Neil Todd

Ocular vestibular evoked myogenic potentials (OVEMPs) produced by air-and bone-conducted sound

OBJECTIVEnTo determine the origin and properties of short latency extraocular potentials produced by activation of the vestibular apparatus using two modes of acoustic stimulation.nnnMETHODSnExtraocular potentials were measured in 10 normal subjects using a bipolar montage to increase selectivity. Three dimensional eye movements were also recorded in five subjects. The subjects were stimulated with both air-conducted (AC) and bone-conducted (BC) sound using a single cycle of a 500Hz sine wave.nnnRESULTSnShort latency positive and negative potentials that peaked at 8.1-12.7ms for AC and 7.5-13.9ms for BC stimulation were recorded, which were distinct for the two eyes and for the two modes of stimulation. The extraocular potentials began prior to the onset of eye movements, which peaked at 16.5-20.1ms for AC, 17.8-25.0ms for BC stimulation.nnnCONCLUSIONSnThe pattern of short latency eye movements and extraocular potentials induced by AC and BC vestibular stimulation are distinct. As the potentials preceded the eye movements and were not correlated morphologically with them, the source of the observed potentials is not an eye movement and thus we refer to them as ocular vestibular evoked myogenic potentials (OVEMPs).nnnSIGNIFICANCEnThe potentials had properties consistent with modulation of the electromyogenic activity of the extraocular muscles and if interpreted as originating from displacement of the eye will give misleading results. AC and BC acoustic stimulation are likely to activate differing profiles of vestibular end organs.

A saccular origin of frequency tuning in myogenic vestibular evoked potentials?: implications for human responses to loud sounds

Previous research has indicated that an early component of click-evoked myogenic potentials in the sternocleidomastoid muscle is vestibularly mediated, since it can be obtained in subjects with loss of cochlear function, but is absent in subjects with loss of vestibular function (Colebatch et al., 1994). We report here the results of an experiment to investigate whether this response shows any tuning properties. In a sample of 11 subjects, we obtained acoustically evoked EMG from the sternocleidomastoid muscle in response to 110 dB SPL 10 ms tone pips with frequencies of 100 Hz, 200 Hz, 400 Hz, 800 Hz, 1600 Hz and 3200 Hz. The results of this experiment indicate that this response does indeed have a well-defined frequency tuning which may be modelled as a resonance with a maximum response at frequencies between 300-350 Hz. The possible saccular origin of the tuning response and the consequences that this may have in human responses to loud sounds is discussed. Also discussed are the consequences of particular electrode arrangements in relation to the innervation and anatomy of sternocleidomastoid.

A utricular origin of frequency tuning to low-frequency vibration in the human vestibular system?

Recent work has demonstrated that the human vestibular system displays a remarkable sensitivity to low-frequency vibration. To address the origin of this sensitivity we compared the frequency response properties of vestibular reflexes to 10ms bursts of air-conducted sound and transmastoid vibration, which are thought to be differentially selective for the saccule and utricle, respectively. Measurements were made using two separate central pathways: vestibular evoked myogenic potentials (VEMPs), which are a manifestation of vestibulo-collic projections, and ocular vestibular evoked myogenic potentials (OVEMPs), which are a manifestation of vestibulo-ocular projections. For both response pathways air-conducted sound and vibration stimuli produced the same patterns of quite different tuning. Sound was characterised by a band-pass tuning with best frequency between 400 and 800Hz whereas vibration showed a low-pass type response with a largest response at 100Hz. Our results suggest that the tuning is at least in part due to properties of end-organs themselves, while the 100Hz best frequency may be a specifically utricular feature.

Tuning and sensitivity of the human vestibular system to low-frequency vibration

Mechanoreceptive hair-cells of the vertebrate inner ear have a remarkable sensitivity to displacement, whether excited by sound, whole-body acceleration or substrate-borne vibration. In response to seismic or substrate-borne vibration, thresholds for vestibular afferent fibre activation have been reported in anamniotes (fish and frogs) in the range -120 to -90 dB re 1g. In this article, we demonstrate for the first time that the human vestibular system is also extremely sensitive to low-frequency and infrasound vibrations by making use of a new technique for measuring vestibular activation, via the vestibulo-ocular reflex (VOR). We found a highly tuned response to whole-head vibration in the transmastoid plane with a best frequency of about 100 Hz. At the best frequency we obtained VOR responses at intensities of less than -70 dB re 1g, which was 15 dB lower than the threshold of hearing for bone-conducted sound in humans at this frequency. Given the likely synaptic attenuation of the VOR pathway, human receptor sensitivity is probably an order of magnitude lower, thus approaching the seismic sensitivity of the frog ear. These results extend our knowledge of vibration-sensitivity of vestibular afferents but also are remarkable as they indicate that the seismic sensitivity of the human vestibular system exceeds that of the cochlea for low-frequencies.

Ocular vestibular evoked myogenic potentials (OVEMPs) produced by impulsive transmastoid accelerations

OBJECTIVEnRecent work has demonstrated the existence of ocular vestibular evoked myogenic potentials (OVEMPs), which likely reflect projections underlying the translational vestibular ocular reflex (TVOR). We examined extraocular muscle activity associated with impulsive acceleration of the head in the transmastoid plane.nnnMETHODSnAccelerometry was measured in 4 subjects in response to acceleration impulses produced by a gamma function delivered with a Minishaker (4810, Bruel & Kjaer). This stimulus produced peak head accelerations of 0.13-0.14 g occurring at between 3.1 and 4.0 ms at the mastoids for both right and left head movement. OVEMPs were recorded in 10 normal subjects with 5 directions of gaze, using electrode pairs placed lateral to, above and below the eyes.nnnRESULTSnOVEMPs occurred at short latency, with initial peaks between 10.3 ms (p10) and 15.3 ms (n15). For a given recording site and gaze direction, the responses were determined solely by the direction of imposed acceleration.nnnCONCLUSIONSnWe propose that, given the transtemporal nature of the stimuli, utricular afferents are likely to be powerfully activated. The OVEMPs evoked may be generated by the lateral recti and oblique muscles.nnnSIGNIFICANCEnSudden lateral accelerations of the head evoke the translational VOR and ocular counter rolling reflex and the pattern of muscle activations indicated by the OVEMPs appear to be a manifestation of these reflexes.

Towards an auditory account of speech rhythm: application of a model of the auditory `primal sketch¿ to two multi-language corpora

The worlds languages display important differences in their rhythmic organization; most particularly, different languages seem to privilege different phonological units (mora, syllable, or stress foot) as their basic rhythmic unit. There is now considerable evidence that such differences have important consequences for crucial aspects of language acquisition and processing. Several questions remain, however, as to what exactly characterizes the rhythmic differences, how they are manifested at an auditory/acoustic level and how listeners, whether adult native speakers or young infants, process rhythmic information. In this paper it is proposed that the crucial determinant of rhythmic organization is the variability in the auditory prominence of phonetic events. In order to test this auditory prominence hypothesis, an auditory model is run on two multi-language data-sets, the first consisting of matched pairs of English and French sentences, and the second consisting of French, Italian, English and Dutch sentences. The model is based on a theory of the auditory primal sketch, and generates a primitive representation of an acoustic signal (the rhythmogram) which yields a crude segmentation of the speech signal and assigns prominence values to the obtained sequence of events. Its performance is compared with that of several recently proposed phonetic measures of vocalic and consonantal variability.

A source analysis of short-latency vestibular evoked potentials produced by air- and bone-conducted sound

OBJECTIVEnTo map short-latency vestibular evoked potentials (VsEPs) using air- (AC) and bone-conducted (BC) sound and to perform source analysis to determine their origin.nnnMETHODSnTen normal volunteers, chosen to have low-normal thresholds for acoustic vestibular activation, participated. In the first part, the subjects individual thresholds for vestibular activation (V(T)) were established using vestibular evoked myogenic potentials (VEMPs) recorded from the sternocleidomastoid muscles. AC sound was delivered with headphones and BC sound with a commercial B71 bone vibrator. In the second part, VsEPs were recorded using Ag/AgCl scalp electrodes in a 10-20 montage supplemented by infra-ocular, mastoid and cerebellar electrodes. Stimuli were 2ms pips, consisting of a single cycle of 500 Hz, presented at +18 dB re V(T) (vestibular condition) and -3 dB re V(T) (control condition).nnnRESULTSnFollowing the control stimulus, auditory mid-latency responses (MLRs) were observed. In the vestibular condition, two dominant groups of non-MLR potentials of presumed vestibular origin appeared (vestibular evoked potentials, or VsEPs), which consisted of a P10-N17 complex maximal at Pz, and an N15-P21 complex maximal at Fpz. Large potentials were also recorded from the infra-ocular electrodes at similar latencies. Source analysis indicated that the two complexes were largely accounted for by a combination of ocular vestibular evoked myogenic potentials (OVEMPs) and sub-cortical sources (possibly vestibular cerebellum), with a smaller contribution from anterior cortical and other myogenic sources.nnnCONCLUSIONSnBoth the N15 and P10 potentials appear to receive an ocular myogenic contribution but both appear also to receive a contribution from other central structures.nnnSIGNIFICANCEnThe P10 and N15 complexes appear to represent the activity of otolith-dependent projections.

Ocular vestibular evoked myogenic potentials produced by impulsive lateral acceleration in unilateral vestibular dysfunction

OBJECTIVEnTo deduce the connectivity underlying ocular vestibular evoked myogenic potentials (OVEMPs) recorded from two sites and produced by lateral transmastoid stimulation in patients with unilateral vestibular dysfunction.nnnMETHODSnOVEMPs were recorded using lateral impulsive stimuli delivered by a hand-held minishaker placed at the mastoid. Twelve patients were tested using the typical OVEMP recording montage placed inferior to the eyes. In a subset of 6 patients, recordings were also made using a lateral electrode montage. The majority of patients were tested following surgery for inner ear disease. Patient responses were compared to those in normal subjects under similar recording conditions.nnnRESULTSnFor the inferior montage, regardless of which mastoid was stimulated, deficits were observed only from the eye opposite the affected ear. In contrast, OVEMPs recorded using the lateral electrode montage showed changes on both sides.nnnCONCLUSIONSnOVEMPs produced using lateral transmastoid stimulation and recorded from beneath the eyes are generated by a crossed vestibulo-ocular pathway while the projections underlying the lateral responses are likely to be bilateral.nnnSIGNIFICANCEnThe vestibular-ocular connectivity underlying the OVEMPs recorded from inferior and lateral recording sites differs. For clinical use, the inferior recording site is the simplest to interpret.

Low-frequency tuning in the human vestibular–ocular projection is determined by both peripheral and central mechanisms

We recently reported that a major contribution to the low-frequency tuning and sensitivity of the human vestibular system is the biomechanical properties of the vestibular end-organs. In the current paper, we investigate the contribution of additional mechanisms to low-frequency tuning. We compared the response properties of the vestibular system in 6 human volunteers to trains of 2 ms pulses of sound and transmastoid vibration using pulse repetition frequencies of 12.5, 25, 50, 100, 200 and 400 Hz. Measurements were made using two separate pathways arising from the vestibular apparatus: to the neck using vestibular evoked myogenic potentials (VEMPs), and to the eyes using ocular vestibular evoked myogenic potentials (OVEMPs). For both sound and vibration the two response pathways produced different tuning to pulse trains. The vestibulo-ocular pathway was characterised by a band-pass tuning with best frequency of 100 Hz whereas the vestibulo-collic pathway showed a peak at 400 Hz with sound only. These results suggest that properties of the vestibulo-ocular pathway also contribute to the low-frequency tuning that occurs for the OVEMP, in addition to previously reported end-organ effects.

Vestibular receptors contribute to cortical auditory evoked potentials.

Acoustic sensitivity of the vestibular apparatus is well-established, but the contribution of vestibular receptors to the late auditory evoked potentials of cortical origin is unknown. Evoked potentials from 500 Hz tone pips were recorded using 70 channel EEG at several intensities below and above the vestibular acoustic threshold, as determined by vestibular evoked myogenic potentials (VEMPs). In healthy subjects both auditory mid- and long-latency auditory evoked potentials (AEPs), consisting of Na, Pa, N1 and P2 waves, were observed in the sub-threshold conditions. However, in passing through the vestibular threshold, systematic changes were observed in the morphology of the potentials and in the intensity dependence of their amplitude and latency. These changes were absent in a patient without functioning vestibular receptors. In particular, for the healthy subjects there was a fronto-central negativity, which appeared at about 42 ms, referred to as an N42, prior to the AEP N1. Source analysis of both the N42 and N1 indicated involvement of cingulate cortex, as well as bilateral superior temporal cortex. Our findings are best explained by vestibular receptors contributing to what were hitherto considered as purely auditory evoked potentials and in addition tentatively identify a new component that appears to be primarily of vestibular origin.