Middle Ear Muscle Contractions Research Articles

Middle ear muscle effects during gunfire noise exposures

A previous study using evoked response audiometry in anesthetized cats [Price and Wansack, J. Acoust. Soc. Am. 86, 2185 (1989)] showed that an impulse with its peak energy in the 4.0-kHz region was surprisingly hazardous. The surprisingly great effect could have been due to the anesthesia itself or the anesthesia could have prevented middle ear muscle contractions (normally not thought to affect the ear's response to gunfire). In order to help discriminate, three groups of ten cats were exposed to 50 midrange impulses at 145 dB peak, fired 3–5 s apart. Group I was anesthetized at the time of the exposure, and groups II and III were not; however, group II was anesthetized for threshold testing immediately after exposure (measured by evoked response audiometry at 2.0, 4.0, 8.0, and 16.0 kHz). Mean threshold shifts at 4.0 kHz 20–30 min post-exposure were 72 dB for group I and 19 dB for group II. Permanent threshold shifts were 44 dB for group I and 5.9 and 5.3 dB for groups II and III, respectively. These data suggest that the anesthetic itself probably does not potentiate the exposure and that the middle ear muscles should be suspected as being protective in any waking cat.

Binaural interaction in brainstem auditory evoked potentials elicited by frequency-specific stimuli.

The frequency specificity of the binaural interaction in brainstem auditory evoked potentials (BAEP) was investigated in ten normal-hearing young adults. A novel stimulus paradigm was devised to reduce the influence of the acoustic reflex (middle ear muscle contraction) on the BAEP, and to minimize the effect of variations in noise level. Sequences of six stimuli (rarefaction clicks or Gaussian-shaped tone pulses with carrier frequencies of 1, 2, 4 and 6 kHz) were periodically presented in the following order: right monaural, left monaural, binaural, left monaural, right monaural, binaural, with an interstimulus interval of 22 ms. Since the sequence of monaural stimuli with binaural stimuli interposed produces a uniform loudness and since the acoustic reflex is a consensual reflex, the relative high stimulus repetition rate (approx. 45/s) causes a muscle contraction which is equal on both sides and rather constant in time. This paradigm turned out to be usable for stimulus intensities as high as 80 dB nHL. The binaural difference potential (BDP) was computed by subtracting the sum of the monaurally (ipsilateral and contralateral) evoked potentials from the binaurally evoked potential. The major binaural interaction occurred in the latency range of BAEP waves V and VI, and there was no evidence of interaction in the earlier portion of the BAEP. Both latency and amplitude of the BDP components were evaluated statistically. The latency of the BDP components - except of the lasted one - showed an almost linear dependence both on stimulus intensity and stimulus frequency. The amplitude grew larger with decreasing frequency, and the visual detection threshold elevated as the stimulus frequency increased. Click stimuli, however, produced the largest amplitudes with lowest visual detection threshold. This novel stimulus paradigm appears to be most suitable for routine clinical investigations since high stimulus intensities can be used.

Auditory self-stimulation by vocalization in the CF-FM bat,Rhinolophus rouxi

1. Cochlear microphonic potentials (CM) were recorded from the cochlear aqueduct ofRhinolophus rouxi in response to vocalizations and artificial stimuli. 2. Vocal self-stimulation was evaluated by comparing the amplitude of the vocalization-induced CM to the calibration curve of the CM determined with artificial stimuli. The equivalent sound pressure level for self-stimulation at the resting frequency of the bat ranged between 78 and 83 dB SPL. 3. The same value of self-stimulation was obtained when we measured the maximum amplitude of beats produced by interference between acoustic self-stimulation and a slightly frequency shifted (500 Hz) playback signal. 4. Middle ear muscle (MEM) contractions occurred in synchrony with vocalizations even though the vocal self-stimulation level was below the threshold for the acoustic MEM reflex. The vocalization-induced MEM contractions had little effect on energy transfer to the inner ear for frequencies around the resting frequency but led to a reduction of the response at lower frequencies. 5. The importance of the vocal self-stimulation for different behavioural conditions of echolocation in CF-FM bats is discussed.

Modulation of activity in starling cochlear ganglion units by middle-ear muscle contractions, perilymph movements and lagena stimuli.

In the present study three groups of cochlear ganglion neurons were detected which differed in respect to their tone-evoked and spontaneous activity: auditory units which showed an irregular spontaneous discharge, non-auditory neurones with regular activity and such with an irregular spontaneous discharge pattern. Electrically-elicited contractions of the middle-ear muscle influenced the tone-evoked and/or the spontaneous activity of the auditory and the non-auditory neurones with irregular spontaneous discharge but not, however, the regularly firing units. Similar results were obtained with imposed perilymph movements in the cochlea (evoked via the vestibular system. Fractions of all three groups of cochlear ganglion neurones were responsive to direct deformations of the membraneous lagena. Several (auditory and non-auditory) units with irregular discharge were excited during a basilar membrane displacement towards scala vestibuli whereas a basilar membrane motion towards scala tympani resulted in a decrease of the discharge rate. A few units showed a different reaction. The results provide evidence that the neurones with periodic spontaneous discharge innervate the lagena and that this sense organ has no auditory significance in birds. The peripheral origin of the 'non-auditory' neurones with irregular spontaneous activity remains undecided and might be the macula lagenae or the apical portion of the basilar papilla.

Automatic gain control in the bat's sonar receiver and the neuroethology of echolocation

The sensitivity of the echolocating bat, Eptesicus fuscus, to sonar echoes at different time delays after sonar emissions was measured in a two-choice echo detection experiment. Since echo delay is perceptually equivalent to target range, the experiment effectively measured sensitivity to targets at different ranges. The bat's threshold for detecting sonar echoes at a short delay of only 1.0 msec after emissions (corresponding to a range of 17 cm) was 36 dB SPL (peak to peak), but the threshold decreased to 8 dB SPL at a longer delay of 6.4 msec (a range of 1.1 m). Prior research has shown that, at even longer delays (corresponding to ranges of 3 to 5 m), the bat's threshold is in the region of 0 dB SPL. Contractions of the bat's middle ear muscles synchronized with the production of echolocation sounds cause a transient loss in hearing sensitivity which appears to account for the observed echo detection threshold shifts. The bat's echo detection thresholds increase by approximately 11 dB for each reduction in target range by a factor of 2 over the span from 17 cm to 1.1 m. As range shortens, the amplitude of echoes from small targets also increases, by 12 dB for each 2-fold reduction in range. Thus, when approaching a target, the bat compensates for changes in echo strength as target range shortens by changing its hearing threshold. Since this compensation appears to occur in the middle ear, the bat regulates echoes reaching the cochlea to a stable amplitude during its approach to a target such as a flying insect. In addition to this automatic gain control linked to target range, the bat aims its head to track a target's position during approach, thus stabilizing echo amplitude even if the target's direction changes. We hypothesize that the bat's directional emissions, directional hearing, middle ear muscle contractions, and head aim response collectively create a three-dimensional spatial tracking filter which the bat locks onto targets to stabilize echo amplitudes during interception of prey. We further hypothesize that this regulation, which cancels echo amplitude changes caused by the target's changing spatial position, leaves the bat free to observe echo amplitude changes caused by the target's own actions, such as insect wing beats. Elimination of spatially dependent echo amplitude changes removes the cause of potentially troublesome changes in neural response latency and keeps stimulation from echoes in the "tip" region of auditory nerve fiber tuning curves.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical and acoustical activation of the middle ear muscle in a songbird

The effect of electrically-elicited middle ear muscle (MEM) contractions on cochlear microphonic potentials (CM) was studied in anesthetized starlings. In addition, the presence or absence of acoustic MEM reflexes was explored in awake starlings and one tawny owl, using the CM for detecting MEM contractions. The most commonly observed effect of electrically-elicited contractions in the anesthetized starlings was a frequency-dependent decrease in CM-amplitude (Fig. 2a) and a shift of the CM input-output functions towards higher SPL's (Fig. 3). It is concluded that the MEM might act as a bandpass filter exerting a (limited) protective effect against acoustic overstimulation of the ear and be capable of extending the ear's dynamic range to higher SPL's. A further physiological significance is discussed. Characteristic CM amplitude modulations, which have previously been shown to result from acoustic reflex contractions of the MEM, were observed in the awake tawny owl, but not in the starlings (Fig. 4).

Analysis of the avian middle ear muscle contraction by strain gauge and volume and impedance change measures

1. 1. Aves, unlike mammals, possess only one middle ear muscle, the stapedius. This muscle, which is innervated by a branch of the facial nerve, is exceptional in the respect that it alone exerts its effects on an entire physiological system, viz. the middle ear. 2. 2. Measurements of the physiological effects of this muscle in situ revealed both fast and slow components: the fast component results from the active contractile machinery of the muscle while the slow component derives from certain passive, visco-elastic attachments. 3. 3. The use of middle ear volume and impedance change measures in situ revealed a broader range of the muscle's physiological actions than was predictable by conventional strain gauge recording and/or histochemical studies.

Effects of prolonged noise exposure in chinchillas with severed middle ear muscles

Using the round window-recorded cochlear microphonic as the index of acoustic reflex activity, we noted a decay in the strength of middle ear muscle contraction in the chinchilla following an eight hour exposure to octave band noise (500 Hz. center frequency, 95 dB. sound pressure level). On the basis of this observation it was concluded that the prolonged exposure reduced the effectiveness of the acoustic reflex in protecting the cochlea. This reduction, however, may have been underestimated if the exposure was also sufficient to reduce cochlear output. The present investigation examined this possibility by comparing the effects of a similar exposure in chinchillas with intact (normal) and severed middle ear muscles. Following exposure, the cochlear microphonic magnitude increased slightly in the normal group. Decreases in the cochlear microphonic were observed in the animals with severed muscles even when the overall exposure level was reduced to simulate the effects of middle ear muscle contraction. These findings indicate that although the muscles did afford some degree of protection to the cochlea during the exposure, the protective effects of the acoustic reflex may have been reduced even beyond our original observations.

The Role of Efferent Inhibition in Human Auditory Attention: An Examination of the Auditory Brainstem Potentials

This study examined whether the efferent olivocochlear bundle (OCB) functions to attenuate irrelevant auditory stimuli during visual attention. Subjects either counted 8000 Hz, 50 dB SL target tone pips, or target letters flashed rapidly on a visual display. The target stimuli were randomly interspersed among nontarget stimuli to prevent ther subjects anticipating their occurrence (Naatanen, 1975). The auditory brainstem potentials were tape recorded for separate analyses of the target and nontarget tone pips. The nontarget tone pips were not significantly affected suggesting they were attenuated in the auditory task (intramodal effect) as well as during the visual task. During focused visual attention, the auditory nerve component to the target tone pips was significantly reduced in amplitude by 37.4% and increased in latency by 90 musec. Wave V generated partially from the inferior colliculus was also reduced in amplitude by 12.9%; however, this decrement, was not statistically significant. The use of 8000 Hz tone pips and random target presentation precluded middle ear muscle contractions producing these results. It was concluded that the efferent OCB which synapses on and inhibits the hair cells and axons within the cochlea may function during visual attention by attenuating irrelevant acoustic stimuli.

The acoustic reflex latency test: clinical application.

Acoustic reflex latency is the time interval between onset of an intense auditory stimulus and onset of middle-ear muscle contraction. The hypothesis was proposed that a retrocochlear lesion involving the afferent (VIIIth nerve) portion of the reflex arc should result in a prolongation of the reflex latency. A clinical procedure, the Acoustic Reflex Latency Test (ARLT), its protocol, methods and equipment were developed in order to test this hypothesis clinically. Absolute latency values and interaural latency differences (ILD) were established on 4 populations which included normals, cochlear lesions due to Ménière's disease, cochlear lesions of variable etiology, and 11 cases of surgically confirmed acoustic tumor. Results reveal a dramatic prolongation of latency in the presence of retrocochlear lesions (all tumors). The test was found to be most effective when 1. both ipsilateral and contralateral measurements were made, 2. stimulus frequencies of 100 Hz and 2000 Hz were employed and 3. both Absolute Latency and ILD were considered. The ARLT, while utilizing an indirect method of measurement, is a valid, cost-effective, and simple clinical procedure, yielding relative latency values and requiring only minor modifications of standard impedance equipment. The ARLT appears to have a higher efficiency rate than BERA, while providing comparable diagnostic information at a considerable saving of time, cost and personnel.

Reflex threshold shift in chinchillas following a prolonged exposure to noise.

The acoustic reflex is considered to reduce transmission across the middle ear and thereby protect the inner ear from intense sounds. The dynamic properties of this reflex seem to be a function of the duration of the eliciting stimulus. Assessment of the protective action afforded by middle-ear muscle contractions for long-term noise exposures requires the knowledge of how these dynamic properties change under such conditions. Round window electrodes were implanted in eight chinchillas. Changes in the threshold of the acoustic reflex were measured during an eight-hour exposure at 95 dB SPL to an octave-band noise centered at 0.5 kHz. The criterion measure of the acoustic reflex was a change in the amplitude of the cochlear microphonic generated by a 0.5 kHz eliciting tone. Thresholds of the acoustic reflex increased systematically throughout the noise exposure up to approximately 14 dB after 8 hours. The time course of the changes in the threshold of the acoustic reflex was nearly identical to the time course of behaviorally measured changes in the auditory sensitivity as reported by Carder and Miller (1972).

Study of the Change of Transmission Efficiencies of Human Ear by Acoustic Reflective Contraction of Middle Ear Muscles

Study of the Change of Transmission Efficiencies of Human Ear by Acoustic Reflective Contraction of Middle Ear Muscles

The influence of middle-ear muscle contraction on auditory threshold for selected pure tones.

The influence of middle-ear muscle (MEM) contraction on auditory threshold has been measured for pure tones of 0.25, 0.5, and 1.5 kHz. The reflex-activating signal was a 3-kHz pure tone. Signal paradigms were chosen to reduce or eliminate the effects of binaural loudness summation, contralateral direct masking, and contralateral remote and backward masking effects, and to maximize the influence of MEM contraction. Results indicate that under no condition was behavioral threshold affected by the MEM contraction induced using a pure-tone stimulus of 3 kHz, 105 dB SPL.

An Objective Predictor of Cochlear Function in the Speech Clinic

A rationale is given for measuring acoustic reflexes in order to establish an estimation of cochlear funtion. Anatomical and physiological descriptions of the normal middle ear are outlined. Particular attention is paid to changes of the acoustic transmitting properties of the middle ear due to middle ear muscle contractions to acoustic stimuli, and the predictive value of these measures in the assessment of sensorineural hearing impairment.

The effects of middle ear muscle contraction on auditory and overload thresholds.

The present investigation examined psychophysically the frequency-specific and nonlinear attenuation of sound energy provided by middle ear muscle contraction in normal hearers. Aural overload and absolute auditory thresholds were measured at 500, 1 000 and 2 000 Hz prior to and during acoustic stapedial reflex contraction. Reflex contraction was elicited with contralateral broad-band noise. Results revealed that overload thresholds were elevated at 500 and 1 000 Hz and a slight decrease in overload threshold was apparent at 2 000 Hz during reflex contraction while absolute thresholds remained essentially unchanged. These data are consistent with previous psychophysical and physiological findings. The aural overload test having been derived directly from observations of cochlear microphonic saturation, moreover, affords direct comparison to results of electrophysiological experimentation.

Activity of the auditory system related to cortical arousal

To determine whether or not the central auditory pathway shows variations in its activity which parallel changes in arcusal, multiple unit activity was recorded from the medial geniculate body, inferior colliculus, superior olivary complex, and cochlear nucleus in cats and was compared with simultaneously recorded electrocorticogram (ECoG) fluctations. Animals were paralyzed to assure auditory activity unconfounded by middle ear muscle contractions and movement-induced noise; extraneous outside noises were also carefully controlled. Under those conditions the medial geniculate body shows activity, both spontaneous and scund-elicited, which increases, usually linearly, as the amplitude of the cortical ECoG decreases. Lower auditory nuclei also vary their discharge, but in a more variable and unpredictable manner. The most common response observed among these lower nuclei was an increase in discharge during ECoG desynchronization, but with no change during the wide range of moderate- to high-amplitude ECoG. Present anatomical knowledge suggests the cortex as the most likely source of the geniculate effect. The mechanism responsible for modifying the activity of the lower auditory nuclei is not known, but the “descending auditory system” and efferents from the reticular formation are likely candidates.

Effects of human middle-ear muscle contractions on speech intelligibility

The present study was designed to further assess the relationship between the acoustic reflex and speech intelligibility. Monosyllabic words mixed with noise were presented to 35 subjects at −6, −3, 0, and +3 dB S/N ratios. The word lists were presented with a 2000-Hz tone in the contralateral ear at a level either 15 dB above or 20 dB below the acoustic reflex threshold. This was performed to evaluate the intelligibility of the monosyllabic words with the acoustic reflex contracted or relaxed. The results indicated that at S/N ratios of −3 and 0 dB a significant decrement in speech intelligibility occurred. A slight but not significant decrement was seen at −6 and +3 dB S/N. The results of the present study show that contraction of the acoustic reflex causes a decrease in speech intelligibility. These results are discussed along with possible reasons to explain the lack of agreement with previous research.

Effects of reflex middle-ear muscle contractions on cochlear responses to bone-conducted sound.

The effects of contralaterally elicited middle-ear muscle (MEM) reflexes on cochlear microphonic responses to air- and bone-conducted tones were examined in decerobrate cats. Stapedius effects on bone condn air conduction were almost identical in configuration and amplitude to those on air conduction at all frequencies. However, tensor tympani effects were more complex, the configuration of the bone-conduction effects varying with the location of the transducer on the skull and with frequency. The relative contributions of the two muscles to the effects of joint contractions varied markedly between animals. It is suggested that non-reflex MEM contractions associated with activity of the facial musculature might provide protection against masking of environmental sounds by the low-frequency bone-conducted sound generated by such activity.

Response plasticity of neurons in auditory cortex of the untrained rhesus monkey

Evoked responses from single neurons in auditory cortex were examined in untrained rhesus monkeys under three states: (1) monkeys' eyes open and occasionally tracking a moving object, (2) eyes closed for at least 30 sec, and (3) anesthetized with 5 or 10 mg/kg ketamine. Pure-tone or white-noise stimuli were presented monaurally to the contralateral ear. The subjects' heads were rigidly restrained. Marked changes in evoked response were frequently seen as the animal drifted from state 1 to state 2. Most commonly the response weakened or disappeared altogether, but occasionally more complex changes occurred in a unit's response pattern, threshold, or tuning characteristics. Ketamine produced various effects, depending on the unit observed: in some cases the evoked response disappeared while in others it became very stable. The changes in responsiveness could not be accounted for by small changes in stimulus intensity which might result from pinna movements or middle-ear muscle contractions, since they were often more dramatic than changes produced by even large changes (>30 dB) in intensity. The results reemphasize the need for careful specification of the subject's behavioral and physiological state in studies of neural coding. [Supported by Grants NS-08181, RR-00166, and GM-00260 from NIH.]

Influence of middle-ear muscle contraction on pure-tone suprathreshold loudness judgments.

Some effects of middle−ear muscle (MEM) contraction on TTS and suprathreshold loudness judgments have been assessed in three experiments. Experiment I was designed to demonstrate the effect of MEM contraction on TTS while in Experiments II and III the influence of MEM contraction on the loudness of suprathreshold pure−tone stimuli was measured. In Experiment I, MEM contraction decreased the effective level of a high−intensity TTS−inducing stimulus reaching the cochlea. In Experiments II and III signal paradigms were chosen to reduce or eliminate the effects of binaural loudness summation, and contralateral remote and backward masking effects, and to maximize the influence of MEM contraction. Results of the loudness judgments indicated no effect of binaural interaction, whereas the MEM contraction affected the loudness of pure tones only when the stimuli were low frequencye (250, 500 Hz) and high intensity (≳100 dB SPL). The results further imply that at high intensity levels there was an interaction between the effects of the intensity of the loudness−judgment stimulus and the intensity of the reflex−inducing stimulus. That is, when the reflex was induced the effect on loudness increased as the intensity of the loudness−judgment stimulus was raised; likewise, when the loudness−judgment stimulus was maintained at a constant sound−pressure level but the intensity of the reflex−inducing stimulus was increased, the effect on loudness increased. Subject Classification: 65.50, 65.22, 65.24.