The role of cochlear and vestibular afferents in long-latency cervical vestibular evoked myogenic potentials.

OBJECTIVE: To examine the origin of cervical vestibular evoked myogenic potential (cVEMP) late waves (n34-p44) elicited with air-conducted click stimuli. DESIGN: Using a retrospective design, cVEMPs from normal volunteers were compared to those obtained from patients with vestibular and auditory pathologies. STUDY SAMPLE: (1) Normal volunteers (n = 56); (2) severe-to-profound sensorineural hearing loss (SNHL) with normal vestibular function (n = 21); (3) peripheral vestibular impairment with preserved hearing (n = 16); (4) total vestibulocochlear deficit (n = 23). RESULTS: All normal volunteers had ipsilateral-dominant early p13-n23 peaks. Late peaks were present bilaterally in 78%. The p13-n23 response was present in all patients with SNHL but normal vestibular function, and 43% had late waves. Statistical comparison of these patients to a subset of age-matched controls showed no significant difference in the frequencies, amplitudes or latencies of their ipsilateral early and late peaks. cVEMPs were absent in all patients with vestibular impairment. CONCLUSION: The presence of long-latency cVEMP waves was not dependent on the integrity of sensorineural hearing pathways, but instead correlated with intact vestibular function. This finding conflicts with the view that these late waves are cochlear in origin, and suggests that vestibular afferents may assume a more prominent role in their generation.

Electrophysiological Activity from the Eye Muscles, Cerebellum and Cerebrum During Reflexive (Classical Pavlovian) Versus Voluntary (Ivanov-Smolensky) Eye-Blink Conditioning.

We report an experiment to investigate the role of the cerebellum and cerebrum in motor learning of timed movements. Eleven healthy human subjects were recruited to perform two experiments, the first was a classical eye-blink conditioning procedure with an auditory tone as conditional stimulus (CS) and vestibular unconditional stimulus (US) in the form of a double head-tap. In the second experiment, subjects were asked to blink voluntarily in synchrony with the double head-tap US preceded by a CS, a form of Ivanov-Smolensky conditioning in which a command or instruction is associated with the US. Electrophysiological recordings were made of extra-ocular EMG and EOG at infra-ocular sites (IO1/2), EEG from over the frontal eye fields (C3'/C4') and from over the posterior fossa over the cerebellum for the electrocerebellogram (ECeG). The behavioural outcomes of the experiments showed weak reflexive conditioning for the first experiment despite the double tap but robust, well-synchronised voluntary conditioning for the second. Voluntary conditioned blinks were larger than the reflex ones. For the voluntary conditioning experiment, a contingent negative variation (CNV) was also present in the EEG leads prior to movement, and modulation of the high-frequency EEG occurred during movement. US-related cerebellar activity was prominent in the high-frequency ECeG for both experiments, while conditioned response-related cerebellar activity was additionally present in the voluntary conditioning experiment. These results demonstrate a role for the cerebellum in voluntary (Ivanov-Smolensky) as well as in reflexive (classical Pavlovian) conditioning.

Extended source analysis of movement related potentials (MRPs) for self-paced hand and foot movements demonstrates opposing cerebral and cerebellar laterality: a preliminary study.

The cerebellum is known to have extensive reciprocal connectivity with the cerebral cortex, including with prefrontal and posterior parietal cortex, which play an important role on the planning and execution of voluntary movement. In the present article we report an exploratory non-invasive electrophysiological study of the activity of the cerebellum and cerebrum during voluntary finger and foot movements. In a sample of five healthy adult subjects, we recorded EEG and the electro-cerebellogram (ECeG) with a 10% cerebellar extension montage during voluntary left and right index finger and foot movements. EMG was recorded from finger extensors and flexors and from the tibialis anterior and soleus muscles and was used to generate triggers for movement related averaging (-2000 to +2000 ms). Source analysis was conducted over five epochs defined relative to EMG onset: whole epoch (-1000 to +1000 ms), pre-move 1000 (-1000 to 0 ms), pre-move 500 (-500 to 0 ms), post-move 500 (0 to +500 ms) and post-move 1000 (0 to +1000 ms). This yielded a total of 123 cerebral and 65 cerebellar dipole clusters from across all epochs, including the pre-movement epochs, which were then subject to statistical analysis. These demonstrated predominantly contralateral dominance for the cerebral clusters, but predominantly ipsilateral dominance for the cerebellar clusters. In addition, both cerebral and cerebellar clusters showed evidence of a somatotopic gradient, medially (X-axis) for the cerebral clusters, and medially and dorso-ventrally (Z-axis) for the cerebellar clusters. These findings support the value of recording cerebellar ECeG and demonstrate its potential to contribute to understanding cerebellar function.

Head Orientation Modulates Vestibular Cerebellar Evoked Potentials (VsCEPs) and Reflexes Produced by Impulsive Mastoid and Midline Skull Stimulation.

The cerebellum plays a critical role in the modulation of vestibular reflexes, dependent on input from proprioceptive afferents. The mechanism of this cerebellar control is not well understood. In a sample of 11 healthy human subjects, we investigated the effects of head orientation on ocular, cervical, postural and cerebellar short latency potentials evoked by impulsive stimuli applied at both mastoids and midline skull sites. Subjects were instructed to lean backwards with the head positioned straight ahead or held rotated in different degrees of yaw towards the right and left sides. Impulsive mastoid stimulation, a potent method of utricular stimulation, produced localised vestibular cerebellar evoked potentials (VsCEPs: P12-N17) which were strongly modulated by head orientation. The response was larger on the side opposite to the direction of head rotation and with stimulation on the side of rotation. In contrast, ocular VEMPs (oVEMPs: n10-p16) were present but showed little change with head posture, while cervical VEMPs (cVEMPs: p15-n23) were larger with the head held rotated away from the side of the recording. Postural effects with lateral vestibular stimulation were strongly modulated by head rotation, with more powerful effects occurring bilaterally with stimulation on the side of rotation. The duration of the postural EMG changes was similar to the post-excitation inhibition of the electrocerebellogram (ECeG), consistent with cerebellar participation. We conclude that head rotation selectively affects evoked vestibular reflexes towards different targets, consistent with their physiological roles. Changes in VsCEPs may contribute to the modulation of postural reflexes by the cerebellum.

Magnitude, variability and symmetry in head acceleration and jerk and their relationship to cervical and ocular vestibular evoked myogenic potentials.

BACKGROUND: Acceleration and changes in acceleration (jerk) stimulate vestibular otolith afferents. Bone-conducted (BC) vibration applied to the skull accelerates the head and produces short latency reflexes termed vestibular evoked myogenic potentials (VEMPs). OBJECTIVE: To determine the magnitude, variability and symmetry in head acceleration/jerk during VEMP recordings and investigate the relationship between head acceleration/jerk and VEMP properties. METHODS: 3D head accelerometery (sagittal, interaural and vertical axes) was recorded bilaterally in thirty-two healthy subjects during cervical (cVEMP) and ocular (oVEMP) recordings. BC 500 Hz sinusoidal tones were applied to the midline forehead using a positive polarity stimulus. RESULTS: The direction of induced acceleration/jerk was predominately backward, outward and downward on either side of the head during cVEMP and oVEMP recordings.Overall, acceleration/jerk was larger in the sagittal and interaural axes and peaked earlier in the interaural axis bilaterally. Acceleration was more symmetric in the sagittal and interaural axes whereas jerk symmetry did not differ between axes. Regression models did not show a systematic relationship between acceleration/jerk and either VEMP reflex. CONCLUSIONS: The pattern of skull acceleration/jerk was relatively consistent between the two sides of the head and across subjects, but there were differences in magnitude, leading to inter-side and inter-subject variability.

Electrophysiological activity from over the cerebellum and cerebrum during eye blink conditioning in human subjects.

We report the results of an experiment in which electrophysiological activity was recorded from the human cerebellum and cerebrum in a sample of 14 healthy subjects before, during and after a classical eye blink conditioning procedure with an auditory tone as conditional stimulus and a maxillary nerve unconditional stimulus. The primary aim was to show changes in the cerebellum and cerebrum correlated with behavioral ocular responses. Electrodes recorded EMG and EOG at peri-ocular sites, EEG from over the frontal eye-fields and the electrocerebellogram (ECeG) from over the posterior fossa. Of the 14 subjects half strongly conditioned while the other half were resistant. We confirmed that conditionability was linked under our conditions to the personality dimension of extraversion-introversion. Inhibition of cerebellar activity was shown prior to the conditioned response, as predicted by Albus (1971). However, pausing in high frequency ECeG and the appearance of a contingent negative variation (CNV) in both central leads occurred in all subjects. These led us to conclude that while conditioned cerebellar pausing may be necessary, it is not sufficient alone to produce overt behavioral conditioning, implying the existence of another central mechanism. The outcomes of this experiment indicate the potential value of the noninvasive electrophysiology of the cerebellum.

Collic evoked potentials, myogenic potentials (CEMPs) and postural responses produced by brief 100 Hz vibration of the sternocleidomastoid muscle.

We present an initial report using 5 subjects, of short and long latency collic evoked responses following a half cycle of 100 Hz vibration (5 ms) applied to the sternocleidomastoid (SCM) tendon. These were detected in EEG and extraocular and leg muscles and compared with vestibular-dependent responses from direct mastoid stimulation. The responses from the extraocular recording site are likely to be evoked myogenic potentials, thus "collic evoked myogenic potentials" (CEMPs). An n19/p24 presumed ocular CEMP (oCEMP) was followed by a P22/N28 response over the posterior fossa, referred to as a collic cerebellar evoked potential (CoCEP), with responses in leg muscles starting around 55 ms. In contrast to their vestibular analogues, the oCEMP and CoCEP were predominantly ipsilateral to the side of stimulation, consistent with a double-crossed projection. In addition, their thresholds were just above the threshold of vibrotactile sensation, implying a low threshold, oligo-synaptic projection of SCM afferents to both extraocular and cerebellar targets. Following these short latency responses, SCM tendon stimulation evoked prolonged EMG responses in postural muscles of the legs, consistent with a role in the afferent limb of a short latency, spino-bulbar-spinal postural response to sternal perturbations. These collic evoked responses are likely to be of value in understanding the functions of cervical muscle afferents and have clinical value, for example in monitoring compensation after vestibular loss.

Effects of posture on cerebellar evoked potentials (CEPs) following brief impulsive stimuli at the mastoid and trunk.

Recordings from over the posterior fossa following impulsive acceleration stimuli have shown short latency evoked potentials of presumed cerebellar origin. In this study, we investigated the effect of posture on these cerebellar evoked potentials (CEPs) and their relationship to postural reflexes recorded from the leg muscles evoked by the same stimuli. Nine healthy subjects were tested during lying (supine and prone), sitting and standing. Impulsive accelerations were applied at the mastoid and to truncal (both C7 and sternal) stimulation sites. The effect of vision, eyes open or closed, was investigated for all three stimuli. For the truncal stimuli, the effect of differing leaning conditions during standing was also recorded. CEP amplitudes were correlated for the three stimuli. For C7 stimulation during standing, both CEPs and postural reflexes scaled as the threat to postural stability increased. However, CEPs for all stimuli were present during lying, sitting and standing with amplitude and latency parameters mainly unaffected by posture or vision. In contrast, postural reflexes from the leg muscles were attenuated when not standing, with the effect being more marked for truncal stimuli. We conclude that CEPs evoked by axial and vestibular stimuli are not systematically gated by posture, in contrast to the reflex responses evoked by the same stimuli.

Non-invasive recording from the human cerebellum during a classical conditioning paradigm using the otolith-evoked blink reflex.

We studied nine normal volunteers with a classical conditioning paradigm using a mastoid tap, believed to activate otolith receptors, as an unconditional stimulus (US) and the consequent blink as the unconditioned response (UR). Both visual (alternation of stripes) and an auditory tone were used as conditional stimuli (CS). Recordings were made below the eyes at IO1 and IO2, from over the frontal eye fields (C3' and C4') and over the posterior fossa, the latter at sites we have previously reported that we were able to record an evoked climbing fibre response (CFR) at short latency. Behavioural analysis confirmed that weak conditioning did occur early, which subsequently showed extinction on repeated CS alone trials. Further, a UR was more likely to occur following a preceding CFR when preceded by a CS, supporting a correlation between the CFRs and behaviour. For further statistical analysis, the time period of interest was divided into a series of epochs, based around the events occurring at the time. Grand averages, plus analysis of variance, confirmed evidence of weak conditioning for the blink response following both modalities. The EMG associated with the eyeblink for the UR occurred at a similar time to the expected post-CFR pause in the spontaneous cerebellar activity, or electrocerebellogram (ECeG), while hypothesised conditioned pausing in the ECeG was also observed in CS alone trials. A correlation was found between the size of the CFR and the RMS amplitudes of the segments covering the ocular vestibular evoked myogenic potential oVEMP, (periocular) EMG and the EOG. The slope was greater for the non-oVEMP segments than for the oVEMP segment suggesting the correlation was not simply due to differing sizes of the vestibular volley. We suggest that these recorded events fit with the proposed role of the CFR in Purkinje neurons in classical conditioning, gating the excitability of the cerebellar nuclei, and thereby neurons in the reticular formation mediating the otolith blink reflex. This effect appears to apply to polysynaptic reflexes only as there was no evidence of changes to the oVEMP.

Comparison of the Effects of Matching and Normalization on the Cervical Vestibular Evoked Myogenic Potential.

HYPOTHESIS: We compared two means of mitigating the effect of sternocleidomastoid (SCM) contraction strength on the cervical vestibular evoked myogenic potential (cVEMP): contraction matching and amplitude normalization. BACKGROUND: SCM muscle contraction strength affects the amplitude of the cVEMP which can impact measures of inter-side asymmetry and diagnostic outcomes. METHODS: In 19 normal subjects, we investigated the effect of muscle contraction variation within a cVEMP recording. We then compared cVEMP recordings on the right and left sides with matched and unmatched muscle contraction strength using raw amplitudes and amplitude ratios (i.e., normalized amplitudes). RESULTS: Contraction variability had significant effects on small sections of a cVEMP recording, but there was no significant effect on overall cVEMP amplitude, suggesting that the cVEMP is relatively unaffected by variable effort during a recording. Matching the contraction across the two sides (d = 0.53, p = 0.016) and amplitude normalization (d = 0.43, p = 0.004) both significantly reduced inter-side asymmetry, but normalization had no additional benefit once the sides were matched (interaction effect, p = 0.019). cVEMPs recorded with matched contractions had the smallest range of asymmetry values. CONCLUSION: The study shows that controlling the background contraction during a cVEMP recording, either by using similar contractions for each trial or by normalizing the amplitude, reduces cVEMP asymmetry and can prevent incorrect results in the minority of subjects who make asymmetric muscle contractions.

Source analyses of axial and vestibular evoked potentials associated with brainstem-spinal reflexes show cerebellar and cortical contributions.

In this work we examine the possible neural basis for two brainstem-spinal reflexes using source analyses of brain activity recorded over the cortex and posterior fossa. In a sample of 5 healthy adult subjects, using axial and vestibular stimulation by means of applied impulsive forces, evoked potentials were recorded with 63 channels using a 10 % cerebellar extension montage. In parallel, EMG was recorded from soleus and tibialis anterior muscles and accelerometry from the lower leg. Recordings over the cerebellum (ECeG) confirmed the presence of short latency (SL) potentials and these were associated with changes in high-frequency power. The SL responses to the two stimulus modalities differed in that the axial stimulation produced an initial pause and then a burst in the high-frequency ECeG, followed by excitation/inhibition in soleus while vestibular stimulation produced an initial burst then a pause, followed by inhibition/excitation in soleus. These short latency responses were followed by longer latency N1/P2/N2 responses in the averaged EEG, which were maximal at FCz. Brain Electrical Source Analysis (BESA) demonstrated both cerebellar and cerebral cortical contributions to the short-latency responses and primarily frontal cortex contributions to the long-latency EPs. The latency and polarity of the SL EPs, in conjunction with changes in high-frequency spontaneous activity, are consistent with cerebellar involvement in the control of brainstem-spinal reflexes. The early involvement of frontal cortex and subsequent later activity may be an indicator of the activation of the cortical motor-related system for rapid responses which may follow the reflexive components. These findings provide evidence of the feasibility of non-invasive electrophysiology of the human cerebellum and have demonstrated cerebellar and frontal activations associated with postural-related stimuli.

Quantifying the effects of electrode placement and montage on measures of cVEMP amplitude and muscle contraction.

BACKGROUND: The cervical vestibular evoked myogenic potential (cVEMP) can be affected by the recording parameters used to quantify the response. OBJECTIVE: We investigated the effects of electrode placement and montage on the variability and symmetry of sternocleidomastoid (SCM) contraction strength and cVEMP amplitude. METHODS: We used inter-side asymmetries in electrode placement to mimic small clinical errors in twenty normal subjects. cVEMPs were recorded at three active electrode sites and referred to the distal SCM tendon (referential montages: upper, conventional and lower). Additional bipolar montages were constructed offline to measure SCM contraction strength using closely-spaced electrode pairs (bipolar montages: superior, lower and outer). RESULTS: The conventional montage generally produced the largest cVEMP amplitudes (P < 0.001). SCM contraction strength was larger for referential montages than bipolar ones (P < 0.001). Inter-side electrode position errors produced large variations in cVEMP and SCM contraction strength asymmetries in some subjects, producing erroneous abnormal test results. CONCLUSION: Recording locations affect cVEMP amplitude and SCM contraction strength. In most cases, small changes in electrode position had only minor effects but, in a minority of subjects, the different montages produced large changes in cVEMP and contraction amplitudes and asymmetry, potentially affecting test outcomes.

Effects of viewing distance on ocular vestibular evoked myogenic potentials (oVEMPs) for air- and bone-conducted stimuli at multiple sites.

BACKGROUND: The ocular vestibular evoked myogenic potential is otolith-dependent and has been suggested to be a manifestation of the linear vestibulo-ocular reflex (L-VOR). A characteristic feature of the translational LVOR (t-LVOR) is its dependence on the distance of a target. OBJECTIVE: To assess if viewing distance affects amplitude and latency properties of the ocular vestibular evoked myogenic potential (oVEMPs). METHODS: Bone- and air-conducted (BC and AC) stimuli were used to evoke oVEMPs in 10 healthy subjects. BC stimuli consisted of impulsive accelerations applied at the mastoids, AFz, Oz and Iz. AC stimuli consisted of 500 Hz tones delivered unilaterally to each ear. Target distances of 40 cm (near), 190 cm (intermediate) and 340 cm (far) were used for all stimuli. RESULTS: The largest amplitude oVEMP was obtained from Iz and the latency for AFz was shorter than for BC stimulation at other sites. We found no significant effect of target distance on oVEMP amplitudes for any of the stimuli used. There was a small but significant effect on latency with the nearest target having a longer latency (overall 12.4 ms vs 12.0 ms for the 2 more distant sites). CONCLUSIONS: Previously reported differences between latencies and stimulus sites for midline BC stimulation were confirmed. Target distance had no significant effect on oVEMP amplitude, which suggests it is not modified like other components of the t-LVOR.

Correction to: Mapping the vestibular cerebellar evoked potential (VsCEP) following air- and bone-conducted vestibular stimulation.

In the original publication of the article, Fig. 3 is incorrect.

Bone-conducted vestibular and stretch reflexes in human neck muscles.

In normal humans, tapping the forehead produces a neck muscle reflex that is used clinically to test vestibular function, the cervical vestibular evoked myogenic potential (cVEMP). As stretch receptors can also be activated by skull taps, we investigated the origin of the early and late peaks of the bone-conducted cVEMP. In twelve normal participants, we differentially stimulated the vestibular and neck stretch receptors by applying vibration to the forehead (activating both vestibular and stretch receptors) and to the sternum (activating mainly stretch receptors). Patients with bilateral vestibulopathy (BVP; n = 26) and unilateral vestibular loss (uVL; n = 17) were also investigated for comparison. Comparison of peaks in normal subjects suggested that the early peaks were vestibular-dependent, while the later peaks had mixed vestibular and stretch input. The late peaks were present but small (1.1 amplitude ratio) in patients with BVP and absent VEMPs, confirming that they do not strictly depend on vestibular function, and largest in age-matched controls (1.5 amplitude ratio, p = 0.049), suggesting that there is an additional vestibular reflex at this latency (approx. 30 ms). Patients with uVL had larger late peaks on the affected than the normal side (1.4 vs 1.0 amplitude ratio, p = 0.034). The results suggest that the early responses in SCM to skull vibration in humans are vestibular-dependent, while there is a late stretch reflex bilaterally and a late vestibular reflex in the contralateral muscle.

Axial perturbations evoke increased postural reflexes in Parkinson's disease with postural instability.

OBJECTIVE: To measure axially-evoked postural reflexes in 11 Parkinson's disease (PD) subjects, both stable and unstable, and to compare these with 13 age-matched controls. METHODS: We measured the short-latency electromyography (EMG) reflex effects of brief impulsive displacements applied to the upper sternum or C7 for tibialis anterior (TA) and soleus. Our subjects were studied standing normally and when leaning both forwards and backwards. RESULTS: The initial mechanical effects of the stimuli were similar but the reflex responses for the unstable PD group were increased, even after allowing for the increased levels of tonic activation. For TA, unstable PD subjects had significantly larger responses than the stable PD group whose responses were in turn significantly larger than controls. For soleus, unstable PD subjects had significantly greater responses than controls. CONCLUSIONS: These findings are consistent with previous evidence that exaggerated postural responses are characteristic of unstable PD subjects. SIGNIFICANCE: Increased postural reflexes are characteristic of unstable PD subjects and may contribute to the instability seen for these patients in response to larger perturbations.

Mapping the vestibular cerebellar evoked potential (VsCEP) following air- and bone-conducted vestibular stimulation.

Vestibular cerebellar evoked potentials (VsCEPs) were recorded from over the occipital and cerebellar regions of the scalp using bone-conducted (BC) stimuli applied at the mastoids (impulsive accelerations and 500 Hz) and 500 Hz acoustic tones (AC). Ten healthy subjects were tested. Electrodes were positioned over the midline (Oz, Iz, CBz) and at 3, 6 and 9 cm intervals lateral to the midline electrodes bilaterally. Additional electrodes were also positioned over posterior neck muscles (SPL1 and SPL2). The largest evoked potentials on average were recorded from the electrodes 3 and 6 cm lateral to the Iz and CBz midline locations. BC stimuli produced short latency potentials on the side contralateral to the stimulated mastoid and were dependent on stimulus polarity. Positive polarity stimuli produced biphasic VsCEPs at approximately 12 and 17 ms (P12-N17) for BC impulses and 10 and 15 ms (P10-N15) for BC 500 Hz stimuli. Following the initial excitation, there was a period of suppression of background activity lasting an average of 16.8 ms for positive polarity BC impulses. Negative polarity stimuli produced later VsCEPs both for BC impulses (P20-N26) and BC 500 Hz (P13-N18). VsCEPs to AC 500 Hz stimuli lateralised to the contralateral side and were larger for right than left ear stimulation. Stimulus polarity (condensation and rarefaction) did not alter the timing of the VsCEPs to AC 500 Hz tones. No evoked response was recorded to somatosensory (median and radial nerve) stimulation. Four patients with cerebellar disease were tested and two showed abnormal VsCEPs with initial negativities. VsCEPs show distinct mapping over the posterior fossa and are likely to reflect climbing fibre responses via crossed otolith-cerebellar pathways.

Modulation of the human electro-cerebellogram (ECeG) during vestibular and optokinetic stimulation.

We have recently shown that it is possible to record vestibular cerebellar evoked potentials (VsCEPs) in the form of a contralateral short-latency biphasic positive-negative wave with surface electrodes placed over the posterior fossa and, in parallel with this, the spontaneous electrocerebellogram (or ECeG). VsCEPs were further shown to exhibit considerable plasticity depending on visual context and in this article we show additionally that the ECeG power and cerebro-cerebellar coherence which accompanies the VsCEPs also changes systematically during vestibular and optokinetic stimuli. In a sample of nine healthy human subjects, half of whom had VsCEPs, we recorded in parallel the ECeG and for comparison the EEG from central scalp sites. The results showed that during vection with vestibular stimulation there is actually a reduction in the power of the ECeG accompanied by an increase in cerebro-cerebellar coherence, both of which correlate with the subjective sensation of vection. Only the VsCEPs present group showed the significant power reduction and increase in coherence, indicating a link between the VsCEP and plasticity in the ECeG. We suggest that these data are consistent with a human cerebro-cerebellar network associated with control of the VOR. The network exhibits the plasticity expected from the known inhibitory properties of Purkinje cells and the pausing of spontaneous Purkinje cell activity following a climbing-fibre evoked response with conjunctive inputs from the mossy-fibre system.

Responses to anterior and posterior perturbations in Parkinson's disease with early postural instability: role of axial and limb rigidity.

We studied 12 patients with Parkinson's disease (PD): 6 with postural instability (Hoehn and Yahr Stage 3) and 6 without (Stage 2 or 2.5), using a quantitative test based on the clinical pull test. Their findings were compared with those for 12 healthy controls. The patients on their usual medications were pulled either forwards or backwards at the level of the shoulders and asked not to take a step in a series of five trials. Acceleration was monitored for the upper trunk, sacrum, and both tibias. EMG was measured in soleus and tibialis anterior (TA) muscles in all and for thigh and truncal muscles in a subgroup. A target of 0.2 g trunk acceleration was used, but smaller perturbations were used in very unstable patients. All the Stage 3 patients lost balance in at least one trial for the posterior perturbations but none for the anterior ones. None of the Stage 2 patients lost balance. There was increased tonic EMG and agonist activity but no difference in EMG onset or initial force production compared to healthy controls. For posterior perturbations, there were two related disorders that separated the PD patients from controls. There was a significantly higher ratio of sacral-to-applied acceleration and both PD groups showed reduced knee acceleration and shortened latency, more so for the Stage 3 group. The increased sacral-to-C7 acceleration ratio was correlated with the tonic level of activation of the hamstrings (HS), quadriceps, and lumbar paraspinal muscles (PS), while the tibial acceleration latency was also correlated with the level of tonic PS activation. We also found that the size of balance responses, 0-200 ms post-perturbation, correlated significantly with the level of tonic activation in nearly all the muscles studied. We confirmed that PD patients show greater instability posteriorly than anteriorly to applied perturbations. Our findings support increasing axial and limb rigidity as the cause of the impaired pull test rather than postural bradykinesia and suggest that tonic truncal and thigh muscle activation may be an important underlying cause.

Vestibular evoked myogenic potentials in practice: Methods, pitfalls and clinical applications.

Vestibular evoked myogenic potentials (VEMPs) are a useful and increasingly popular component of the neuro-otology test battery. These otolith-dependent reflexes are produced by stimulating the ears with air-conducted sound or skull vibration and recorded from surface electrodes placed over the neck (cervical VEMPs) and eye muscles (ocular VEMPs). VEMP abnormalities have been reported in various diseases of the ear and vestibular system, and VEMPs have a clear role in the diagnosis of superior semicircular canal dehiscence. However there is significant variability in the methods used to stimulate the otoliths and record the reflexes. This review discusses VEMP methodology and provides a detailed theoretical background for the techniques that are typically used. The review also outlines the common pitfalls in VEMP recording and the clinical applications of VEMPs.