Threshold-duration function of the acoustic reflex in man

The middle ear muscle reflex: Current and future role in assessing noise-induced cochlear damage

The middle ear muscle reflex (MEMR) in humans is a bilateral contraction of the middle ear stapedial muscle in response to moderate-to-high intensity acoustic stimuli. Clinically, MEMR thresholds have been used for differential diagnosis of otopathologies for decades. More recently, changes in MEMR amplitude or threshold have been proposed as an assessment for noise-induced synaptopathy, a subclinical form of cochlear damage characterized by suprathreshold hearing problems that occur as a function of inner hair cell (IHC) synaptic loss, including hearing-in-noise deficits, tinnitus, and hyperacusis. In animal models, changes in wideband MEMR immittance have been correlated with noise-induced synaptopathy; however, studies in humans have shown more varied results. The discrepancies observed across studies could reflect the heterogeneity of synaptopathy in humans more than the effects of parametric differences or relative sensitivity of the measurement. Whereas the etiology and degree of synaptopathy can be carefully controlled in animal models, synaptopathy in humans likely stems from multiple etiologies and thus can vary greatly across the population. Here, we explore the evolving research evidence of the MEMR response in relation to subclinical noise-induced cochlear damage and the MEMR as an early correlate of suprathreshold deficits.

Accounting for nonmonotonic precursor duration effects with gain reduction in the temporal window model

The mechanisms of forward masking are not clearly understood. The temporal window model (TWM) proposes that masking occurs via a neural mechanism that integrates within a temporal window. The medial olivocochlear reflex (MOCR), a sound-evoked reflex that reduces cochlear amplifier gain, may also contribute to forward masking if the preceding sound reduces gain for the signal. Psychophysical evidence of gain reduction can be observed using a growth of masking (GOM) paradigm with an off-frequency forward masker and a precursor. The basilar membrane input/output (I/O) function is estimated from the GOM function, and the I/O function gain is reduced by the precursor. In this study, the effect of precursor duration on this gain reduction effect was examined for on- and off-frequency precursors. With on-frequency precursors, thresholds increased with increasing precursor duration, then decreased (rolled over) for longer durations. Thresholds with off-frequency precursors continued to increase with increasing precursor duration. These results are not consistent with solely neural masking, but may reflect gain reduction that selectively affects on-frequency stimuli. The TWM was modified to include history-dependent gain reduction to simulate the MOCR, called the temporal window model-gain reduction (TWM-GR). The TWM-GR predicted rollover and the differences with on- and off-frequency precursors whereas the TWM did not.

Temporal integration of the contralateral acoustic-reflex threshold and its age-related changes

Although numerous studies have investigated temporal integration of the acoustic-reflex threshold (ART), research is lacking on the effect of age on temporal integration of the ART. Therefore the effect of age on temporal integration of the ART was investigated for a broad-band noise (BBN) activator. Subjects consisted of two groups of adults with normal-hearing sensitivity: one group of 20 young adults (ten males and ten females, ages 18-29 years, with a mean age of 24 years) and one group of 20 older adults (ten males and ten females, ages 59-75 years, with a mean age of 67.5 years). Activating stimulus durations were 12, 25, 50, 100, 200, 300, 500, and 1000 ms. Significant main effects for duration and age were obtained. That is, as the duration increased, the acoustic reflex threshold for BBN decreased. The interactions of duration x age group and duration x hearing level were not significant. The result of pair-wise analysis indicated statistically significant differences between the two age groups at durations of 20 ms and longer. The observed age effect on temporal integration of the ART for the BBN activator is interpreted in relation to senescent changes in the auditory system.

Auditory system: Peripheral nonlinearity and central additivity, as revealed in the human stapedius-muscle reflex

Human stapedius-muscle contractions in response to 3-kHz, 20-msec tone bursts were determined indirectly by measuring the associated acoustic-impedance changes at the tympanic membrane with an acoustic bridge. The measurement was possible because the bridge practically eliminates the effect of the ear-canal air volume interposed between the tympanic membrane and the tip of the measuring tube. By using burst pairs, temporal additivity of the muscle responses was demonstrated both when the stimulus bursts were presented contralaterally to the measured impedance changes and when the first burst was presented ipsilaterally. The summation time constant was on the order of 200 msec, much longer than the twitch time constant of the muscle fibers. Therefore, the summation had to take place in a nucleus preceding the stapedius muscle. The magnitude of the muscle response obeyed a compressive function paralleling the loudness function up to sound pressure levels of at least 120 dB.

Effect of probe frequency and gender on click-rate-induced facilitation of the acoustic reflex thresholds

This study evaluated the effects of probe frequency and gender on the click-rate-induced facilitation of the acoustic reflex thresholds (ARTs). ARTs were measured by delivering clicks at the repetition rates of 60, 120, 180, 240 and 300/sec. The probe tones were 226, 678 and 1000 Hz. Rate-induced facilitation (RIF) was calculated for each subject, for each of the probe tones, by subtracting the minimum ART from the maximum ART. The mixed MANOVA on the RIF revealed no significant main effects or interactions for gender or probe tone frequency. However, paired comparisons revealed that the RIF values obtained from male subjects with the 1000 Hz probe tones were significantly lower than those obtained from the female subjects with all the probe tone frequencies. The results suggest that RIF can be studied with higher probe tones, but gender differences need to be considered if the probe tone is 1000 Hz.

Ipsilateral acoustic reflex thresholds at varying click rates in humans

The effects of the click repetition rates on the ipsilateral acoustic reflex thresholds were investigated in 17 normal female subjects (total 34 ears) within the age-range of 20 to 26 years. The acoustic reflexes were elicited in response to condensation clicks (100 microseconds duration) at repetition rates of 50, 100, 150, 200, 250 and 300/sec. The probe tone frequency was 226 Hz and the intensity was 85dB SPL. The thresholds improved significantly with increase in the repetition rates. The mean threshold advantage was 11.2dB with an increase in the rate from 50 to 100/sec, it was 7.8dB when the rate was increased from 100 to 200/sec and was further reduced to 2.6dB when the rate was increased from 200 to 300/sec. These findings are discussed with reference to the temporal and the spatial summation and facilitation. Possible clinical implications of the findings are presented.

Effect of stimulus duration on stapedius reflex threshold in electrical stimulation via cochlear implant

The effect of stimulus duration on the threshold of the contralateral stapedius reflex was investigated in patients supplied with a Vienna cochlear implant (analog stimulation via CI) and compared to results of a normal-hearing reference group in case of acoustic stimulation. Changes in reflex threshold were determined at four frequencies (125, 500, 1,000 and 2,000 Hz in case of electrostimulation and at 500, 1,000, 2,000 and 4,000 Hz for acoustic stimulation) and for five durations (30, 50, 100, 300 and 500 ms). The comparison of the two stimulation modes was accomplished by using the same instrumentation and procedure. Reflex threshold was evaluated according to an objective criterion based on the individual noise distribution of recordings without reflex and by subjective judgement. For both stimulation modes a strong effect of stimulus duration on reflex threshold was observed (p < 0.001). The amount of temporal integration reflected by the threshold difference between 500 and 50 ms was approximately 2-4 dB for electrical stimulation via CI and 6 dB for acoustic stimulation in normal-hearing individuals. In case of electrostimulation, the reflex threshold for stimuli of 30 ms was most often above the limit of uncomfortable loudness sensation; the increase in reflex threshold for acoustic stimulation between 500 and 30 ms was approximately 14 dB. There was no evidence of frequency effect on reflex threshold nor an interaction between frequency and stimulus duration for either stimulation mode.

The acoustic middle ear muscle reflex in albino rats

The acoustic middle ear muscle reflex was studied in albino rats anesthetized with chloralose. The best frequency of the reflex and the threshold at this frequency were on average about 3 kHz and 57 dB SPL, respectively. The threshold increased as frequency increased above, and decreased below, the best frequency at a rate of about 20 dB/octave. Above about 12 kHz, the muscular response showed instability and habituation. Thresholds were similar between stapedius and tensor tympani reflexes and between ipsilateral and contralateral reflexes. The middle ear transmission loss due to the reflex was the greatest and nearly constant below about 1 kHz, where the loss was about 18 dB at the maximal stimulation. Above this frequency the loss decreased as frequency increased up to 20 kHz. Thus the reflex, unlike that in other animals, suppressed transmission over the whole range of reflex-eliciting frequencies. The transfer function of the reflex had a well damped low-pass characteristic with a cut-off frequency of about 20 Hz. From the above characteristics of the reflex, the role of the rat's tympanic muscles in improving ultrasonic hearing under ambient noises was suggested.

Temporal summation in acoustic reflex growth functions

Acoustic reflex growth functions for pure tone activators of 500, 1000 and 2000 Hz and a broad-band noise (BBN) were obtained with activator durations of 20, 200 and 1000 msec (10 msec rise/decay) in four young, normal subjects. The lowest reflex thresholds were obtained for the 1000 msec activators, followed by 200 msec and then 20 msec. The reflex growth functions revealed lower reflex magnitudes for the shorter duration activators. When normalized to account for temporal summation at reflex threshold, reflex magnitude growth was similar among the three durations at 500 Hz, slower for 20 msec than for 200 and 1000 msec at 1000 Hz, and became progressively slower with decreasing duration from 1000 to 200 to 20 msec at 2000 Hz. When expressed in SPL, the BBN growth functions reasonably resembled those at 2000 Hz. No particular pattern was revealed for the normalized BBN growth functions.

Validation of an automatic otoadmittance middle ear analyzer

Tympanometry and acoustic reflex measurements were performed with an automatic otoadmittance middle ear analyzer (MEA) on 40 otologically normal children and 172 children with a history of recurrent acute otitis media or otoscopic evidence of persistent otitis media with effusion (OME), or both. For children with OME, the measurements were taken within a one-hour period prior to myringotomy. Myringotomy findings were used to validate predictive schema aimed at determining admittance values associated with OME. Predictive accuracy was quantitifed with percent sensitivity and specificity values. The results show that the MEA demonstrated suitable diagnostic predictability, but this was significantly influenced by the schema chosen to interpret obtained admittance measures. The simple presence or absence of a tympanometric peak (regardless of gradient) yielded the highest sensitivity and specificity values, whereas the manufacturer's suggested approach, with or without the acoustic reflex, was not as successful. The implications of these results are discussed.

The normal middle ear reflex thresholds towards white noise and acoustic clicks in young adults

The acoustic middle ear muscle reflex thresholds towards white noise and fast sequences of clicks were determined in 46 normal-hearing young adults using electronic averaging. For white noise the thresholds were normally distributed around an average value of 72.7 dB SPL. With sequences of clicks the threshold depended above all on the click repetition rate within each stimulus sequence. When the repetition was 128/sec the threshold distribution was almost normal around an average value of 73.9 dB SPL. For the individual test subject there was good correlation between the white noise and the click reflex thresholds. It was our impression that the click generated reflexes had a more well defined threshold than with white noise stimuli, but this impression needs further validation.

Acoustic reflex measures of cochlear damage--a normative study

It has been argued that certain reflex measures may indicate the presence of end-organ hearing loss. It has been noticed that the difference between wide band noise and pure tone thresholds is diminished, that the reflex threshold duration function is flattened and that the reflex relaxation index, as specified by Norris et al. (1974b), is reduced in cases of end-organ hearing loss. These aspects of the acoustic stapedial reflex, together with its growth with stimulus intensity, have been investigated in 25 normally hearing subjects. The overriding feature of the measures taken of the acoustic stapedial reflex, was the high inter-subject variability. The differences between the acoustic reflex thresholds for pure tone and WBN was found to be normally distributed about a mean value of 12 dB. The threshold-duration functions again were highly variable and encompassed the entire range of results from normal and end-organ impaired groups as reported by Woodford et al. (1975). The variability encountered in the RRI measures was found to increase with frequency. The growth of the acoustic stapedial reflex with intensity was found to be frequency dependent: the gradient of the function increasing with frequency for the long duration (500 ms) pulses and decreasing with the short duration (50 ms) pulses. It is concluded that considerable caution must be exercised in extrapolation from any one of these measures in isolation.