Vestibulocollic Reflex Research Articles

Vestibular sensitivity in vestibular migraine: VEMPs and motion sickness susceptibility

Vestibular migraine (VM) has gained recognition as a distinct clinical entity in recent years. The pathophysiology is unclear. Vestibular evoked myogenic potential (VEMP) is a validated method to test the vestibulocollic reflex and peripheral vestibular hypersensitivity to noise. The aim of our study was to evaluate the vestibular function and sensitivity in a series of VM patients in comparison to ordinary migraine (M) patient and healthy controls. Thirty-seven patients diagnosed with VM according to the Neuhauser criteria, 32 migraineurs, and 30 healthy persons underwent VEMP testing (response and sound intensity threshold) and a motion sickness susceptibility questionnaire. We found absence of unilateral or bilateral VEMP response at 90 dB normal hearing level (nHL) in 44% of the VM patients, in 25% of the migraineurs, as compared to in 3% of the healthy controls (p = 0.001). The sound intensity threshold and latencies were similar in all the groups. Migraineurs (VM > M) reported more motion sickness than healthy controls (p = 0.006). The results indicate more pathology in the VEMP circuitry in migraineurs than in healthy controls. We did not find support for peripheral vestibular hypersensitivity in terms of lower VEMP threshold among VM patients, but they are more sensitive for motion triggers than other migraineurs.

F010 Functional status of hearing aids in bimodal (CI-HA) users

Short tone bursts trigger a vestibular evoked myogenic potential (VEMP), an inhibitory potential which reflects a component of the vestibulocollic reflex (VCR). These potentials arise as a result of activation of the sacculus and are expressed through the vestibulo-collic reflex (VCR). Up to now, the ascending projections of the sacculus are unknown in humans, only the representation of the semicircular canals or the entire vestibular nerve has been demonstrated. The aim of this study was to determine whether a sacculus stimulus that evoked VEMPs could activate vestibular cortical areas in fMRI.To determine this, we studied the differential effects of unilateral VEMP stimulation in 21 healthy right-handers in a clinical 1.5 T scanner while wearing piezo electric headphones. A unilateral VEMP stimulus and two auditory control stimuli were given in randomized order over the stimulated ear. A random effects statistical analysis was done with SPM2 (p < 0.05, corrected). After exclusion of the auditory effects, the major findings were as follows: (i) significant activations were located in the multisensory cortical vestibular network within both hemispheres, including the posterior insular cortex, the middle and superior temporal gyri, and the inferior parietal cortex. (ii) The activation pattern was elicited bilaterally with a predominance of the right hemisphere in right-handers. (iii) Saccular vestibular projection was predominantly ipsilateral, whereas (iv) pure acoustic stimuli were processed with a predominance of the respective contralateral and mainly in the left hemisphere.This is the first demonstration by means of fMRI of the cortical representation of the saccular input at cortical level. The activation pattern is similar to that known from the stimulation of the entire vestibular nerve or the horizontal semicircular canal. Our data give evidence of a task-dependent separation of the processing within the vestibular otolith and the auditory systems in the two hemispheres.

Ataxia telangiectasia: a “disease model” to understand the cerebellar control of vestibular reflexes

Experimental animal models have suggested that the modulation of the amplitude and direction of vestibular reflexes are important functions of the vestibulocerebellum and contribute to the control of gaze and balance. These critical vestibular functions have been infrequently quantified in human cerebellar disease. In 13 subjects with ataxia telangiectasia (A-T), a disease associated with profound cerebellar cortical degeneration, we found abnormalities of several key vestibular reflexes. The vestibuloocular reflex (VOR) was measured by eye movement responses to changes in head rotation. The vestibulocollic reflex (VCR) was assessed with cervical vestibular-evoked myogenic potentials (cVEMPs), in which auditory clicks led to electromyographic activity of the sternocleidomastoid muscle. The VOR gain (eye velocity/head velocity) was increased in all subjects with A-T. An increase of the VCR, paralleling that of the VOR, was indirectly suggested by an increase in cVEMP amplitude. In A-T subjects, alignment of the axis of eye rotation was not with that of head rotation. Subjects with A-T thus manifested VOR cross-coupling, abnormal eye movements directed along axes orthogonal to that of head rotation. Degeneration of the Purkinje neurons in the vestibulocerebellum probably underlie these deficits. This study offers insights into how the vestibulocerebellum functions in healthy humans. It may also be of value to the design of treatment trials as a surrogate biomarker of cerebellar function that does not require controlling for motivation or occult changes in motor strategy on the part of experimental subjects.

Vestibular control of the head: possible functions of the vestibulocollic reflex

Here, we review the angular vestibulocollic reflex (VCR) focusing on its function during unexpected and voluntary head movements. Theoretically, the VCR could (1) stabilize the head in space during body movements and/or (2) dampen head oscillations that could occur as a result of the head's underdamped mechanics. The reflex appears unaffected when the simplest, trisynaptic VCR pathways are severed. The VCR's efficacy varies across species; in humans and monkeys, head stabilization is ineffective during low-frequency body movements in the yaw plan. While the appearance of head oscillations after the attenuation of semicircular canal function suggests a role in damping, this interpretation is complicated by defects in the vestibular input to other descending motor pathways such as gaze premotor circuits. Since the VCR should oppose head movements, it has been proposed that the reflex is suppressed during voluntary head motion. Consistent with this idea, vestibular-only (VO) neurons, which are possible vestibulocollic neurons, respond vigorously to passive, but not active, head rotations. Although VO neurons project to the spinal cord, their contribution to the VCR remains to be established. VCR cancelation during active head movements could be accomplished by an efference copy signal negating afferent activity related to active motion. Oscillations occurring during active motion could be eliminated by some combination of reflex actions and voluntary motor commands that take into account the head's biomechanics. A direct demonstration of the status of the VCR during active head movements is required to clarify the function of the reflex.

Anatomical observations of the caudal vestibulo-sympathetic pathway.

The vestibular system senses the movement and position of the head in space and uses this information to stabilize vision, control posture, perceive head orientation and self-motion in three-dimensional space, and modulate autonomic and limbic activity in response to locomotion and changes in posture. Most vestibular signals are not consciously perceived and are usually appreciated through effector pathways classically described as the vestibulo-ocular, vestibulo-spinal, vestibulo-collic and vestibulo-autonomic reflexes. The present study reviews some of the recent data concerning the connectivity and chemical anatomy of vestibular projections to autonomic sites that are important in the sympathetic control of blood pressure.

Technical pitfalls of vestibular-evoked myogenic potentials

In humans, intense sound evokes ipsilateral myogenic potentials in the sternocleidomastoid muscle (SCM) with a short latency positive-negative (p13-n23) waveform. This sound-evoked vestibulocollic reflex is termed the vestibular-evoked myogenic potential (VEMP). In the present study, the technical and diagnostic pitfalls of the VEMP are introduced.An electromyograph (EMG) is used to record signals from a pair of surface electrodes mounted on the upper-half (active electrode) and the sternal head (reference electrode) of the SCM. The positions of the electrodes influence the p13-n23 amplitude and the VEMP waveform. Head rotation around the yaw axis brings the proximal end of the unilateral SCM muscle close to the distal end, making the outline of the SCM muscle more visible on the body surface. This rotation is useful for the precise positioning of the electrodes.The muscle background activity during the test influences the p13-n23 amplitude of the VEMP. Subjects are instructed to rotate their heads around the yaw axis contralateral to the sound stimulation so as to contract their unilateral SCM muscles. During the VEMP test, the background EMG activity of the tested SCM is continuously monitored, and the peak-to-peak amplitude in the interference pattern of the background EMG should be maintained at more than 200 μV. When the attenuation of the amplitude in the interference pattern begins, the subjects should be instructed to rotate their heads once again so as to recover tonic contraction.The muscle background activity during the test is electromyographically estimated based on the integral value of the rectified EMG activity level for 20 milliseconds preceding the stimulus onset. When the integral value of the rectified EMG activity is less than 1000 μV×ms, the background EMG activity of the tested SCM is objectively evaluated as being too low to induce the evoked response.To correct the amplitude according to the muscle background activity, the p13-n23 amplitude is divided by the integral value of the rectified EMG activity level. This correction is important for the test-retest reproductivity of the VEMP amplitude. To diagnose a right-left asymmetry in the VEMP amplitude, the clinical use of the corrected amplitude is recommended.

Melatonin attenuates the vestibulosympathetic but not vestibulocollic reflexes in humans: selective impairment of the utricles

Melatonin has been reported to decrease nerve activity of medial vestibular nuclei in the rat and is associated with attenuated muscle sympathetic nerve activity (MSNA) responses to baroreceptor unloading in humans. The purpose of this study was to determine if melatonin alters the vestibulosympathetic reflex (VSR) and vestibulocollic reflex (VCR) in humans. In study 1, MSNA, arterial blood pressure, and heart rate were measured in 12 healthy subjects (28 ± 1 yr; 6 men, 6 women) during head-down rotation (HDR) before and 45 min after ingestion of either melatonin (3 mg) or placebo (sucrose). Subjects returned at least 2 days later at the same time of day to repeat the trial after ingesting the opposite treatment (melatonin or placebo). Melatonin significantly attenuated MSNA responses during HDR compared with placebo (burst frequency Δ 4 ± 1 vs. Δ 7 ± 1 bursts/min, and total MSNA Δ 51 ± 20 and Δ 96 ± 15%, respectively; P < 0.02). In study 2, vestibular evoked myogenic potentials (VEMP) were measured in 10 healthy subjects (26 ± 1 yr; 4 men and 6 women) before and after ingestion of 3 mg melatonin. Melatonin did not alter the timing of the p13 and n23 peaks (pre-melatonin 13.2 ± 0.4 and 21.3 ± 0.6 ms vs. post-melatonin 13.5 ± 0.4 and 21.4 ± 0.7 ms, respectively) or the p13-n23 interpeak amplitudes [pre-melatonin 22.5 ± 4.6 arbitrary units (au) and post-melatonin 22.7 ± 4.6 au]. In summary, melatonin attenuates the VSR and supports the concept that melatonin negatively affects orthostatic tolerance. However, melatonin does not alter the VCR in humans suggesting melatonin's effect on the VSR appears to be mediated by the utricles.

Clicking the eye muscles?

Peripheral vestibular function used to be tested by caloric irrigation of the external auditory canals. Functional testing significantly improved, however, when Halmagyi and Curthoys1 devised the head-impulse test for swift and reliable bedside testing of the vestibulo-ocular reflex. Malfunction of a single semicircular canal, or more than one, can be detected by head movements in the plane of the individual semicircular canal. Vestibular-evoked potential testing became part of neurootology assessments in the early 1990s. Brief pulses of sound (clicks) can be used to record short-latency vestibular-evoked myogenic potentials from the sternocleidomastoid muscles (cVEMP), which are part of the vestibulo-collic reflex. The cVEMPs originate in the saccular macula and travel via a trisynaptic pathway through the inferior division of the vestibular nerve, the vestibular nucleus, and the medial vestibulo-spinal tract to the spinal accessory nerve.2 The diagnostic value for their clinical application is mainly based on the selective evaluation of saccular function. In vestibular neuritis preserved cVEMPs in two-thirds of the patients show sparing of the inferior part of the vestibular nerve, which …

Eye–head coordination in the guinea pig I. Responses to passive whole-body rotations

Vestibular reflexes act to stabilize the head and eyes in space during locomotion. Head stability is essential for postural control, whereas retinal image stability enhances visual acuity and may be essential for an animal to distinguish self-motion from that of an object in the environment. Guinea pig eye and head movements were measured during passive whole-body rotation in order to assess the efficacy of vestibular reflexes. The vestibulo-ocular reflex (VOR) produced compensatory eye movements with a latency of approximately 7 ms that compensated for 46% of head movement in the dark and only slightly more in the light (54%). Head movements, in response to abrupt body rotations, also contributed to retinal stability (21% in the dark; 25% in the light) but exhibited significant variability. Although compensatory eye velocity produced by the VOR was well correlated with head-in-space velocity, compensatory head-on-body speed and direction were variable and poorly correlated with body speed. The compensatory head movements appeared to be determined by passive biomechanical (e.g., inertial effects, initial tonus) and active mechanisms (the vestibulo-collic reflex or VCR). Chemically induced, bilateral lesions of the peripheral vestibular system abolished both compensatory head and eye movement responses.

Otolith stimulation induces c-Fos expression in vestibular and precerebellar nuclei in cats and squirrel monkeys

Vestibular information is critical for the control of balance, posture, and eye movements. Signals from the receptors, the semicircular canals and otoliths, are carried by the eighth nerve and distributed to the four nuclei of the vestibular nuclear complex, the VNC. However, anatomical and physiological data suggest that many additional brainstem nuclei are engaged in the processing of vestibular signals and generation of motor responses. To assess the role of these structures in vestibular functions, we have used the expression of the immediate early gene c-Fos as a marker for neurons activated by stimulation of the otoliths or the semicircular canals. Excitation of the otolith organs resulted in widespread c-Fos expression in the VNC, but also in other nuclei, including the external cuneate nucleus, the postpyramidal nucleus of the raphé, the nucleus prepositus hypoglossi, the subtrigeminal nucleus, the pontine nuclei, the dorsal tegmental nucleus, the locus coeruleus, and the reticular formation. Rotations that excited the semicircular canals were much less effective in inducing c-Fos expression. The large number of brainstem nuclei that showed c-Fos expression may reflect the multiple functions of the vestibular system. Some of these neurons may be involved in sensory processing of the vestibular signals, while others provide input to the vestibulo-ocular, vestibulocollic, and vestibulospinal reflexes or mediate changes in autonomic function. The data show that otolith stimulation engages brainstem structures both within and outside of the VNC, many of which project to the cerebellum.

Presynaptic and postsynaptic ion channel expression in vestibular nuclei neurons after unilateral vestibular deafferentation

Vestibular compensation refers to the recovery of function occurring after unilateral vestibular deafferentation, but some patients remain uncompensated. Similarly, more than half of the operated chickens compensate three days after unilateral vestibular ganglionectomy (UVG), but the rest remain uncompensated. This review focuses on the studies performed on the principal cells of the chick tangential nucleus after UVG. The tangential nucleus is a major avian vestibular nucleus whose principal cells are all second-order, vestibular reflex projection neurons participating in the vestibuloocular and vestibulocollic reflexes controlling posture, balance, and eye movements. Using whole-cell patch-clamp approach in brain slice preparations, spontaneous spike firing, ionic conductances, and spontaneous excitatory postsynaptic currents (sEPSCs) are recorded in principal cells from controls and operated chickens three days after UVG. In compensated chickens, the proportion of spontaneous spike firing principal cells and their spike discharge rate are symmetric on the lesion and intact sides, with the rates increased over controls. However, in the uncompensated chickens, the spike discharge rate increases on the lesion side, but not on the intact side, where only silent principal cells are recorded. In all the experimental groups, including controls, silent principal cells are distinguished from spontaneous spiking cells by smaller persistent sodium conductances and higher activation thresholds for the fast sodium channel. In addition, silent principal cells on the intact side of uncompensated chickens have larger dendrotoxin-sensitive potassium conductances, with a higher ratio of immunolabeling for surface/cytoplasmic expression of a dendrotoxin-sensitive, potassium channel subunit, Kv1.1. Finally, in compensated chickens, sEPSC frequency is symmetric bilaterally, but in uncompensated chickens sEPSC frequency increased only on the lesion side, where the expression of Kv1.2 decreased in synaptotagmin-labeled terminal profiles on the principal cell bodies. Altogether, the specific sodium and potassium channels important for the development of spike firing pattern and/or presynaptic glutamate release on vestibular reflex projection neurons may be critically involved in changing postsynaptic neuron excitability after vestibular deafferentation.

OVEMP to air-conducted tones reflects functions of different vestibular populations from cVEMP?

Vestibular evoked myogenic potentials recorded on the sternocleidomastoid muscle (SCM) have been called cVEMP (cervical VEMP) and have drawn the attention of clinicians, because cVEMPs can reflect functions of the eighth cranial nerve that differ from those shown by caloric tests and auditory brainstem responses (ABRs), predominantly saccular functions [1, 2]. Recently, myogenic potentials around the eyes have been studied, having also been regarded as of vestibular origin [3, 4]. These are called ocular vestibular evoked myogenic potentials (oVEMPs). While Iwasaki et al. [5] have proposed that oVEMPs to bone conducted vibration could be of utricular origin, the origin of oVEMPs to air-conducted sound remains unknown. Herein, we present a patient in whom oVEMPs in response to air-conducted sound might provide suggestions concerning the origin. A 52-year-old woman consulted our clinic with a complaint of right tinnitus. The tympanic membranes were normal and pure tone hearing was also normal on both sides. ABRs (air-conducted 80 dB nHL clicks) showed prolongation of the interpeak interval between waves 1 and 5 to the right ear stimulation. She did not show gaze nystagmus or positional nystagmus. Caloric tests to cold water were normal bilaterally. Vestibular evoked myogenic potentials on the SCM (cVEMPs) in response to air-conducted 500 Hz short tone bursts (STB, rise/fall time = 1 ms, plateau time = 2 ms, 95 dB nHL) were normal (Fig. 1). Then, ocular VEMPs (oVEMPs) were measured. The methods of recording of oVEMPs were almost the same as those in previously reported except for bandpass-filter [6]. The electrodes were placed just beneath the lower eye lids (active) and 2 cm below the active electrodes (indifferent). Stimuli were presented through the headphone. The stimulation rate was 5 Hz. During recording, the subject was asked to maintain an upward-gaze. The signals were bandpass-filtered (20–2,000 Hz) and 100 responses were averaged. In this patient, recording conditions for oVEMP were the same as those for cVEMP except for the sites of electrodes. According to the previous report [4], recordings under the eye contralateral to the stimulated ear were assessed. Then, oVEMPs in response to right ear stimulation were absent (Fig. 1). Finally, MRI demonstrated right vestibular schwannoma (VS) (20 mm in diameter) (Fig. 1). In this particular patient, oVEMPs in response to airconducted 500 Hz STB were absent on the VS affected side, while cVEMPs using the same stimuli were normal. How can these findings be explained? One of the simplest explanations may be a difference in the contribution rate of utricular afferents between vestibulo-collic reflexes and vestibulo-ocular reflexes. Concerning the vestibulo-ocular reflexes, utricular afferents could provide a greater contribution than saccular afferents [7]. Then, dysfunctions of utricular afferents affected oVEMPs while they did not affect cVEMP because of the rich contribution of saccular afferents to cVEMPs. To justify this hypothesis, the responsiveness of utricular afferents to air-conducted 500 Hz STB must be proven. Currently, there is no clear evidence. However, at the early stage of VEMP study, T. Murofushi (&) K. Wakayama Department of Otolaryngology, Teikyo University School of Medicine Mizonokuchi Hospital, 3-8-3 Mizonokuchi, Takatsu-ku, Kawasaki 213-8507, Japan e-mail: murofush@med.teikyo-u.ac.jp

Effect of melatonin on the vestibulocollic reflex in humans

Melatonin has been reported to decrease nerve activity of the medial vestibular nuclei in rats. The purpose of the present study was to determine if melatonin alters the vestibulocollic reflex (VCR) in humans. Based on our previous findings with melatonin's affect on the vestibulosympathetic reflex (VSR), we hypothesized that exogenous melatonin ingestion would attenuate the VCR. To test this hypothesis, vestibular evoked myogenic potentials (VEMP) were recorded in 10 healthy subjects (26 ± 1 yr; 4 male, 6 female) before and 45 min after ingestion of 3 mg melatonin. Additionally, we recorded VEMP in 8 healthy subjects (25 ± 1 yr; 1 male, 7 female) who underwent the same experiments but without melatonin to serve as time controls. Melatonin did not alter the timing of the first VEMP peak (13 ± 0.4 and 14 ± 0.3 ms, pre and post melatonin, respectively) nor the second VEMP peak (21 ± 0.6 and 21 ± 0.7 ms, pre and post melatonin, respectively). More importantly, VEMP interpeak amplitude was not altered by melatonin (before: 22 ± 5 vs. after: 23 ± 5 a.u.). Comparable results were observed in the control group (before: 26 ± 6 vs. after: 26 ± 7 a.u.) These data indicate that melatonin does not alter the VCR in humans. In conjunction with our previous findings demonstrating an attenuated VSR with exogenous melatonin, the present findings suggest that melatonin's affect on the VCR and VSR are different in humans.NIH HL077670, NASA Space Fellowship

Distribution of vestibulospinal contacts on the dendrites of ipsilateral splenius motoneurons: An anatomical substrate for push-pull interactions during vestibulocollic reflexes

Excitatory and inhibitory synapses may control neuronal output through a push-pull mechanism—that is, increases in excitation are coupled to simultaneous decreases in inhibition or vice versa. This pattern of activity is characteristic of excitatory and inhibitory vestibulospinal axons that mediate vestibulocollic reflexes. Previously, we showed that medial vestibulospinal tract (MVST) neurons in the rostral descending vestibular nucleus (DVN), an excitatory pathway, primarily innervate the medial dendrites of contralateral splenius motoneurons. In the present study, we tested the hypothesis that the counterparts of the push-pull mechanism, the ipsilateral inhibitory MVST synapses, are distributed on the dendritic tree such that the interactions with excitatory MVST synapses are enhanced. We combined anterograde tracing and intracellular staining in adult felines and show that most contacts (∼ 70%) between inhibitory MVST neurons in the rostral DVN and ipsilateral splenius motoneurons are also located on medial dendrites. There was a weak bias towards proximal dendrites. Using computational methods, we further show that the organization of excitatory and inhibitory MVST synapses on splenius motoneurons increases their likelihood for interaction. We found that if either excitatory or inhibitory MVST synapses were uniformly distributed throughout the dendritic tree, the proportion of inhibitory contacts in close proximity to excitatory contacts decreased. Thus, the compartmentalized distribution of excitatory and inhibitory MVST synapses on splenius motoneurons may be specifically designed to enhance their interactions during vestibulocollic reflexes. This suggests that the push-pull modulation of motoneuron output is based, in part, on the spatial arrangement of synapses on the dendritic tree.

Vestibular evoked myogenic potentials: Past, present and future

Since the first description of sound-evoked short-latency myogenic reflexes recorded from neck muscles, vestibular evoked myogenic potentials (VEMPs) have become an important part of the neuro-otological test battery. VEMPs provide a means of assessing otolith function: stimulation of the vestibular system with air-conducted sound activates predominantly saccular afferents, while bone-conducted vibration activates a combination of saccular and utricular afferents. The conventional method for recording the VEMP involves measuring electromyographic (EMG) activity from surface electrodes placed over the tonically-activated sternocleidomastoid (SCM) muscles. The "cervical VEMP" (cVEMP) is thus a manifestation of the vestibulo-collic reflex. However, recent research has shown that VEMPs can also be recorded from the extraocular muscles using surface electrodes placed near the eyes. These "ocular VEMPs" (oVEMPs) are a manifestation of the vestibulo-ocular reflex. Here we describe the historical development and neurophysiological properties of the cVEMP and oVEMP and provide recommendations for recording both reflexes. While the cVEMP has documented diagnostic utility in many disorders affecting vestibular function, relatively little is known as yet about the clinical value of the oVEMP. We therefore outline the known cVEMP and oVEMP characteristics in common central and peripheral disorders encountered in neuro-otology clinics.

Characteristics of vestibular evoked myogenic potentials as an otolith function test

In humans, a positive followed by a negative waveform with short latency is reproducibly evoked in the sternocleidomastoid (SCM) muscle in response to intense sound. This sound-evoked vestibulocollic reflex is termed the vestibular evoked myogenic potential (VEMP). We investigated the characteristics of the VEMP as an otolith function test. The human VEMP has its best frequency located between 500 Hz and 1 kHz that resembles a V-shaped tuning curve of the saccular nerve in cats, recorded in the inferior vestibular nerve. The amplitude of the human VEMP was enhanced under microgravity during parabolic flight and was attenuated by the rostro-caudal linear acceleration on Earth. These findings suggest that the human VEMP is an otolith function test and originates from the saccule. After vestibular deafferentiation, unilateral VEMP dysfunction in the affected side appears and lasts for a long time. In patients with inner ear and retrolabyrinthine disorders, abnormality of the VEMP was not always coincident with that of the caloric test. These findings suggested that the VEMP, like the caloric test, is another important tool for the diagnosis of the affected side, and that a test battery of the VEMP and caloric test may help to increase the rate of abnormal findings among patients with the peripheral vestibular disorders. In the human VEMP, activation of acoustically responsive saccular afferents triggers an acoustic reflex in the SCM muscle. Future studies are needed to clarify in detail, the clinical usefulness of the VEMP as an otolith function test.

Die elektrophysiologische Untersuchung des vestibulo-collischen Reflexes (VCR)

The vestibulocollic reflex (VCR) and/or the vestibular-evoked myogenic potentials (VEMP) have become a routine diagnostic tool to assess otolith function in recent years. The VCR belongs to the group of vestibulo-spinal reflexes that control posture and stance. The reflex is usually recorded from the ipsilateral sternocleidomastoid muscle (SCM) after monaural acoustic stimulation. It is elicited by intense square-wave click-tones (0.1 ms duration, 95 dB sound pressure level (SPL) above the normal hearing threshold) or short tone-bursts (STB) (80 dB SPL above normal hearing threshold). The VCR represents an unphysiological stimulation of the saccculus that reaches the vestibular nuclei in the lateral pons via the inferior vestibular nerve. After a switch-over in the medial vestibular nuclei, it travels by means of the medial vestibulospinal tract to the ipsilateral motorneurons of the SCM. There, it modulates a tonic baseline activity of the muscle. The VCR has its strength in the detection of fistulas of the semicircular canals, the dysfunction of the sacculus in Meniere's disease as well as the involvement of the inferior vestibular nerve in vestibular neuritis. It is also a good clinical method to evaluate acoustic neurinoma, the extent of a bilateral vestibulopathy and to monitor the course of therapy in basilary migraine.

Vestibulocervical reflexes in idiopathic Parkinson disease

The mechanism of gait instability in Parkinson disease (PD) is not completely understood. We examined the saccular part of the otolith function and its possible contribution to gait difficulties in idiopathic PD. Fifty-four PD patients (mean age 66 years, 32 men) were included. These were characterized with respect to disease severity, duration, treatment, as well as the presence of disease complications, dementia and depression. Vestibular evoked myogenic potentials (VEMP) were recorded in patients and 53 healthy controls. VEMP responses were recorded in all controls. Unilaterally absent VEMP responses were found in 20 (37%) of PD patients and bilaterally absent responses in four (7.4%). All patients with preserved peaks had normal latencies as compared with controls. The number of PD patients with abnormal/absent VEMP was thus significantly higher than in controls (p<0.00). There were no correlations between VEMP abnormality and disease stage, falls or medication. A correlation was found between abnormal VEMP and depression/antidepressant treatment. PD patients often have absent vestibulocollic reflexes. Further investigations are needed to elucidate the significance of this finding for postural stability and gait in this disorder.

Impact of endolymphatic mastoid shunt surgery on saccule and lateral semicircular canal function

Endolymphatic mastoid shunt surgery (EMSS) is widely performed in patients with medically intractable Meniere's disease. Although many patients report an improvement of symptoms after surgery, the mechanisms which are responsible for the relief of complaints are not known. To date, only few studies exist which studied the influence of EMSS on vestibular function. The present study examines the effect of EMSS on saccule function by measuring vestibular evoked myogenic potentials and the effect on lateral semicircular canal function by sinusoidal harmonic acceleration (SHA) testing. No changes in vestibulo-collic reflexes were found after surgery compared to before surgery. SHA testing resulted in comparable phase lag and gain pre- and postoperatively. Although central compensation is clinically evident no effect in specific vestibular diagnostic testing is seen. Modulations of canal-otolith interaction might suggest a change of symptoms. The only method so far to evaluate the success of EMSS is the patient's subjective assessment.

Effect of Canal Plugging on Quadrupedal Locomotion in Monkey

The vestibular system plays an important role in controling gait, but where in the labyrinths relevant activity arises is largely unknown. After the semicircular canals are plugged, low frequency (0.01-2 Hz) components of the angular vestibulo-ocular reflex (aVOR) and angular vestibulo-collic reflex (aVCR) are lost, but high frequency (3-20 Hz) components remain. We determined how loss of low frequency canal afference affects limb and head movements during quadrupedal locomotion. Head, body, and limb movements were recorded in three dimensions (3-D) in a cynomolgus monkey with a motion detection system, while the animal walked on a treadmill. All six canals were plugged, reducing the canal time constants from approximately 4.0 sec to approximately 0.07 sec. Major changes in the control of the limbs occurred after surgery. Fore and hind limbs were held farther from the body, producing a broad-based gait. Swing-phase trajectories were inaccurate, and control of medial-lateral limb movement was erratic. These changes in gait were present immediately after surgery, as well as 15 months later, when the animal had essentially recovered. Thus, control of the limbs in the horizontal plane was defective after loss of the low-frequency semicircular canal input and never recovered. Cycle-averaged pitch and roll head rotations, and 3-D head translations were also significantly larger and more erratic after than before surgery. Head rotations in yaw could not be quantified due to intrusion of voluntary head turns. These findings indicate that the semicircular canals provide critical low frequency information to maximize the accuracy of stepping and stabilize the head during normal quadrupedal locomotion.