Greg A. O'Beirne

Effects of eye rotation on the sound-evoked post-auricular muscle response (PAMR).

One objective electrophysiological test for deafness involves presenting a brief acoustic stimulus to a subject and measuring the electrical activity evoked in the muscle located just behind the ear (the post-auricular muscle or PAM). We describe a method for enhancing this post-auricular muscle response (PAMR) using lateral eye movement, which increases both the tonic EMG activity in the PAM and the magnitude of the PAMR, and decreases response latency. EMG activity in most subjects tested (more than 30) increased almost instantly on rotation of the eyes, and thereafter grew more slowly with maintained lateral gaze, with the largest increase occurring with eye rotation towards rather than away from the measurement electrodes over the PAM. The EMG activity returned rapidly to near pre-rotation levels when the eyes were returned to the forwards position, with full recovery taking some minutes. While there was a similar increase and return of the PAMR amplitude with eye rotation, the time-course of these changes was somewhat different, largely because the EMG activity and the PAMR amplitude were not proportional. Rather the PAMR amplitude was a saturating function of EMG level, so that the PAMR response did not fall as markedly as the EMG when the eyes were returned to a forwards gaze, and the recovery of the PAMR amplitude to pre-rotation levels appeared to take longer. We discuss the neural mechanisms that may be responsible for this PAMR potentiation with eye movement and discuss its probable role in increasing variability in early studies which did not control for eye movement. We also discuss the utility of eye rotation in potentiating and stabilising the PAMR to allow its use in screening for deafness.

Test–retest reliability of motor evoked potentials (MEPs) at the submental muscle group during volitional swallowing

Motor evoked potentials (MEPs) recorded from pharyngeal and anterior hyo-mandibular (submental) muscles at rest have been used to evaluate treatment effects on neural pathways underlying swallowing. This study documents a novel methodological approach of recording reliable intra- and inter-session MEPs at the submental muscle group during task-related volitional swallowing. MEPs were elicited by single-pulse transcranial magnetic stimulation (TMS), triggered by a custom-made system when a pre-set level of surface electromyographic activity in the target muscles was breached. Fifteen MEPs were recorded during each of four sessions. Intraclass correlation coefficients (ICCs) were used to assess test-retest reliability within and across sessions for blocks of 3, 5, 10 and 15 trials. Highly reliable intra-session reliability was achieved, maximal for blocks of five trials (0.915). Inter-session reliability varied between 0.474 (three trials per block) and 0.909 (10 trials per block). Surface electromyography-triggered TMS allows reliable measurement of MEP amplitude at the submental muscle group within and across sessions when muscles are pre-activated during volitional swallowing. This methodology will be useful for future investigations on the effects of pathology and modulation of swallowing neural pathways.

The role of linguistic and indexical information in improved recognition of dysarthric speech

This investigation examined perceptual learning of dysarthric speech. Forty listeners were randomly assigned to one of two identification training tasks, aimed at highlighting either the linguistic (word identification task) or indexical (speaker identification task) properties of the neurologically degraded signal. Twenty additional listeners served as a control group, passively exposed to the training stimuli. Immediately following exposure to dysarthric speech, all three listener groups completed an identical phrase transcription task. Analysis of listener transcripts revealed remarkably similar intelligibility improvements for listeners trained to attend to either the linguistic or the indexical properties of the signal. Perceptual learning effects were also evaluated with regards to underlying error patterns indicative of segmental and suprasegmental processing. The findings of this study suggest that elements within both the linguistic and indexical properties of the dysarthric signal are learnable and interact to promote improved processing of this type and severity of speech degradation. Thus, the current study extends support for the development of a model of perceptual processing in which the learning of indexical properties is encoded and retained in conjunction with linguistic properties of the signal.

Transtympanic Electrocochleography for the Diagnosis of Ménière's Disease

This paper evaluated the diagnostic power of electrocochleography (ECochG) in detecting Ménières disease (MD) as compared with two subjective assessment methods, including the clinical guidelines provided by the American Academy of Otolaryngology—Head and Neck Surgery Committee on Hearing Equilibrium and the Gibson score. A retrospective study of 250 suspected MD cases was conducted. The agreement between the three assessment methods was found to be relatively high, with a total reliability being higher than 70%. Participants who tested “positive” with ECochG exhibited a higher occurrence rate of asymmetric hearing threshold as well as the four MD symptoms, namely, vertigo, hearing loss, tinnitus, and aural fullness. The “positive” ECochG group also showed a high correlation between the ECochG measures in response to stimuli at adjacent frequency ranges, suggesting that the interfrequency ECochG correspondence may be sensitive to the presence of endolymphatic hydrops and thus may serve as a useful diagnostic marker for MD.

The post-auricular muscle response: an objective electrophysiological method for evaluating hearing sensitivity

Post-auricular muscle responses (PAMRs) were recorded in sixteen adults with normal hearing and twenty adults with sensorineural hearing loss. Click stimuli were presented at 20 to 80 dB nHL via insert earphones. Only one ear was tested in hearing-impaired subjects, but normal-hearing subjects were tested monaurally and binaurally. PAMR amplitudes declined and latencies increased with decreasing click intensity. Both binaural stimulation and eye turn enhanced the PAMR. In hearing-impaired subjects, PAMR thresholds were correlated with audiometric thresholds for the eyes-turned condition. All normal-hearing subjects had PAMR when recording conditions were optimized and half had responses for the least optimal condition (20 dB nHL, monaural, eyes front). With eyes turned and monaural clicks at 35 dB nHL, the level widely used for infant hearing screening, most normal-hearing adults had a PAMR. Thus the PAMR is a robust response that may be a useful adjunct to ABR for objective hearing assessment. Sumario Las respuestas del músculo post-auricular (PAMR) fueron registradas en 16 adultos con audición normal y 20 adultos con hipoacusia sensorineural. Se presentaron estímulos con clics a 20 y 80 dB nHL por medio de auriculares de inserción. Solo se evaluó un oído en los hipoacúsicos pero los sujetos normales fueron evaluados mono y binauralmente. Las amplitudes del PAMR declinaron y las latencias aumentaron conforme la intensidad del clíc disminuyó. Tanto la estimulación binaural y el giro ocular aumentaron el PAMR. En hipoacusicos los umbrales PAMR se correlacionaron con los umbrales audiométricos en la condición de ojos girados. Todos los normales tuvieron PAMR cuando se optimizaron las condiciones de registro y la mitad tuvieron respuestas en la condición menos óptima (20 dB nHL, monoaural y con los ojos al frente). Con los ojos girados y con clics monoaurales en 35dB nHL, el nivel mas ampliamente usado para la identificación auditiva neonatal, la mayoría de los adultos con audición normal tuvieron PAMR. Por ello se considera que el PAMR es una respuesta robusta que puede ser un útil accesorio para las ABR en la evaluación objetiva de la audición.

Cortical representation of tympanic membrane movements due to pressure variation: an fMRI study.

Middle ear sensory information has never been localized in the homunculus of the somatosensory cortex (S1). We investigated the somatosensory representation of the middle ear in 15 normal hearing subjects. We applied small air pressure variations to the tympanic membrane while performing a 3T‐fMRI study. Unilateral stimulations of the right ear triggered bilateral activations in the caudal part of the postcentral gyrus in Brodmann area 43 (BA 43) and in the auditory associative areas 42 (BA 42) and 22 (BA 22). BA 43 has been found to be involved in activities accompanying oral intake and could be more largely involved in pressure activities in the oropharynx area. The tympanic membrane is indirectly related to the pharynx area through the action of tensor tympani, which is a Eustachian tube muscle. The Eustachian tube muscles have a role in pressure equalization in the middle ear and also have a role in the pharyngeal phase of swallowing. Activation of BA 42 and BA 22 could reflect activations associated with the bilateral acoustic reflex triggered prior to self‐vocalization to adjust air pressure in the oropharynx during speech. We propose that BA 43, 42, and 22 are the cortical areas associated with middle ear function. We did not find representation of tympanic membrane movements due to pressure in S1, but its representation in the postcentral gyrus in BA 43 seems to suggest that at least part of this area conveys pure somatosensory information. Hum Brain Mapp, 2011.

Mathematical model of outer hair cell regulation including ion transport and cell motility

To understand the regulatory processes within the cochlea, and outer hair cells (OHCs) in particular, we have developed a mathematical model of OHC regulation that takes into account their known electrical properties, and includes fast and slow somatic motility. We model how cytosolic Ca(2+) is involved in regulation of (i) the OHC membrane potential, (ii) the operating point of OHC mechano-electrical transduction (MET) channels via slow motility; (iii) basolateral wall K(+) permeability via Ca(2+)-sensitive K(+) channels; and (iv) cytosolic concentrations of Ca(2+) itself, via Ca(2+)-ATPase-mediated sequestration within the OHCs and Ca(2+)-induced Ca(2+)-release (CICR) from the same intracellular Ca(2+) storage organelles. To account for some aspects of the cochleas transient response to experimental perturbations, we have included a putative intracellular second-messenger cascade based on cytosolic Ca(2+). Overall, the OHC basolateral permeability determines the resting membrane potential of the OHCs and their standing current, which influences the endocochlear potential, and also affects the AC receptor potential that drives the prestin-mediated somatic electromotility and active cochlear gain. The model we have developed provides a physiologically-plausible and internally-consistent explanation for the time-courses of the cochlear changes we have observed during a number of different experimental perturbations, including a slow oscillatory behaviour presumed due to oscillations in cytosolic Ca(2+) concentration. We also show how the known ionic mechanisms within OHCs act to regulate membrane potential and hair bundle angle over a very wide range of strial current and intracochlear hydrostatic pressure. Not included in the model are osmotic effects, the nonlinear aspects of prestins electromotility, the intracellular role of Cl(-) in modifying this motility, nor adaptation of MET at the apex of OHCs. Only one Ca(2+) sequestration compartment has been included in this implementation of the model, with the two types of basolateral Ca(2+) cisternae combined into a single compartment. Despite these limitations, the model as presented offers insights into the regulation of OHC membrane potential and MET at the hair cell apex, and is our first step in understanding in a quantitative way the integrated function of the molecular components of ion transport and motility in these cells.

A correlation method for detecting the sound-evoked post-auricular muscle response (PAMR).

We have made detailed measurements of the sound-evoked post-auricular muscle response (PAMR) in four adults and two infants, in an attempt to understand the inter-relationships between sound level, potentiation of the PAMR with voluntary PAM contraction or eye rotation, electromyographic (EMG) noise, amplitude of the PAMR, and a correlation measure of the presence of the PAMR. We have found that the amplitude of the PAMR is a simple linear function of the decibel level of a monophasic click (0.1 ms duration), and that the PAMR amplitude is also a saturating power function of the level of tonic EMG. As a result, PAMR=PAM(o).SL. (EMG-EMG(noise))(2)/[(EMG-EMG(noise))(2)+beta(2)], where SL is the decibel level of a click above subjective threshold, PAM(o) is a parameter accounting for the differing PAMR amplitude across individuals or with altered electrode placement, EMG(noise) is the component of EMG not associated with PAMR potentiation, and beta determines the initial rate of growth of PAMR at low levels of PAM activation. We have also found that the correlation measure (C) of the PAMR follows a saturating power function of the signal-to-noise ratio (SNR=PAMR/EMG), with C=SNR(2)/(SNR(2)+delta(2)), where delta determines the onset of saturation in the correlation as a function of SNR. The combination of these two relationships means that correlation is a non-monotonic function of the EMG (PAM activation): it can be large for moderate levels of EMG, but small for high levels of EMG, because the PAMR amplitude saturates but the EMG does not. The correlation is a fast, convenient means of detecting the PAMR, whether using clicks or tone-bursts, and can be used effectively in adults or infants, as long as the reflex is moderately activated. This moderate activation is most effectively produced by eye rotation towards the recording electrodes.

Rising-frequency chirps and earphones with an extended high-frequency response enhance the post-auricular muscle response

The purpose of this study was to determine whether rising-frequency chirps presented via earphones with an extended high-frequency response would optimize the post-auricular muscle response (PAMR). The PAMR was recorded in adults using three different stimuli (a click, a rising-frequency chirp, and a truncated speech stimulus, /t/). Conventional ER-3A insert earphones were compared to ER-2 insert earphones to determine whether the PAMR is enhanced by the ER-2s extended high-frequency response. There were significant stimulus and earphone effects on PAMR amplitudes. The PAMR was largest for the chirp stimulus and the ER-2 earphones. The poorest responses were obtained using the /t/ stimulus and conventional ER-3A earphones. The results support previous ABR studies that have demonstrated a significant advantage of chirps over clicks for evoked response audiometry, and indicate that the PAMR is enhanced by inclusion of additional high-frequency stimulus energy. Sumario El propósito de este estudio fue determinar si los gorjeos de frecuencia ascendente presentados con auriculares de respuesta extendida en las frecuencias agudas, optimizan la respuesta del músculo post-auricular (PAMR). El PAMR se registró en adultos que usaron tres estímulos diferentes (clic, gorjeo de frecuencia ascendente y estimulo truncado de lenguaje, /t/). Se compararon auriculares de inserción convencionales ER-3A con auriculares de inserción ER-2 para determinar si el PAMR mejora con las respuestas de alta frecuencia extendida del ER-2. Se apreciaron efectos significativos en el estímulo y en los auriculares sobre las amplitudes del PAMR. EL PAMR fue más grande para los estímulos con gorjeo y con los auriculares ER-2. Las respuestas mas pobres se dieron usando el estimulo /t/ y los auriculares convencionales ER-3A. Estos resultados apoyan estudios previos de ABR que demostraron una ventaja significativa de los gorjeos sobre los clics en la audiometría de respuestas evocadas e indican que el PAMR se incrementa con la inclusión adicional de la energía de estímulos de altas frecuencias.

Boltzmann analysis of CM waveforms using virtual instrument software

We describe a modification to our technique for the rapid analysis of low-frequency cochlear microphonic (CM) waveforms in the basal turn of the guinea pig cochlea (Patuzzi and Moleirinho, 1998). The transfer curve relating instantaneous sound pressure in the ear canal to instantaneous receptor current through the outer hair cells (OHCs) is determined from the distorted microphonic waveform generated in the extracellular fluid near the hair cells, assuming a first-order Boltzmann activation curve. Previously, the analysis was done in real time using custom-built electronic circuitry. Here, the same task is performed numerically using virtual instrument software (National Instruments LabVIEW 4.1) running on a personal computer. The assumed theoretical function describing the CM waveform is Vcm = Voff + Vsat/[1 + exp[(Eo+Z.Po.sin(2pi f + phi(tot)))/kT]], where the six parameters are (i) a DC offset voltage (Voff); (ii) the frequency of the sinusoidal stimulus (f); (iii) the phase of the sinusoidal stimulus (phi(tot)); (iv) the maximal amplitude of the distorted microphonic signal (Vsat); (v) the sensitivity of the transduction process (Z); and (vi) the operating point on the sigmoidal transfer curve (Eo). The software obtains the least-squares fit to the CM waveforms by continuously deriving the six parameters at a speed of about one determination per second. The independent fitting of the frequency and phase allows the data to be analysed off-line from data previously recorded to tape (i.e. the frequency and phase of the microphonic response need not be known accurately beforehand). We present here an outline of the software we have used, and give an example of the changes which can be monitored using the technique (transient asphyxia). The methods advantages and limitations have been discussed in our previous paper. The virtual instrument described here is available from the authors on request.