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

Electromyography of Extrinsic and Intrinsic Ear Muscles in Healthy Probands and Patients with Unilateral Postparalytic Facial Synkinesis

There are currently no data on the electromyography (EMG) of all intrinsic and extrinsic ear muscles. The aim of this work was to develop a standardized protocol for a reliable surface EMG examination of all nine ear muscles in twelve healthy participants. The protocol was then applied in seven patients with unilateral postparalytic facial synkinesis. Based on anatomic preparations of all ear muscles on two cadavers, hot spots for the needle EMG of each individual muscle were defined. Needle and surface EMG were performed in one healthy participant; facial movements could be defined for the reliable activation of individual ear muscles’ surface EMG. In healthy participants, most tasks led to the activation of several ear muscles without any side difference. The greatest EMG activity was seen when smiling. Ipsilateral and contralateral gaze were the only movements resulting in very distinct activation of the transversus auriculae and obliquus auriculae muscles. In patients with facial synkinesis, ear muscles’ EMG activation was stronger on the postparalytic compared to the contralateral side for most tasks. Additionally, synkinetic activation was verifiable in the ear muscles. The surface EMG of all ear muscles is reliably feasible during distinct facial tasks, and ear muscle EMG enriches facial electrodiagnostics.

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.

The effect of interaural timing on the posterior auricular muscle reflex in normal adult volunteers

The posterior auricular muscle (PAM) reflex to sounds has been used clinically to determine hearing threshold as an alternative to other audiological diagnostic measures such as the auditory brainstem response. We have shown that the PAM response is also sensitive to interaural timing differences in normally hearing adults. PAM responses were evoked by both ipsilateral/ contralateral monaural stimulation and by binaural stimulation. Introducing sound delays ipsilaterally or contralaterally decreased the PAM response amplitude and increased its latency. The PAM response in this study shows a qualitatively similar pattern to that seen by the binaural interaction component (BIC) of the auditory brainstem potential to binaural clicks described in previous studies, in that both: have their shortest latency and maximal amplitudes centred around zero interaural timing differences, have response latencies increase with increasing interaural delays up to 1.2 ms and have response amplitudes decrease with increasing interaural delays of up to 1.2 ms. Our data show that the PAM response may be useful in measuring binaural integration in humans non-invasively for diagnostic or research studies.

Postauricular reflexes elicited by soft acoustic clicks and loud noise probes: Reliability, prepulse facilitation, and sensitivity to picture contents

The startle blink reflex is facilitated during early picture viewing, then inhibited by attention during pleasant and aversive pictures compared to neutral pictures, and finally potentiated during aversive pictures specifically. However, it is unclear whether the postauricular reflex, which is elicited by the same loud acoustic probe as the startle blink reflex but enhanced by appetitive instead of defensive emotion, has the same pattern and time course of emotional modulation. We examined this issue in a sample of 90 undergraduates using serially presented soft acoustic clicks that elicited postauricular (but not startle blink) reflexes in addition to standard startle probes. Postauricular reflexes elicited by both clicks and probes correlated during food and nurturant contents, during which they were potentiated compared to neutral pictures, suggesting clicks effectively elicit emotionally modulated postauricular reflexes. The postauricular reflex was initially facilitated during the first 500 ms of picture processing but was larger during pleasant than neutral pictures throughout picture processing, with larger effect sizes during the latter half of picture processing. Across reflexes and eliciting stimuli, measures of emotional modulation had higher coefficient alphas than magnitudes during specific picture contents within each valence, indicating that only emotional modulation measures assess higher-order appetitive or defensive processing.

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.