Responses to Brief Perturbations of Stance: EMG, Midline Cortical and Subcortical Changes.

We studied simultaneous EMG and midline EEG responses, the latter including over the cerebellum, in 10 standing subjects (35 ± 15 yrs; 5 females) following repeated taps to the sternum. We confirmed previous reports that this evokes short latency EMG responses in leg muscles, consistent with postural reflexes. EEG power had relatively more high frequency components (> 30 Hz) when recorded from electrodes over the cerebellum (Iz, SIz) compared to other midline electrodes. We showed clear midline evoked EEG potentials occurring at short latency over the cerebellum (P23, N31, N42, P54) and frontally (N28, N57) prior to the previously-described perturbation evoked potential (P1/N1/P2). The P23 response correlated with the subsequent EMG response in the tibialis anterior muscles. We conclude that early activity occurs from electrodes over the cerebellum in response to a brief tap to the sternum. This is likely to represent cerebellar activity and it appears to modulate short latency postural EMG responses.

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.

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.

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.

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.

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.

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.

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.

Location and phase effects for ocular and cervical vestibular-evoked myogenic potentials evoked by bone-conducted stimuli at midline skull sites.

Our object was to investigate the effect of location and phase on the properties of oVEMPs and cVEMPs evoked by two bone conducted (BC) stimuli, 500 Hz and an impulsive stimulus for midline skull sites from Nz to Iz, in normal volunteers. Compressive and rarefactive onset phases were used and the induced linear and rotational accelerations measured. We confirmed our previous finding of marked changes in the polarity of oVEMPs with location. For cVEMPs using the 500Hz stimulus there were few changes with location or phase, but the impulsive stimulus showed clear phase-related changes at several locations, with the shortest latencies occurring with compressive stimuli at AFz and Fz and the largest amplitudes at Iz. For oVEMPs, both stimuli showed clear effects of phase, with the shortest latencies with compressive stimuli at AFz and Fz and with the largest negativity at Oz or Iz. Whereas the effectiveness at Iz is consistent with a role in the linear VOR, the inversion of polarity and shorter latency around AFz and Fz is not and could not be explained by changes in acceleration of the head. The latency for BC 500Hz oVEMPs for AFz was the same as that for air-conducted (AC) stimuli. We suggest that whereas BC stimuli at most sites work through displacement of the otolith membrane, BC oVEMPs evoked at AFz and Fz may work through a direct action on utricular hair cells. Our findings have implications for clinical testing of VEMPs using midline BC stimuli. NEW & NOTEWORTHY We investigated VEMPs evoked from multiple midline skull sites. Large oVEMP responses were obtained with compressive stimuli at Iz, consistent with a role in the linear VOR, but we also showed inversion of polarity and the shortest latency for stimuli given at AFz and Fz. We propose that BC stimuli given at AFz and Fz may have a direct effect on otolith hair cells, whereas at other sites they work through displacement of the otolith membrane.

Vestibular cerebellar evoked potentials in humans and their modulation during optokinetic stimulation.

We recorded evoked potentials (EPs) from over the posterior fossa and in parallel ocular vestibular evoked myogenic potentials (OVEMPs) during visuo-vestibular stimulation in a sample of 7 male and 11 female human subjects. In 9 of the 18 subjects we were able to record EPs reliably in the form of an early biphasic positive-negative wave with latencies ~12 and 17 ms ipsilateral to head acceleration direction (P12-N17) and a slightly later, contralateral, biphasic positive-negative wave with latencies ~19 and 23 ms (P19-N23). The amplitudes of the responses varied widely between subjects. Both P12 and N23 EPs were modulated by the mode of visual stimulation, larger for vection (sense of movement) compared with optokinetic nystagmus and for congruent movement. We suggest that the EPs measured over the posterior fossa are a manifestation of climbing fiber responses of cerebellar cortical Purkinje cells, i.e., a form of vestibular cerebellar EP (VsCEP). The two subject groups with and without VsCEPs were distinguished by the magnitude of their OVEMPs and their subjective experience of vection. The modulation of VsCEPs by visual context may be a manifestation of cerebellar control of linear vestibular ocular reflex gain. NEW & NOTEWORTHY We report likely vestibular cerebellar evoked potentials (VsCEPs) produced by lateral head impulses recorded in intact humans over the posterior fossa. VsCEPs occurred as short-latency P12-N17 waves ipsilateral to the direction of head motion and as P19-N23 contralaterally and were present in half our subjects. Their properties suggest that the VsCEPs may be of a climbing-fiber origin. VsCEPs are related to the perception of motion and, possibly, control of linear vestibular ocular reflex gain.

Axial reflexes are present in older subjects and may contribute to balance responses.

We studied the response to axial taps (mini-perturbations) of a group of 13 healthy older subjects (mean age 63 ± 12 years, 7 females, 6 males), 12 of whom were also studied using larger applied (macro-) perturbations requiring active postural responses. The mini-perturbation consisted of a brief impulsive force produced by a mini-shaker applied to the trunk at the level of the shoulders and anteriorly at the upper sternum which was perceived as a tap. Acceleration, force platform, and EMG measurements were made. The average peak accelerations for the mini-perturbations were 108 mG (anterior) and - 78.9 mG (posterior). Responses overall were very similar to those previously reported for younger subjects: the perturbation evoked short latency responses in leg muscles, modulated by degree and direction of lean, and were largest for the muscle most relevant for the postural correction. The increases in the amplitude for the main agonist were greater than the increase in tonic activity. With both anterior and posterior lean, co-contraction responses were present. The size of the EMG response to the mini-perturbations correlated with the corresponding earliest EMG responses (0-100, 100-200 ms intervals) to the larger postural perturbations, timing which corresponds to balance responses. The balance responses evoked by the larger imposed postural perturbations may, therefore, receive a contribution through the reflex pathway mediating the axial tap responses, whose efferent limb appears to be the reticulospinal tract.

Properties of short-latency responses in the upper limbs evoked by axial impulses during leaning: evidence for reticulospinal projections.

We studied the short-latency (SL) effects of postural perturbations produced by impulses applied over the spine of the C7 vertebra or the sternum ("axial impulses") in 12 healthy subjects. EMG recordings were made bilaterally from the triceps brachii, biceps brachii, soleus, and tibialis anterior muscles, and unilaterally from the deltoid, forearm flexors, forearm extensors, and first dorsal interosseous (FDI) muscles. Sternal impulses evoked short-latency responses in the biceps when subjects leaned posteriorly to support approximately 12% of their body weight with the arms, but these responses were only modestly larger than for isometric contraction of the arms (26.3 vs. 14.7%). In contrast, clear excitatory responses could be evoked in the deltoid, triceps, forearm muscles, and FDI when leaning anteriorly to support similar amounts of body weight. These responses were significantly larger than during isometric contraction. The deltoid (42.5%) and triceps (44.7%) had the largest responses in supported anterior lean and onset latencies increased distally in this condition (mean 31.8 ms in deltoid to 53.7 ms in FDI). There was a disproportionate delay between the forearm muscles and FDI. For both directions of lean, postural reflex responses normally present in the legs were severely attenuated. SL upper limb excitatory responses were bigger in proximal muscles as well as larger and more widespread for anterior axial perturbations compared to posterior axial perturbations when using the arms to support body weight. Our findings also provide further evidence of a role for reticulospinal pathways in mediating these rapid postural responses to accelerations of the trunk.

The inion response revisited: evidence for a possible cerebellar contribution to vestibular-evoked potentials produced by air-conducted sound stimulation.

This study investigated the effect of eye gaze and head position on vestibular-evoked potentials (VsEPs). Head position would be expected to affect myogenic sources, and eye position is known to affect ocular myogenic responses (ocular vestibular-evoked myogenic potentials), whereas a neurogenic source should behave otherwise. Eleven healthy subjects were recruited, and VsEPs, using 72-channel EEG, were recorded at a fixed intensity above the vestibular threshold. Three eye gaze and three head positions were tested (-20°, 0°, and +20° to the horizontal). Short-latency potentials showed that in addition to the expected effect of gaze on infraocular (IO') leads, where up-gaze gives a maximum response, significant changes in amplitude were also observed in electrodes remote from the eyes and in particular, from contralateral parietal-occipital (PO) and neck (CB') leads. Short-latency potentials of similar latency were observed (p10/n17 and n10/p17, respectively). The pattern of change with gaze in the PO leads was distinct from that observed for the IO' leads. For the PO leads, the maximum response was obtained with neutral gaze, and this was also distinct from that observed for CB' electrodes, where a maximal response was observed with head flexion in the second wave but not the first. Evidence of modulation of N42 and N1 potentials with both eye and head position was also observed. Head- and eye-position manipulation thus suggests that the inion response consists of an early neurogenic component, as well as myogenic responses. The p10/n17 at PO, in particular, may be an indicator of vestibulocerebellar projections.NEW & NOTEWORTHY Loud sounds were used to activate vestibular receptors in human volunteers and the effects of head and eye position studied for short-latency responses. A potential (p10/n17) recorded in the parieto-occipital leads showed behavior not expected for a response with a myogenic origin. Source modeling suggested a possible origin from the cerebellum. It may represent a new indicator of human vestibulocerebellar function.

Postural responses in the upper limbs evoked by axial impulses: a role for reticulospinal projections.

We studied the short-latency (SL) postural effects of axial impulses in 11 subjects (22 ± 2 years old). Recordings were made bilaterally from soleus and tibialis anterior (TA) muscles. We confirmed that with leaning anteriorly and posteriorly, reflex EMG increases occurred in both muscle groups at short latency following brief perturbations applied over C7 or the sternum (soleus mean latencies 57.5 and 66.4 ms; TA mean 51.7 and 55.4 ms, respectively). While the size of the SL reflexes was affected by the direction of lean when standing we found that light touch did not affect the amplitudes or latencies significantly. We investigated the presence of SL responses in the upper limb muscle triceps brachii during an isometric contraction and when the arm muscles had a direct role in supporting approximately 40% of the body weight. Similar levels of tonic EMG activity occurred in triceps in both conditions but significantly larger SL reflexes occurred when used posturally compared to the isometric contraction (23.0 vs 3.3%) while the reverse occurred for SL responses in soleus and TA, which were significantly attenuated. The responses were present with the head in the neutral position but with head rotation were larger contralateral to the direction of rotation. Calculations based upon the relative latencies suggest that the pathway responsible is not the corticospinal tract. We conclude that axially evoked SL postural reflexes are unaffected by light tactile input but are present in upper limb muscles when used for postural support. We propose that the pathway mediating these responses is the reticulospinal tract.

Frequency and phase effects on cervical vestibular evoked myogenic potentials (cVEMPs) to air-conducted sound.

Few previous studies of tuning using air-conducted (AC) stimuli and the cervical vestibular evoked myogenic potential (cVEMP) have compensated for the effects of middle ear (ME) attenuation. Zhang et al. (Exp Brain Res 213:111-116, 2011a) who did allow for ME effects were able to show a secondary peak around 100 Hz for the ocular VEMP (oVEMP). Recently, it has become clear that the otolith afferents responsible for the cVEMP and oVEMP differ and thus the nature of tuning may be more related to the reflex studied determining which otolith receptors are activated rather than the properties of the stimulus. We wished to reinvestigate the tuning for the cVEMP using AC stimuli, to establish whether the low-frequency peak is specific for the oVEMP or a consequence of the stimulus modality itself. In response to recent evidence using a 500 Hz AC stimulus that there was no effect of stimulus phase, we also investigated whether phase (condensation or rarefaction) had an effect at any frequency. We measured corrected cVEMP amplitudes and latencies in response to stimuli between 50 and 1200 Hz in 10 normal volunteers using an AC stimulus adjusted for ME attenuation. We confirmed earlier reports of the similarity of the tuning for both the cVEMP and oVEMP reflexes but found no separate 100 Hz peak for the cVEMP. AC stimulus phase did not affect either amplitude or latency. Both the tuning pattern and the phase effects contrast with those previously reported for bone-conducted (BC) stimuli. Unlike BC stimulation, which shows tuning consistent with an action on the otolith membrane, AC stimuli are likely to act through a different mechanism, most likely directly at the hair cell level.

Postural responses to anterior and posterior perturbations applied to the upper trunk of standing human subjects.

This study concerned the effects of brisk perturbations applied to the shoulders of standing subjects to displace them either forwards or backwards, our aim being to characterise the responses to these disturbances. Subjects stood on a force platform, and acceleration was measured at the level of C7, the sacrum and both tibial tuberosities. Surface EMG was measured from soleus (SOL), tibialis anterior (TA), the hamstrings (HS), quadriceps (QUAD), rectus abdominis (RA) and lumbar paraspinal (PS) muscles. Trials were recorded for each of four conditions: subjects' eyes open (reference) or closed and on a firm (reference) or compliant surface. Observations were also made of voluntary postural reactions to a tap over the deltoid. Anterior perturbations (mean C7 acceleration 251.7 mg) evoked activity within the dorsal muscles (SOL, HS, PS) with a similar latency to voluntary responses to shoulder tapping. Responses to posterior perturbations (mean C7 acceleration -240.4 mg) were more complex beginning, on average, at shorter latency than voluntary activity (median TA 78.0 ms). There was activation of TA, QUAD and SOL associated with initial forward acceleration of the lower legs. The EMG responses consisted of an initial phasic discharge followed by a more prolonged one. These responses differ from the pattern of automatic postural responses that follow displacements at the level of the ankles, and it is unlikely that proprioceptive afferents excited by ankle movement had a role in the initial responses. Vision and surface properties had only minor effects. Perturbations of the upper trunk evoke stereotyped compensatory postural responses for each direction of perturbation. For posterior perturbations, EMG onset occurs earlier than for voluntary responses.

Contrasting phase effects on vestibular evoked myogenic potentials (VEMPs) produced by air- and bone-conducted stimuli.

We have studied the effects of stimulus phase on the latency and amplitude of cVEMPs and oVEMPs by reanalysing data from Lim et al. (Exp Brain Res 224:437-445, 2013) in which alternating phase was used. Responses for the different initial stimulus phase, either positive or negative, were separated and reaveraged. We found that the phase (compressive or rarefactive) of AC 500-Hz stimuli had no significant effect on either latency or amplitude of the responses. Conversely, phase (positive = motor towards subjects) did alter the effects of BC 500-Hz stimulation. For cVEMPs, phase consistently affected initial latency with earlier responses for positive stimuli, while, for stimulation at the mastoid, negative onset phase gave larger responses. For the oVEMP, effects were different for the two sites of BC stimulation. At the forehead, the response appeared to invert, whereas at the mastoid there appeared to be a delay of the initial response. Related to this, the effect of phase for the two sites was opposite: at the mastoid, positive responses were earlier but negative were larger (particularly for long stimuli). At the forehead, the effect was the opposite: negative onset stimuli evoked earlier responses, whereas positive onset evoked larger responses. These findings indicate a basic difference in the way that AC and BC stimuli activate vestibular receptors and also indicate that the effects of phase of BC stimulation depend on location. Stimulus alternation does little to affect the response to AC stimulation but obscures the effects of BC stimuli, particularly for the oVEMP.

Selective changes of ocular vestibular myogenic potentials in Parkinson's disease.

BACKGROUND: Vestibular evoked myogenic potentials represent electrophysiological tools to measure vestibular reflex actions at different levels of the brainstem in Parkinson's disease. OBJECTIVE: To investigate cervical and ocular vestibular myogenic potentials in Parkinsonian patients with mild disability. METHODS: In 13 Parkinsonian patients and 13 age-matched healthy controls, cervical and ocular vestibular myogenic potentials were recorded after unilateral air-conducted tone bursts and bone-conducted stimuli delivered at the forehead or mastoids. RESULTS: In contrast to relatively preserved cervical vestibular evoked myogenic potentials, ocular vestibular evoked myogenic potentials were significantly delayed and of reduced amplitude, particularly after impulsive stimulation in Parkinsonian patients. Levodopa had no significant effect on either type of response. CONCLUSION: In mild to moderate Parkinson's disease, altered ocular vestibular myogenic potentials may indicate early functional involvement of the upper brainstem, in contrast to preserved lower brainstem function as reflected by normal cervical vestibular myogenic potentials.

Axially evoked postural reflexes: influence of task.

Postural reflexes were recorded in healthy subjects (n = 17) using brief axial accelerations and tap stimuli applied at the vertebra prominens (C7) and manubrium sterni. Short latency (SL) responses were recorded from the soleus, hamstrings and tibialis anterior muscles and expressed as a percentage of the background EMG prior to stimulus onset. In the majority of postural conditions tested, subjects were recorded standing erect and leaning forward with their feet together. The SL response was larger for soleus than for the hamstrings during standing (soleus vs hamstrings; 70.4 vs 28.1%), whereas the opposite occurred during kneeling (25.3 vs 127.3%). Concordant head and trunk accelerations produced larger SL responses than discordant accelerations for soleus and hamstrings, but the evoked excitatory response was independent of head direction and as expected for the direction of truncal acceleration. Postural reflexes for soleus and tibialis anterior were strongly affected by conditions that posed a significant threat to postural stability; stimulation at C7 was associated with significant SL enhancement for soleus during anterior lean while sternal stimulation showed SL enhancement for tibialis anterior during posterior lean. Cutaneous anaesthesia applied over the C7 stimulation site had no significant effect on EMG responses, nor did vision or surface type (rigid or compliant). This study provides further evidence that postural reflexes produced by brief axial accelerations are independent of cutaneous receptors, vestibular afferents and ankle proprioceptors, and demonstrates that postural tasks and truncal orientation significantly affect the evoked response, consistent with a role in stabilising posture.