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

Myogenic artifacts masquerade as neuroplasticity in the auditory frequency-following response

The frequency-following response (FFR) is an evoked potential that provides a neural index of complex sound encoding in the brain. FFRs have been widely used to characterize speech and music processing, experience-dependent neuroplasticity (e.g., learning and musicianship), and biomarkers for hearing and language-based disorders that distort receptive communication abilities. It is widely assumed that FFRs stem from a mixture of phase-locked neurogenic activity from the brainstem and cortical structures along the hearing neuraxis. In this study, we challenge this prevailing view by demonstrating that upwards of ~50% of the FFR can originate from an unexpected myogenic source: contamination from the postauricular muscle (PAM) vestigial startle reflex. We measured PAM, transient auditory brainstem responses (ABRs), and sustained frequency-following response (FFR) potentials reflecting myogenic (PAM) and neurogenic (ABR/FFR) responses in young, normal-hearing listeners with varying degrees of musical training. We first establish that PAM artifact is present in all ears, varies with electrode proximity to the muscle, and can be experimentally manipulated by directing listeners' eye gaze toward the ear of sound stimulation. We then show this muscular noise easily confounds auditory FFRs, spuriously amplifying responses 3-4-fold with tandem PAM contraction and even explaining putative FFR enhancements observed in highly skilled musicians. Our findings expose a new and unrecognized myogenic source to the FFR that drives its large inter-subject variability and cast doubt on whether changes in the response typically attributed to neuroplasticity/pathology are solely of brain origin.

Myogenic artifacts masquerade as neuroplasticity in the auditory frequency-following response (FFR)

The frequency-following response (FFR) is an evoked potential that provides a "neural fingerprint" of complex sound encoding in the brain. FFRs have been widely used to characterize speech and music processing, experience-dependent neuroplasticity (e.g., learning, musicianship), and biomarkers for hearing and language-based disorders that distort receptive communication abilities. It is widely assumed FFRs stem from a mixture of phase-locked neurogenic activity from brainstem and cortical structures along the hearing neuraxis. Here, we challenge this prevailing view by demonstrating upwards of ~50% of the FFR can originate from a non-neural source: contamination from the postauricular muscle (PAM) vestigial startle reflex. We first establish PAM artifact is present in all ears, varies with electrode proximity to the muscle, and can be experimentally manipulated by directing listeners' eye gaze toward the ear of sound stimulation. We then show this muscular noise easily confounds auditory FFRs, spuriously amplifying responses by 3-4x fold with tandem PAM contraction and even explaining putative FFR enhancements observed in highly skilled musicians. Our findings expose a new and unrecognized myogenic source to the FFR that drives its large inter-subject variability and cast doubt on whether changes in the response typically attributed to neuroplasticity/pathology are solely of brain origin.

Assessment of Vestigial Auriculomotor Activity to Acoustic Stimuli Using Electrodes In and Around the Ear

Recently, it has been demonstrated that electromyographic (EMG) activity of auricular muscles in humans, especially the postauricular muscle (PAM), depends on the spatial location of auditory stimuli. This observation has only been shown using wet electrodes placed directly on auricular muscles. To move towards a more applied, out-of-the-laboratory setting, this study aims to investigate if similar results can be obtained using electrodes placed in custom-fitted earpieces. Furthermore, with the exception of the ground electrode, only dry-contact electrodes were used to record EMG signals, which require little to no skin preparation and can therefore be applied extremely fast. In two experiments, auditory stimuli were presented to ten participants from different spatial directions. In experiment 1, stimuli were rapid onset naturalistic stimuli presented in silence, and in experiment 2, the corresponding participant's first name, presented in a "cocktail party" environment. In both experiments, ipsilateral responses were significantly larger than contralateral responses. Furthermore, machine learning models objectively decoded the direction of stimuli significantly above chance level on a single trial basis (PAM: ≈ 80%, in-ear: ≈ 69%). There were no significant differences when participants repeated the experiments after several weeks. This study provides evidence that auricular muscle responses can be recorded reliably using an almost entirely dry-contact in-ear electrode system. The location of the PAM, and the fact that in-ear electrodes can record comparable signals, would make hearing aids interesting devices to record these auricular EMG signals and potentially utilize them as control signals in the future.

Physiological oculo-auricular-facial-mandibular synkinesis elicited in humans by gaze deviations

Integrated motor behaviors involving ocular motion-associated movements of the head, neck, pinna, and parts of the face are commonly seen in animals orienting to a visual target. A number of coordinated movements have also been observed in humans making rapid gaze shifts to horizontal extremes, which may be vestiges of these. Since such integrated mechanisms point to a non-pathological co-activation of several anatomically separate cranial circuits in humans, it is important to see how the different pairs of integrative motor behaviors with a common trigger (i.e., ocular motion) manifest in relation to one another. Here, we systematically examined the pattern of eye movement-induced recruitment of multiple cranial muscles in humans. Simultaneous video-oculography and bilateral surface electromyograms of transverse auricular, temporalis, frontalis, and masseter muscles were recorded in 15 healthy subjects (8 females; 29.3±5.2 years) while they made head-fixed, horizontal saccadic, pursuit and optokinetic eye movements. Potential chin laterotrusion linked to contractions of masticator muscles was captured with a yaw-fixed accelerometer. Our findings objectively show an orchestrated aural-facial-masticatory muscle response to a range of horizontal eye movements (prevalence of 21-93%). These responses were most prominent during eccentric saccades. We further reveal distinctions between the various observed activation patterns in terms of their profile (transient or sustained), laterality (with respect to direction of gaze) and timing (with respect to saccade onset). Possible underlying neural substrates, their atavistic behavioral significance, and potential clinical applications for monitoring sensory attention and designing attention-directed hearing aids in the future are discussed.

Muscles in and around the ear as the source of "physiological noise" during auditory selective attention: A review and novel synthesis

The sensitivity of the auditory system is regulated via two major efferent pathways: the medial olivocochlear system that connects to the outer hair cells, and by the middle ear muscles-the tensor tympani and stapedius. The role of the former system in suppressing otoacoustic emissions has been extensively studied, but that of the complementary network has not. In studies of selective attention, decreases in otoacoustic emissions from contralateral stimulation have been ascribed to the medial olivocochlear system, but the acknowledged problem is that the results can be confounded by parallel muscle activity. Here, the potential role of the muscle system is examined through a wide but not exhaustive review of the selective attention literature, and the unifying hypothesis is made that the prominent "physiological noise" detected in such experiments, which is reduced during attention, is the sound produced by the muscles in proximity to the ear-including the middle ear muscles. All muscles produce low-frequency sound during contraction, but the implications for selective attention experiments-in which muscles near the ear are likely to be active-have not been adequately considered. This review and synthesis suggests that selective attention may reduce physiological noise in the ear canal by reducing the activity of muscles close to the ear. Indeed, such an experiment has already been done, but the significance of its findings have not been widely appreciated. Further sets of experiments are needed in this area.

Vestigial auriculomotor activity indicates the direction of auditory attention in humans

Unlike dogs and cats, people do not point their ears as they focus attention on novel, salient, or task-relevant stimuli. Our species may nevertheless have retained a vestigial pinna-orienting system that has persisted as a 'neural fossil’ within in the brain for about 25 million years. Consistent with this hypothesis, we demonstrate that the direction of auditory attention is reflected in sustained electrical activity of muscles within the vestigial auriculomotor system. Surface electromyograms (EMGs) were taken from muscles that either move the pinna or alter its shape. To assess reflexive, stimulus-driven attention we presented novel sounds from speakers at four different lateral locations while the participants silently read a boring text in front of them. To test voluntary, goal-directed attention we instructed participants to listen to a short story coming from one of these speakers, while ignoring a competing story from the corresponding speaker on the opposite side. In both experiments, EMG recordings showed larger activity at the ear on the side of the attended stimulus, but with slightly different patterns. Upward movement (perking) differed according to the lateral focus of attention only during voluntary orienting; rearward folding of the pinna’s upper-lateral edge exhibited such differences only during reflexive orienting. The existence of a pinna-orienting system in humans, one that is experimentally accessible, offers opportunities for basic as well as applied science.

Dogs, cats, monkeys and other animals perk their ears in the direction of sounds they are interested in. Humans and their closest ape relatives, however, appear to have lost this ability. Some humans are able to wiggle their ears, suggesting that some of the brain circuits and muscles that allow automatic ear movements towards sounds are still present. This may be a ‘vestigial feature’, an ability that is maintained even though it no longer serves its original purpose.

Now, Strauss et al. show that vestigial movements of muscles around the ear indicate the direction of sounds a person is paying attention to. In the experiments, human volunteers tried to read a boring text while surprising sounds like a traffic jam, a baby crying, or footsteps played. During this exercise, Strauss et al. recorded the electrical activity in the muscles of their ears to see if they moved in response to the direction the sound came from. In a second set of experiments, the same electrical recordings were made as participants listened to a podcast while a second podcast was playing from a different direction. The individuals’ ears were also recorded using high resolution video.

Both sets of experiments revealed tiny involuntary movements in muscles surrounding the ear closest to the direction of a sound the person is listening to. When the participants tried to listen to one podcast and tune out another, they also made ear ‘perking’ movements in the direction of their preferred podcast.

The results suggest that movements of the vestigial muscles in the human ear indicate the direction of sounds a person is paying attention to. These tiny movements could be used to develop better hearing aids that sense the electrical activity in the ear muscles and amplify sounds the person is trying to focus on, while minimizing other sounds.

Evidence of a Vestibular Origin for Crossed-Sternocleidomastoid Muscle Responses to Air-Conducted Sound

OBJECTIVES

Small, excitatory surface potentials can sometimes be recorded from the contralateral sternocleidomastoid muscle (SCM) following monaural acoustic stimulation. Little is known about the physiological properties of these crossed reflexes. In this study, we sought the properties of crossed SCM responses and through comparison with other cochlear and vestibular myogenic potentials, their likely receptor origin.

DESIGN

Surface potentials were recorded from the ipsilateral and contralateral SCM and postauricular (PAM) muscles of 11 healthy volunteers, 4 patients with superior canal dehiscence and 1 with profound hearing loss. Air-conducted clicks of 105 dB nHL and tone bursts (250 to 4000 Hz) of 100 dB nHL were presented monaurally through TDH 49 headphones during head elevation. Click-evoked responses were recorded under two conditions of gaze in random order: gaze straight ahead and rotated hard toward the contralateral recording electrodes. Amplitudes (corrected and uncorrected) and latencies for crossed SCM responses were compared with vestibular (ipsilateral SCM) and cochlear (PAM) responses between groups and across the different recording conditions.

RESULTS

Surface waveforms were biphasic; positive-negative for the ipsilateral SCM, and negative-positive for the contralateral SCM and PAM. There were significant differences in the amplitudes and latencies (p = 0.004) for click responses of healthy controls across recording sites. PAM responses had the largest mean-corrected amplitudes (2.3 ± 2.8) and longest latencies (13.0 ± 1.2 msec), compared with ipsilateral (1.6 ± 0.5; 12.0 ± 0.7 msec) and contralateral (0.8 ± 0.3; 10.4 ± 1.0 msec) SCM responses. Uncorrected amplitudes and muscle activation for PAM increased by 104.4% and 46.8% with lateral gaze respectively, whereas SCM responses were not significantly affected. Click responses of patients with superior canal dehiscence followed a similar latency, amplitude, and gaze modulation trend as controls. SCM responses were preserved in the patient with profound hearing loss, yet all PAM were absent. There were significant differences in the frequency tuning of the three reflexes (p < 0.001). Tuning curves of healthy controls were flat for PAM and down sloping for ipsilateral and contralateral SCM responses. For superior canal dehiscence, they were rising for PAM and slightly down sloping for SCM responses.

CONCLUSIONS

Properties of crossed SCM responses were similar, though not identical, to those of ipsilateral SCM responses and are likely to be predominantly vestibular in origin. They are unlikely to represent volume conduction from the PAM as they were unaffected by lateral gaze, were shorter in latency, and had different tuning properties. The influence of crossed vestibulo-collic pathways should be considered when interpreting cervical vestibular-evoked myogenic potentials recorded under conditions of binaural stimulation.

Gap-induced inhibition of the post-auricular muscle response in humans and guinea pigs

A common method for measuring changes in temporal processing sensitivity in both humans and animals makes use of GaP-induced Inhibition of the Acoustic Startle (GPIAS). It is also the basis of a common method for detecting tinnitus in rodents. However, the link to tinnitus has not been properly established because GPIAS has not yet been used to objectively demonstrate tinnitus in humans. In guinea pigs, the Preyer (ear flick) myogenic reflex is an established method for measuring the acoustic startle for the GPIAS test, while in humans, it is the eye-blink reflex. Yet, humans have a vestigial remnant of the Preyer reflex, which can be detected by measuring skin surface potentials associated with the Post-Auricular Muscle Response (PAMR). A similar electrical potential can be measured in guinea pigs and we aimed to show that the PAMR could be used to demonstrate GPIAS in both species.

In guinea pigs, we compare the GPIAS measured using the pinna movement of the Preyer reflex and the electrical potential of the PAMR to demonstrate that the two are at least equivalent. In humans, we establish for the first time that the PAMR provides a reliable way of measuring GPIAS that is a pure acoustic alternative to the multimodal eye-blink reflex. Further exploratory tests showed that while eye gaze position influenced the size of the PAMR response, it did not change the degree of GPIAS.

Our findings confirm that the PAMR is a sensitive method for measuring GPIAS and suggest that it may allow direct comparison of temporal processing between humans and animals and may provide a basis for an objective test of tinnitus.

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Myogenic potentials from the guinea pig pinna show gap induced pre-pulse inhibition.

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Startle inhibition is also shown by gaps in background noise using the Preyer reflex.

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Startle potentials recorded behind the human pinna show gap-induced inhibition.

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Human post-auricular muscle potentials may form an objective test for tinnitus.

Neuroprosthetics for Auricular Muscles: Neural Networks and Clinical Aspects

The mammalian external ear houses extrinsic and intrinsic auricular muscles. There are three extrinsic auricular muscles-the posterior, superior, and anterior auricular muscles-and six intrinsic muscles-the helicis major and minor, tragicus, anti-tragicus, transverse and oblique muscles. These muscles have been considered vestigial in humans. However, numerous therapeutic and diagnostic wearable devices are designed to monitor and alleviate the symptoms of neurological disorders, brainstem injuries, emotional states, and auditory functions, by making use of the neural networks of the auricular muscles and their locations, which are easily accessible for ergonomic wearable biomedical devices. They can also serve as a bio-controller of human neuroprosthetics. The functionality of these auricular muscles remains elusive and requires further experimentation for a more in-depth understanding of their anatomy. The aims of this review are (1) to provide a detailed account of the neural networks of the extrinsic and intrinsic auricular muscles, (2) to describe diagnostic and therapeutic functions of these muscles as demonstrated in the current literature, and (3) to outline existing and potential neuroprosthetic applications making use of the auricular muscles and their neural networks.

Evidence for a vestigial pinna-orienting system in humans

Although some people can voluntarily move their ears, overt reflexive control of the pinnae has been lost during the course of primate evolution. Humans and apes do not move their ears to express emotion, they do not defensively retract them when startled, and they do not point them at novel, salient, or task-relevant stimuli. Nevertheless, it is the thesis of this review that neural circuits for pinna orienting have survived in a purely vestigial state for over 25 million years. There are three lines of evidence: (1) Shifting the eyes hard to one side is accompanied by electromyographic (EMG) activity in certain ear muscles and by a barely visible (2-3 mm) curling of the dorsal edge of the pinna. (2) The capture of attention by a novel, unexpected sound emanating from behind and to one side has been found to trigger a weak EMG response in the muscle behind the corresponding ear. (3) Reflexive EMG bursts recorded during a selective attention task suggested that subjects were unconsciously attempting to orient their ears toward the relevant sounds. In addition to pinna orienting, the possibility that pinna startle might have survived in a vestigial state is also considered. It is suggested that the postauricular reflex to sudden, intense sounds constitutes a vestigial startle response, but that the reflex arc is dominated by a pathway that bypasses the main organizing center for startle.

Rotation of the eyes (not the head) potentiates the postauricular muscle response

OBJECTIVE

The authors investigated how eye and head rotation modulate the human postauricular muscle response (PAMR), to determine the optimal strategy for potentiating the PAMR, or minimizing it to reduce contamination of small neurogenic responses.

DESIGN

The authors recorded the PAMR evoked by binaural bipolar clicks (50 dB SL, 360 µsec per phase with 53-msec interval) from behind the right ear of 12 normally hearing adults, and attempted to enhance it with (a) voluntary ear contraction alone, (b) head rotation alone (with the eyes forward-directed and tracking a target attached to the head), or (c) by lateral eye movement alone (toward the right measurement side, with the head facing forward).

RESULTS

When the head alone was rotated with eyes fixed relative to the head, the PAMR increased only slightly in some subjects, as did the ongoing electromyography (EMG) (probably due to EMG activity from nearby neck muscles). On returning the head to the forward control position, the PAMR dropped rapidly to control amplitudes. When the eyes alone were rotated, both the EMG and the PAMR increased markedly in most subjects, and returning the eyes to their control position produced a gradual return to control amplitudes. Voluntary PAM contraction (in those subjects who could accomplish it) increased the EMG tone and the PAMR amplitude concomitantly, with vigorous voluntary PAM contraction saturating the PAMR but not the EMG.

CONCLUSION

Head rotation alone was not effective in potentiating the PAMR when lateral eye movement relative to the head was avoided during the head rotation maneuver. When lateral eye movement was controlled appropriately, the PAMR could be potentiated reliably, or reliably avoided when recording smaller neurogenic responses. If head rotation was used to optimize the PAMR without explicit control of eye movement, a false impression of variability was produced.

Automatic removal of sonomotor waves from auditory brainstem responses

We have developed a computerized technique for automatic detection and removal of sonomotor waves (SMWs) from auditory brainstem responses (ABRs). Our approach is based on adaptive decomposition using a redundant set of Gaussian and 1-cycle-limited Gabor functions. In order to find optimal parameters and evaluate the efficiency of the methods, simulated data were first used before applying it to clinical data. Results were good and confirmed by an expert with years of clinical experience in ABR evaluation.

The nursing hypothesis: an evolutionary account of emotional modulation of the postauricular reflex

The postauricular reflex (PAR) is anomalous because it seems to be potentiated during positive emotions and inhibited during negative states, unlike eyeblink and other components of the startle reflex. Two evolutionary explanations based on simian facial emotion expressions were tested. Reflexes were elicited while 47 young adult volunteers made lip pursing or grimacing poses and viewed neutral, intimidating, or appetitive photos. The PAR was enhanced during appetitive slides, but only as subjects carried out the lip-pursing maneuver. These results support the nursing hypothesis, which assumes that infant mammals instinctively retract their pinnae while nursing in order to comfortably position the head. Appetitive emotions prime the ear-retraction musculature, even in higher primates whose postauricular muscles are vestigial.

Auditory evoked blink reflex and posterior auricular muscle response: observations in patients with HFS and PFS

Our goal was to investigate the characteristics of the auditory brainstem reflexes in patients with hemifacial spasm (HFS) and postparalytic facial syndrome (PFS). The spasm activities and responses by supraorbital and auditory stimuli were recorded from the orbicularis oculi, the posterior auricular and the mentalis muscles in 27 HFS and 21 PFS patients. The results were compared with those of 20 controls. Blink reflex (BR) was obtained by supraorbital stimulation in normal controls and on both sides of HFS and PFS patients whereas sound evoked bilateral auditory blink reflex (ABR) in 96.3%, 90.5% and 100%, respectively. Both BR and ABR showed synkinetic spread on symptomatic sides in all patients. The posterior auricular muscle response (PAMR) was observed bilaterally in 59.3%, 42.9% and 75.0% of groups, respectively. However, there was no synkinetic spread of PAMR. Since PAMR does not show synkinetic spread even in the presence of synkinetic spread of ABR and BR, we may suggest that a distal origin may be responsible of the synkinetic spread, or PAM is probably governed by a smaller nucleus in the brainstem. Thus it may be speculated that its excitability is insufficient to stimulate the ABR nucleus, whereas the reverse process is possible.

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-2's extended highfrequency 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.

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.

Estimating the operating point of the cochlear transducer using low-frequency biased distortion products

Distortion products in the cochlear microphonic (CM) and in the ear canal in the form of distortion product otoacoustic emissions (DPOAEs) are generated by nonlinear transduction in the cochlea and are related to the resting position of the organ of Corti (OC). A 4.8 Hz acoustic bias tone was used to displace the OC, while the relative amplitude and phase of distortion products evoked by a single tone [most often 500 Hz, 90 dB SPL (sound pressure level)] or two simultaneously presented tones (most often 4 kHz and 4.8 kHz, 80 dB SPL) were monitored. Electrical responses recorded from the round window, scala tympani and scala media of the basal turn, and acoustic emissions in the ear canal were simultaneously measured and compared during the bias. Bias-induced changes in the distortion products were similar to those predicted from computer models of a saturating transducer with a first-order Boltzmann distribution. Our results suggest that biased DPOAEs can be used to non-invasively estimate the OC displacement, producing a measurement equivalent to the transducer operating point obtained via Boltzmann analysis of the basal turn CM. Low-frequency biased DPOAEs might provide a diagnostic tool to objectively diagnose abnormal displacements of the OC, as might occur with endolymphatic hydrops.

Effects of eye position on the vestibular evoked myogenic potential

CONCLUSION

The position of a subject's eyes during vestibular evoked myogenic potential (VEMP) recording significantly alters the magnitude of the response. This change is largely due to an alteration in the tonicity of the sternocleidomastoid muscle (SCM) caused by variations in the position of the eye. However, even if electromyographic (EMG) normalization is conducted effects of eye position remain.

OBJECTIVE

To determine if eye position has a significant effect on the magnitude of the VEMP.

SUBJECTS AND METHODS

VEMPs were collected from 32 ears measured on 16 healthy subjects. The recordings were made unilaterally using the head turn method. The acoustic stimuli were 500 Hz air-conduction short tone bursts. VEMPs were measured in three recording conditions: (i) eyes in the same direction as head turn, (ii) eyes straight ahead, (iii) eyes in the opposite direction to head turn.

RESULTS

All 32 ears tested showed a VEMP response with eyes in all three positions. Repeated measures analysis of variance (RM-ANOVA) verified an overall significant effect of eye position (p<0.001). Post hoc paired t tests revealed statistically significant differences between the eyes opposite and the other two conditions (p<0.001). Normalization of the VEMP magnitude using pre-stimulus EMG reduced the effect; however, some variability remained.

Human auditory nerve compound action potentials and long latency responses

CONCLUSION

The compound action potential (CAP) is followed by a long latency response (LLR), attributable to the post-auricular musculature. The LLR to one pulse may overlap with the CAP to a subsequent one, contributing to the clinically observed reduction in CAP at high pulse rates.

OBJECTIVES

To measure refractory and other influences on CAPs in humans and guinea pigs.

MATERIALS AND METHODS

CAPs were obtained from humans using trans-tympanic and extra-tympanic electrocochleography and from anaesthetized guinea pigs. Stimuli were single pulses presented at a slow rate, pairs of pulses, and 100 ms pulse trains where the inter-pulse interval alternated between 4 and 6 ms.

RESULTS

For single pulses, the CAP shape was similar across species. For pairs of pulses, the CAP to the second pulse was smaller than that to the first, and decreased with increasing inter-pulse interval in a way that was similar across species. For pulse trains, CAPs were observed in response to each pulse in the train for the guinea pigs, but not for humans. For both filtered and unfiltered single pulses, there was a large LLR in humans, but not in guinea pigs, with peaks at latencies of 10-12 and 20-25 ms. Posture affected the LLR in a way consistent with the post-auricular response.

Emotional modulation of the post-auricular reflex

A large literature now exists on emotional modulation of the startle blink reflex. The current study examined affective modulation of the post-auricular reflex, which can be measured in relation to the same noise probe used to evoke the startle reflex. We recorded the post-auricular reflex during viewing of pictures that varied systematically in emotional valence, content, and intensity. A significant linear valence modulation effect was found, with pleasant pictures potentiating and aversive pictures inhibiting the post-auricular reflex in comparison with neutral pictures. This modulatory effect did not vary as a function of picture content, but it was most robust for highly intense emotional pictures. Implications for the assessment of basic emotional action tendencies are discussed.

Human auditory steady-state responses: the effects of recording technique and state of arousal

There is some controversy in the literature about whether auditory steady-state responses (ASSRs) can be reliably recorded in all subjects and whether these responses consistently decrease in amplitude during drowsiness. In 10 subjects, 40-Hz ASSRs became significantly different from background electroencephalogram activity with a probability of P < 0.01 and an average time of 22 s (range, 2-92 s), provided that the responses were analyzed with time-domain averaging rather than spectral averaging. In a second experiment with 10 subjects, 40-Hz ASSRs recorded between the vertex and posterior neck consistently decreased in amplitude during drowsiness and sleep. Findings that the ASSR may occasionally increase during drowsiness may be explained by postauricular muscle responses recorded from a mastoid reference. These may occur during drowsiness in association with rolling-eye movements. ASSRs recorded between the vertex and posterior neck are not distorted by these reflexes. These findings combine with previous literature on the effects of general anesthetics on the ASSR to confirm that the ASSR is a valid option for monitoring the hypnotic effects of general anesthetics.

IMPLICATIONS

Auditory steady-state responses to stimuli presented at rates near 40 Hz can be used to monitor anesthesia. These responses can be quickly and reliably recorded during both sleep and wakefulness, provided that appropriate averaging techniques are used.

Oculo-auricular phenomenon secondary to vestibular dysfunction

The oculo-auricular phenomenon consists of coactivation of the ocular rectus lateralis and the posterior muscles of both ears (transverse auriculae and obliquus auriculae muscles). This coactivation produces a bilateral curling of auricles during extreme lateral gaze that can be observed in as much as an 80% of the normal population. We herein describe a 26-year-old man who presented a transient oculo-auricular phenomenon in the course of a vestibular vertigo.

Basic properties of 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). Although this electrical response has been known for many years, it has been ignored by most clinicians and frequently misreported in the literature. This paper presents the fundamental properties of the PAM electrical response (the PAMR) and examines ways in which its measurement can be improved by altering the standard electrode position and filtering. The response consists of a simple bipolar compound action potential with a first peak latency of between 12.5 and 15 ms, depending on the stimulus intensity and PAM muscle tone. The largest recordings can be made with an active electrode over the PAM and with the reference electrode on the dorsal surface of the pinna. It can be obtained with click and tone-burst stimuli within 20 dB of the subjective detection threshold, can be evoked with tone-bursts between 500 Hz and 16 kHz and grows either linearly with the click level or approximately exponentially with the tone-burst level, reaching a maximum of as large as 250 microV pp in some subjects. It has a frequency spectrum mostly between 25 and 200 Hz. The response is often visible in raw recordings, with as few as 20 averages required for obtaining a stable waveform. There is very little amplitude and latency difference in stimulating the ear on the same side or opposite side to the recording electrodes and the binaurally evoked response is similar to the simple arithmetic sum of the waveforms obtained with monaural stimulation. The response latency and duration are longer in very young infants, but reach adult values by 12 months of age. In a companion paper, we describe a method of enhancing the PAMR using lateral eye movement (Patuzzi and O'Beirne, 1999a).

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.