an outcome and estimates of movement cancellation require hundreds of trials.

Validation of a within-trial measure of the oculomotor stop process.

The countermanding (or stop signal) task requires subjects try to withhold a planned movement upon the infrequent presentation of a stop signal. We have previously proposed a within-trial measure of movement cancellation based on neck muscle recruitment during the cancellation of eye-head gaze shifts. Here, we examined such activity after either a bright or dim stop signal, a manipulation known to prolong the stop signal reaction time (SSRT).

Greater benefits of multisensory integration during complex sensorimotor transformations.

Multisensory integration enables rapid and accurate behavior. To orient in space, sensory information registered initially in different reference frames has to be integrated with the current postural information to produce an appropriate motor response. In some postures, multisensory integration requires convergence of sensory evidence across hemispheres, which would presumably lessen or hinder integration.

Ultrasound-guided insertion of intramuscular electrodes into suboccipital muscles in the non-human primate.

The head-neck system is highly complex from a biomechanical and musculoskeletal perspective. Currently, the options for recording the recruitment of deep neck muscles in experimental animals are limited to chronic approaches requiring permanent implantation of electromyographic electrodes. Here, we describe a method for targeting deep muscles of the dorsal neck in non-human primates with intramuscular electrodes that are inserted acutely.

Neck muscle responses evoked by transcranial magnetic stimulation of the human frontal eye fields.

Transcranial magnetic stimulation (TMS) provides a non-invasive means of investigating brain function. Whereas TMS of the human frontal eye fields (FEFs) does not induce saccades, electrical stimulation of the monkey FEF evokes eye-head gaze shifts, with neck muscle responses evoked at stimulation levels insufficient to evoke a saccade. These animal results motivated us to examine whether TMS of the FEF (TMS-FEF) in humans evokes a neck muscle response.

A within-trial measure of the stop signal reaction time in a head-unrestrained oculomotor countermanding task.

The countermanding (or stop-signal) task, which requires the cancellation of an impending response on the infrequent presentation of a stop signal, enables study of the contextual control of movement generation and suppression. Here we present a novel and empirical measure of the time needed to cancel an impending gaze shift by recording neck muscle activity during a head-unrestrained oculomotor countermanding paradigm. On a subset of stop signal trials, subjects generated small head movements toward a target even though gaze remained stable due to a compensatory vestibular-ocular reflex.

On the relation between ocular torsion and visual perception of line orientation.

A recent study by Poljac et al. [Poljac, E., Lankheet, M.

Cognitive demand affects the gain of the torsional optokinetic response.

Cognitive tasks such as mental arithmetic and fixation of imagined targets are known to affect vestibular nystagmus. Here we show that another cognitive task-subject's active control of the rotation of a single moving visual line in an otherwise darkened room-influences the gain of the torsional optokinetic response to that single moving visual line.

Changes in ocular torsion position produced by a single visual line rotating around the line of sight--visual "entrainment" of ocular torsion.

A large- or full-field visual stimulus slowly rotating around the naso-occipital axis of an observer causes both eyes to tort, and many of the factors controlling this optokinetic torsional response have been identified. The present study reports that a single line rotating about the line of sight can cause both eyes to tort in the same direction as the stimulus but with a low gain. We have used the term 'entrainment' to describe this torsional response.