Prepulse inhibition and facilitation of the postauricular reflex, a vestigial remnant of pinna startle

Gap-induced inhibition of the post-auricular muscle response in miniature pigs

BACKGROUND

Tinnitus, a persistent condition that significantly impairs the quality of life for patients, poses a substantial burden on society. Owing to its complex and poorly understood pathogenesis, the development of effective animal models for tinnitus is imperative to enhance our comprehension of this condition.

OBJECTIVES

The present study aims to establish a large animal model to delineate the mechanisms underlying the onset of tinnitus.

MATERIALS AND METHODS

For the first time, we conducted a comparative analysis of the Post-Auricular Muscle Response (PAMR) and the Eye Blink Response (EBR) in miniature pigs, demonstrating their utility in manifesting Gap Prepulse Inhibition of the Acoustic Startle (GPIAS). Subsequently, we employed the gap-induced inhibition of PAMR to ascertain evidence of tinnitus following noise exposure.

RESULTS AND CONCLUSION

Through comparing the efficacy of PAMR and EBR in GPIAS detection, we have substantiated that PAMR provides a stable and objective reflection of GPIAS. Consequently, PAMR emerges as a reliable method for measuring GPIAS in miniature pigs. Furthermore, the behavioral paradigm of gap-induced inhibition of PAMR successfully identified behavioral evidence of tinnitus subsequent to noise exposure.

Direct cochlear nerve stimulation monitoring through evoked muscle responses during retrosigmoid vestibular schwannoma resection surgery: technical note

OBJECTIVE

Cochlear nerve preservation during surgery for vestibular schwannoma (VS) may be challenging. Brainstem auditory evoked potentials and cochlear compound nerve action potentials have clearly shown their limitations in surgeries for large VSs. In this paper, the authors report their preliminary results after direct electrical intraoperative cochlear nerve stimulation and recording of the postauricular muscle response (PAMR) during resection of large VSs.

METHODS

The details for the electrode setup, stimulation, and recording parameters are provided. Data of patients for whom PAMR was recorded during surgery were prospectively collected and analyzed.

RESULTS

PAMRs were recorded in all patients at the ipsilateral vertex-earlobe scalp electrode, and in 90% of the patients they were also observed in the contralateral electrode. The optimal stimulation intensity was found to be 1 mA at 1 Hz, with a good cochlear response and an absent response from other nerves. At that intensity, the ipsilateral cochlear response had an initial peak at a mean (± SEM) latency of 11.6 ± 1.5 msec with an average amplitude of 14.4 ± 5.4 µV. One patient experienced a significant improvement in his audition, while that of the other patients remained stable.

CONCLUSIONS

PAMR monitoring may be useful in mapping the position and trajectory of the cochlear nerve to enable hearing preservation during surgery.

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.

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.

Vestibular Evoked Myographic Correlation

This work started from the hypothesis that the physiological processes giving rise to the vestibular evoked myogenic potential (VEMP) can be induced not only by transient sounds but also by a continuous stimulation with a stochastic signal. The hypothesis is based on the idea that the number of motor unit action potentials (MUAPs) decreases after a momentary amplitude increase of the effective stimulus, whereas a momentary amplitude decrease has the opposite effect. This concept was theoretically analyzed by assuming that the effective stimulus is closely related to the envelope of the stimulus actually presented. The analysis led to the prediction that the cross-correlation function of the effective stimulus and the measured electromyogram (EMG) has VEMP-like properties. Experiments confirmed this prediction, thus providing evidence of a novel electrophysiological response: the vestibular evoked myographic correlation (VEMCorr). The methodological approach corresponded to a conventional VEMP study, except that the stimulus (delivered with a hand-held minishaker) comprised not only a series of 500-Hz tone pulses (classical VEMP measurement, for comparison) but also sequences of narrow-band noise with a center frequency of 500 Hz (VEMCorr measurement). Each of the 12 test persons showed a clear VEMCorr. Moreover, VEMP and VEMCorr largely resembled each other, as predicted. Apparently they are two different expressions of a more general mechanism that leads to a roughly linear relationship between stimulus envelope and expectation of the EMG. Future applications of the VEMCorr could exploit that a continuous-stimulation paradigm allows for varying the center frequency of the stimulus without changing the relative bandwidth.

Measuring Pavlovian appetitive conditioning in humans with the postauricular reflex

Despite its evolutionary and clinical significance, appetitive conditioning has been rarely investigated in humans. It has been proposed that this discrepancy might stem from the difficulty in finding suitable appetitive stimuli that elicit strong physiological responses. However, this might also be due to a possible lack of sensitivity of the psychophysiological measures commonly used to index human appetitive conditioning. Here, we investigated whether the postauricular reflex—a vestigial muscle microreflex that is potentiated by pleasant stimuli relative to neutral and unpleasant stimuli—may provide a valid psychophysiological indicator of appetitive conditioning in humans. To this end, we used a delay differential appetitive conditioning procedure, in which a neutral stimulus was contingently paired with a pleasant odor (CS+), while another neutral stimulus was not associated with any odor (CS−). We measured the postauricular reflex, the startle eyeblink reflex, and skin conductance response (SCR) as learning indices. Taken together, our results indicate that the postauricular reflex was potentiated in response to the CS+ compared with the CS−, whereas this potentiation extinguished when the pleasant odor was no longer delivered. In contrast, we found no evidence for startle eyeblink reflex attenuation in response to the CS+ relative to the CS−, and no effect of appetitive conditioning was observed on SCR. These findings suggest that the postauricular reflex is a sensitive measure of human appetitive conditioning and constitutes a valuable tool for further shedding light on the basic mechanisms underlying emotional learning in humans.

Understanding Mixed Emotions: Paradigms and Measures

In this review, we examine the paradigms and measures available for experimentally studying mixed emotions in the laboratory. For eliciting mixed emotions, we describe a mixed emotions film library that allows for the repeated elicitation of a specific homogeneous mixed emotional state and appropriately matched pure positive, pure negative, and neutral emotional states. For assessing mixed emotions, we consider subjective and objective measures that fall into univariate, bivariate, and multivariate measurement categories. As paradigms and measures for objectively studying mixed emotions are still in their early stages, we conclude by outlining future directions that focus on the reliability, temporal dynamics, and response coherence of mixed emotions paradigms and measures. This research will build a strong foundation for future studies and significantly advance our understanding of mixed emotions.