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• Transcranial oscillatory currents may stimulate the vestibular cortex in the range of its physiological activity (1 or 2 Hz) • Induced behavioral effects (motion sickness and postural sways) are not due to a peripheral spread of the current towards vestibular receptors. • Motion sickness represents a previously neglected possible side effect of slow-frequency tACS. The vestibular cortex is a multisensory associative region that, in neuroimaging investigations, is activated by slow-frequency (1–2 Hz) galvanic stimulation of peripheral receptors. We aimed to directly activate the vestibular cortex with biophysically modeled transcranial oscillatory current stimulation (tACS) in the same frequency range. Thirty healthy subjects and one rare patient with chronic bilateral vestibular deafferentation underwent, in a randomized, double-blind, controlled trial, to tACS at slow (1 or 2 Hz) or higher (10 Hz) frequency and sham stimulations, over the Parieto-Insular Vestibular Cortex (PIVC), while standing on a stabilometric platform. Subjective symptoms of motion sickness were scored by Simulator Sickness Questionnaire and subjects' postural sways were monitored on the platform. tACS at 1 and 2 Hz induced symptoms of motion sickness, oscillopsia and postural instability, that were supported by posturographic sway recordings. Both 10 Hz-tACS and sham stimulation on the vestibular cortex did not affect vestibular function. As these effects persisted in a rare patient with bilateral peripheral vestibular areflexia documented by the absence of the Vestibular-Ocular Reflex, the possibility of a current spread toward peripheral afferents is unlikely. Conversely, the 10 Hz-tACS significantly reduced his chronic vestibular symptoms in this patient. Weak electrical oscillations in a frequency range corresponding to the physiological cortical activity of the vestibular system may generate motion sickness and postural sways, both in healthy subjects and in the case of bilateral vestibular deafferentation. This should be taken into account as a new side effect of tACS in future studies addressing cognitive functions. Higher frequencies of stimulation applied to the vestibular cortex may represent a new interventional option to reduce motion sickness in different scenarios. [ABSTRACT FROM AUTHOR]

• In unoperated vestibular schwannoma, ipsilesional canals show decreased vestibulo-ocular reflex (VOR) gain and increased saccade amplitude and frequency. • One week after unilateral vestibular deafferentation lesioned canal gains drop by 0.22–0.35 and first saccade frequency and amplitudes increase by 37% and 2.9°. • The video head-impulse test excitation/dis-facilitation VOR asymmetry is less for the intact posterior canal than for the other canals. To examine the vestibulo-ocular reflex (VOR) and compensatory-saccades before and after complete unilateral vestibular deafferentation (UVD). Forty patients were studied before and after surgery for vestibular or facial schwannoma using the video head-impulse test (vHIT) and multivariable regression. Prior to UVD (median(IQR), 14(58.4) days), the average VOR-gain towards the lesioned -ear was lower than in normal for all semicircular canals (lateral, anterior, posterior: 0.69, 0.72, 0.49). One-week after UVD (5(3.0) days) VOR gains were further reduced (0.22, 0.37, 0.27), however, within one-year after UVD (171(125.0) days) the lesioned- ear VOR gains had slightly increased (+0.08, +0.11, +0.03), maximally for the anterior-canal. After UVD, the VOR gain asymmetry (gain towards minus away from intact-ear) was lower for the intact posterior-canal plane (0.56, 0.56, 0.22). For the lesioned canals, the frequency and amplitude of the first compensatory-saccade increased from 61–93% and 1.9–3.6° pre-surgery, to 98–99% and to 3.1–5.9° one-week post-surgery and remained unchanged over one-year; second saccade frequency and amplitude decreased over the same timespan. After UVD the high-acceleration VOR for the intact posterior-canal plane is more symmetrical than the other canals. First compensatory-saccades adapt within one week and subsequently change only marginally. Saccade compensation from surgical UVD is near complete by one-week. [ABSTRACT FROM AUTHOR]

• Video Head Impulse tests of the vestibulo-ocular reflex (VOR) provided the best separation of vestibular neuritis (VN) from posterior circulation stroke (PCS). • Refixation saccade-analysis complemented VOR gain measures in differentiating VN from PCS. • Ocular VEMP (oVEMP) asymmetry demonstrated low sensitivity but high specificity as a marker of VN. To separate vestibular neuritis (VN) from posteriorcirculation stroke (PCS) using quantitative tests of canal and otolith function. Video Head-Impulse tests (vHIT) were used to assess all three semicircular canal pairs; vestibulo-ocular reflex (VOR) gain and saccade metrics were examined. Cervical and ocular-Vestibular-Evoked Myogenic Potentials (c- and oVEMP) and Subjective Visual Horizontal (SVH) were used to assess otolith function. For controls (n = 40), PCS (n = 22), and VN (n = 22), mean horizontal-canal VOR-gains were 0.96 ± 0.1, 0.85 ± 0.3 and 0.40 ± 0.2, refixation-saccade prevalence was 71.9 ± 41, 90.7 ± 57, 209.2 ± 62 per 100 impulses and cumulative-saccade amplitudes were 0.9 ± 0.4°, 2.4 ± 2.2°, 8.0 ± 3.5°. Abnormality-rates for cVEMP, oVEMP and SVH were 38%, 9%, 72% for PCS, and 43%, 50%, 91% for VN. A gain ≤0.68, refixation-saccade prevalence of ≥135% and cumulative-saccade amplitudes ≥5.3° separated VN from PCS with sensitivities of 95.5%, 95.5%, and 81.8%, and specificities of 68.2%, 86.4% and 95.5%. VOR-gain and saccade prevalence when combined, separated VN from PCS with a sensitivity and specificity of 90.9%. Abnormal oVEMP asymmetry-ratios were of low sensitivity (50%) but high specificity (90.9%) for separating VN from PCS. vHIT provided the best separation of VN from PCS. VOR-gain, refixation-saccade prevalence and amplitude were effective discriminators of VN from PCS. vHIT and oVEMP could assist early identification of the aetiology of Acute Vestibular Syndrome in the Emergency Room. [ABSTRACT FROM AUTHOR]

Abstract: Objective: This study utilized bone-conducted vibration (BCV) stimuli for eliciting ocular vestibular-evoked myogenic potentials (oVEMPs) to assess the vestibulo-ocular reflex (VOR) system in healthy children and adults. Methods: Fifteen healthy children aged 3–14years and 18 healthy adults aged 24–28years underwent oVEMP testing. Structural factors such as body weight, body height and body mass index were measured for each healthy subject. Results: All healthy children and adults presented clear oVEMPs, bilaterally. The characteristic parameters (latencies and amplitude) of oVEMPs did not significantly differ between children and adults. The mean interaural (y-axis) acceleration magnitudes for eliciting oVEMPs were 0.37±0.12g for children and 0.41±0.20g for adults, a non-significant difference. As stimulation intensity increased stepwise, interaural acceleration magnitude increased correspondingly, leading to early nI latency and large nI–pI amplitude of oVEMPs. However, no structural factor was statistically correlated with interaural acceleration magnitude. Conclusion: When children aged >3years, the simple and quick oVEMP test via BCV stimulation can be used for investigating the integrity of the VOR system, with the characteristic parameters (latencies and amplitude) unaffected by structural factors. Significance: Establishing the norm of oVEMP is essential for diagnosing VOR deficit in children aged >3years. [Copyright &y& Elsevier]

Abstract: Objective: To assess the effects of vertical and horizontal gaze, head rotation, body position, and vision on the ocular vestibular evoked myogenic potential (OVEMP) produced by air-conducted (AC) sound. Methods: Ten normal subjects were stimulated by 500Hz 2ms AC tone bursts at 136–142dB peak SPL. OVEMPs were recorded from electrodes placed beneath the eyes. Angles of vertical gaze ranged from maximal downward to upward gaze in increments of 5–10°. Horizontal gaze was measured during elevation and ranged from 20° adduction to 20° abduction. Results: Increasing vertical gaze increased OVEMP amplitude, especially for the contralateral eye (neutral vs maximal upward gaze; contra: 1.0 vs 2.6μV; ipsi: 0.8 vs 0.9μV; P <0.001). OVEMPs from the contralateral eye peaked significantly earlier in the upward gaze positions (contra: 9.2ms; ipsi: 10.4ms; P <0.001), but peaked later during downward gaze (contra: 14.2ms; ipsi: 11.4ms; P =0.014). There were small effects of horizontal gaze and supine body position, but no effects of head rotation or vision. Conclusions: OVEMP amplitudes are strongly modulated by gaze position. Truncal position also affects OVEMP amplitude. Significance: This study quantifies the effect of gaze on the OVEMP and demonstrates the importance of controlling for gaze in clinical and experimental studies. [Copyright &y& Elsevier]

Abstract: Objective: Synchronous extraocular muscle activity can be recorded from around the eyes at the beginning of a vestibular-evoked eye movement (ocular vestibular evoked myogenic potentials, OVEMPs). As galvanic vestibular stimulation (GVS) evokes the vestibulo-ocular reflex, we wished to investigate GVS-evoked OVEMPs. Methods: We stimulated 10 normals and 6 patients with unilateral vestibular loss (uVL) with bi/unipolar 4mA, 2ms current steps at the mastoid. OVEMPs were recorded from electrodes placed superior and inferior to the eyes. Results: OVEMPs were present beneath both eyes in all normal subjects: an initial positivity ipsilateral to the cathodal electrode (peak latency 9.9ms, amplitude 1.3μV) and an initial negativity contralateral to the cathode (8.8ms, 2.4μV). In the patients, stimulation of the affected side produced little or no response. Stimulation of the intact side produced only contralateral responses. Conclusions: The infra-orbital response is likely produced primarily by the inferior obliques, producing conjugate torsion away from the cathode. The projection to the ipsilateral eye depends upon normal vestibular function on the contralateral side. Significance: OVEMPs can be evoked by GVS. While bilateral effects are obtained with unilateral stimulation in normals, the primary vestibular pathway to the inferior oblique in humans is crossed. [Copyright &y& Elsevier]