Long-term dance training modifies eye-head coordination in response to passive head impulse.

Long-term dance training is known to improve postural control, especially in challenging postural tasks. However, the effect of dance training on the vestibulo-ocular reflex (VOR) has yet to be properly assessed. This study directly investigated whether VOR parameters are influenced by long-term dance training by testing dancers and controls using the video head impulse test. VOR gains using two of the most common methods (area ratio and instantaneous gains), latency and amplitude of the first saccade, if applicable, were computed. Results revealed a larger VOR gain as measured by area gain and instantaneous gain at 40 ms specifically for left-head impulses, but not right-head impulses. No significant differences in saccade frequency, amplitude, or latency were observed between groups. These differences appear to stem from a modified eye-to-head relationship during high-velocity head impulses in dancers. More specifically, the dancers' eyes lead head movement during passively applied head impulses, which result in higher VOR gain.NEW & NOTEWORTHY This study demonstrates, for the first time, that long-term dance training results in a nonlinear relationship between eye and head velocity within the first milliseconds following passive head impulse. The data also suggest a larger VOR gain in dancers. This finding suggests that dance training may modify eye-head relationship in passive high-frequency head movements. This is of particular interest for vestibular rehabilitation.

Influence of predictability on saccade timing in a head impulse VOR suppression task.

Gaze stabilization performance has been shown to be influenced differently when the head is either passively or actively moved in normal healthy participants. However, for a visual fixation suppression task, it remains unknown if the pattern of coordinated head and eye movement is influenced differently by passive or active head movements. We used a suppression head impulse paradigm (SHIMP), where the subject's goal was to maintain gaze stabilized on a visual target that moved with the head during rapid impulsive head movements, to evaluate gaze fixation performance in three conditions: (1) passive-unpredictable where the examiner applied impulsive head yaw rotations with random timing and direction, (2) passive-predictable where the direction of head rotation was announced and then the examiner repeatedly applied impulses in the same direction, and (3) active where the test subject self-generated their head movements. Thirteen young healthy adults performed all three conditions to assess the percentage of early saccades that initiated the gaze shift toward the final visual target position and the latency of first saccades. Early saccades were defined as those occurring within the duration of the head impulse. Results showed that active head impulses generated the greatest percentage of early saccades, followed by predictable and unpredictable. Among the two passive conditions, predictability shortened the first saccade onset latencies. Active condition onset latencies were shorter than in either of the passive conditions, showing a consistent head-leads-eye pattern defining a specific behavioral pattern that could vary across patient groups leading to insights into central neural mechanisms that control eye-head coordination.

Vestibular function modulates the impact of nGVS on postural control in older adults.

Previous studies have reported an important relationship between increasing age, vestibular impairment, and increased risk of falls. Recently, noisy galvanic vestibular stimulation (nGVS) has been shown to improve postural control in older adults during and potentially following stimulation. However, this effect of nGVS in older adults has not been examined in interaction with the integrity of the vestibular function. We aimed at determining the effect of nGVS on postural control in older adults with and without vestibular impairment and examining the sustained effect of nGVS as compared with a sham stimulation. Thirty-six older adults were assigned to the nGVS group (n = 24) or the sham group (n = 12). In the nGVS group, 12 participants had normal vestibular function and 12 had vestibular impairment. Static postural control was assessed prior to stimulation, during stimulation, and immediately following 30 min of nGVS. Results showed that nGVS induced a significant improvement in sway velocity (P < 0.001) and path length (P < 0.001) compared with sham stimulation. Furthermore, nGVS induced a significantly greater improvement of sway velocity (P < 0.05) and path length (P < 0.05) in older adults with vestibular impairment compared with older adults with normal vestibular function. Improvements in sway velocity (P < 0.001) and path length (P < 0.001) induced by nGVS were sustained immediately following stimulation. These findings suggest that nGVS improves postural control in older adults, and that the effect of nGVS varies depending on the integrity of the vestibular function. Results also show that nGVS effect on postural control, compared with a sham stimulation, can be sustained after the end of stimulation.NEW & NOTEWORTHY The present study is the first study to investigate the impact of vestibular function on the improvement of postural control induced by nGVS in older adults and to compare the improvement of postural control of older adults with and without vestibular impairment. Our results also suggest that nGVS is beneficial for all older adults, and even more for those with a vestibular impairment. Therefore, it could be an approach to reduce falls.

Impact of galvanic vestibular stimulation electrode current density on brain current flow patterns: Does electrode size matter?

BACKGROUND: Galvanic vestibular stimulation (GVS) uses at least one electrode placed on the mastoid process with one or multiple placed over other head areas to stimulate the vestibular system. The exact electrode size used is not given much importance in the literature and has not been reported in several studies. In a previous study, we compared the clinical effects of using different electrode sizes (3 cm2 and 35 cm2) with placebo but with the same injected current, on postural control. We observed significant improvement using the smaller size electrode but not with the bigger size electrode. The goal of this study was to simulate the current flow patterns with the intent to shed light and potentially explain the experimental outcome. METHODS: We used an ultra-high-resolution structural dataset and developed a model to simulate the application of different electrode sizes. We considered current flow in the brain and in the vestibular labyrinth. RESULTS: Our simulation results verified the focality increase using smaller electrodes that we postulated as the main reason for our clinical effect. The use of smaller size electrodes in combination with the montage employed also result in higher induced electric field (E-field) in the brain. CONCLUSIONS: Electrode size and related current density is a critical parameter to characterize any GVS administration as the choice impacts the induced E-field. It is evident that the higher induced E-field likely contributed to the clinical outcome reported in our prior study.

Using Transcranial Electrical Stimulation in Audiological Practice: The Gaps to Be Filled.

The effects of transcranial electrical stimulation (tES) approaches have been widely studied for many decades in the motor field, and are well known to have a significant and consistent impact on the rehabilitation of people with motor deficits. Consequently, it can be asked whether tES could also be an effective tool for targeting and modulating plasticity in the sensory field for therapeutic purposes. Specifically, could potentiating sensitivity at the central level with tES help to compensate for sensory loss? The present review examines evidence of the impact of tES on cortical auditory excitability and its corresponding influence on auditory processing, and in particular on hearing rehabilitation. Overall, data strongly suggest that tES approaches can be an effective tool for modulating auditory plasticity. However, its specific impact on auditory processing requires further investigation before it can be considered for therapeutic purposes. Indeed, while it is clear that electrical stimulation has an effect on cortical excitability and overall auditory abilities, the directionality of these effects is puzzling. The knowledge gaps that will need to be filled are discussed.

The aging-related decrease in cVEMP amplitude cannot be accounted for by normalization.

BACKGROUND: Cervical Vestibular Evoked Myogenic Potentials (cVEMP) is an electromyogenic measure commonly used in clinic to assess saccule function. The main parameters are peak-peak amplitude and interaural asymmetry ratio (IAR). Several non-vestibular factors may influence these parameters. Notably, a greater EMG contraction level prior to stimulation leads to an increased amplitude. As aging impacts both vestibular structures and muscle propreties, it is still a matter of debate whether the decrease in cVEMP amplitude observed in normal aging is due to EMG differences prior to stimulation or to the effect of aging on the sacculo-collic reflex pathway. At the clinical level, understanding the effect of aging on the relationship between EMG activity and cVEMP response (amplitude, asymmetry ratio) and the effect of normalization is crucial to improving the categorization of healthy versus pathological responses. OBJECTIVE: To investigate whether normalization modifies cVEMP amplitude and asymmetry ratios differently in younger and older heatlhy adults. METHOD: cVEMP recordings were conducted in 42 normal healthy participants divided in two age groups: younger (n = 29): mean = 22.79 years old SD = 1.66; and older (n = 13): mean = 69.00 years old SD = 3.61. Air-conducted cVEMP were recorded using Eclipse (Interacoustics, Denmark). The stimulus was a 95 dBnHL tone burst (500 Hz) with rise, plateau and fall time of 1 ms. cVEMP were recorded only when EMG levels were between 50µV and 150µV, using the Eclipse (Interacoustic, Denmark) monitoring system. RESULTS: No significant differences were observed for prestimulus EMG levels between younger and older participants (F(1,83) = 1.13, p = 0.291). However, significant differences between groups were observed for raw cVEMP amplitude (F(1,83) = 14.78; p < 0.001) and corrected cVEMP amplitude (F(1,83) = 21.85; p < 0.0001). A significant positive linear relationship between prestimulus EMG contraction level (RMS) and raw cVEMP amplitude was observed in younger participants (r2 = 0.234; p < 0.001), but not in older adults (r2 = -0.0144; p = 0.056). Finally, no significant differences between younger and older participants were observed for raw amplitude asymmetry ratios (F(1,41) = 0.124, p = 0.726) or normalized asymmetry ratios (F(1,41) = 0.726, p = 0.508). CONCLUSION: Our results suggest that when EMG is monitored and activation of the SCM is sufficient, the observed decline in cVEMP amplitude with normal aging does not seem to be caused by EMG differences and is therefore likely due to the known histopathological modifications of the vestibular system that occurs with normal aging.

Usefulness of Video Head Impulse Test Results in the Identification of Meniere's Disease.

Meniere's disease (MD) is an inner ear disorder inducing tinnitus, aural fullness, sensorineural hearing loss, and vertigo episodes. In the past few years, efforts have been made to develop objective measures able to distinguish MD from other pathologies. Indeed, some authors investigated electrophysiological measures, such as electrocochleography and vestibular evoked myogenic potentials or imaging techniques. More recently, the video head impulse test (vHIT) was developed to assess the vestibulo-ocular reflex (VOR). In the last few years, authors aimed at identifying how vHIT may help to identify MD. The objective of this manuscript is to review the different vHIT results in MD patients. We will discuss the usefulness of these findings in the identification of MD, how these results may be explained by pathophysiological mechanisms associated with MD, and finally provide directions for future studies.

Questioning the lasting effect of galvanic vestibular stimulation on postural control.

Noisy galvanic vestibular stimulation (nGVS) has been shown to enhance postural stability during stimulation, and the enhancing effect has been observed to persist for several hours post-stimulation. However, these effects were observed without proper control (sham condition) and the possibility of experimental bias has not been ruled out. The lasting effect of nGVS on postural stability therefore remains in doubt. We investigated the lasting effect of nGVS on postural stability using a control (sham) condition to confirm or infirm the possibility of experimental bias. 28 participants received either nGVS or a sham stimulation. Static postural control was examined before stimulation, immediately after 30 minutes of nGVS and one-hour post-stimulation. Results showed a significant improvement of sway velocity (p<0.05) and path length (p<0.05) was observed following nGVS, as previously shown. A similar improvement of sway velocity (p<0.05) and path length (p<0.05) was observed in sham group and no significant difference was found between nGVS group and sham group (p>0.05), suggesting that the observed postural improvement in nGVS could be due to a learning effect. This finding suggests the presence of experimental bias in the nGVS effect on postural stability, and highlights the need to use a sham condition in the exploration of the nGVS effect so as to disentangle the direct effect of the electrical stimulation from a learning effect. Furthermore, numerous parameters and populations need to be tested in order to confirm or infirm the presence of a real long-lasting effect of nGVS on postural stability.

State-of-the-art assessment allows for improved vestibular evoked myogenic potential test-retest reliability.

The goal of the present study was to evaluate the test-retest reliability values of myogenic responses using the latest guidelines for vestibular assessment. Twenty-two otologically and neurologically normal adults were assessed twice, on two different days. The analyses were carried out using interclass correlations. The results showed that the latest recommendations for vestibular assessment lead to test-retest reliability values that are as high, or greater, than those reported in previous studies. The results suggest that state-of-the-art testing, using the latest recommendations as well as electromyography control, improves reliability values of myogenic responses, more specifically for the cervical vestibular evoked myogenic potentials. The impact of small differences in experimental procedures on the reliability values of myogenic responses is also addressed.