Subharmonic Distortion in Ear Canal Pressure and Intracochlear Pressure and Motion

When driven at sound pressure levels greater than ~110 dB stimulus pressure level, the mammalian middle ear is known to produce subharmonic distortion. In this study, we simultaneously measured subharmonics in the ear canal pressure, intracochlear pressure, and basilar membrane or round window membrane velocity, in gerbil. Our primary objective was to quantify the relationship between the subharmonics measured in the ear canal and their intracochlear counterparts. We had two primary findings: (1) The subharmonics emerged suddenly, with a substantial amplitude in the ear canal and the cochlea; (2) at the stimulus level for which subharmonics emerged, the pressure in scala vestibuli/pressure in the ear canal amplitude relationship was similar for the subharmonic and fundamental components. These findings are important for experiments and clinical conditions in which high sound pressure level stimuli are used and could lead to confounding subharmonic stimulation.     

Interaural delay sensitivity to tones and broad band signals in the guinea-pig inferior colliculus

We have measured the sensitivity of 243 low-frequency cells in the central nucleus of the guinea pig to the interaural time delay of best frequency (BF) tones, wideband noise and synthetic vowels. The highest rate of firing for the majority of cells occurred when the stimulus to the contralateral ear arrived 100–400 μs before that to the ipsilateral ear. The best delays for tones and noise measured in the same cell were highly correlated. In contrast to the tone delay functions, the majority of the delay functions obtained in response to wideband signals did not cycle, but were characterized by a single dominant peak or trough. The response frequency calculated from the delay functions to the vowel often did not correspond to the unit's BF, suggesting that the unit was responding to a component close to the first formant frequency (730 Hz) of the vowel. Phase-locked responses, on the other hand, only occurred to the fundamental frequency of the vowel (100 Hz) and not to higher frequency components. The responses to delayed tone and noise signals in the guinea pig are very like those obtained in the cat and other mammals. The similarity of the range of best delays for the guinea-pig with those reported for the cat, despite the difference in head size in these two species, suggests that the sensitivity to interaural delays reflects the properties of the binaural pathways rather than an adaptation to the delays normally experienced by the animal.     

Auditory-nerve fiber responses to tones in a noise masker

The phase-locked responses of single auditory-nerve fibers were measured for a continuous tonal stimulus presented in a noise background. The response amplitude, the primary Fourier component of the period histogram, was found to be dependent on the relative levels of the noise and tone. Different noise-to-tone level ratio resulted in quite different response amplitude; changing overall level keeping noise-to-tone ratio constant (constant dB difference) provided little change in response. With transient stimuli, phase-locked response to the tone at noise onset was consistently greater than to a tone presented during the steady-state noise exposure. When responses were normalized to the average rate, the differences between onset and steady-state responses were not clearcut.     

The envelope following response to multiple tone pair stimuli

The envelope following response (EFR) is a steady-state evoked response which follows the envelope of a stimulating waveform. A tone pair with frequencies f₁ and f₂ generates a temporal envelope whose period corresponds to the difference in frequency between the constituent tones (f₂−f₁=f_2,1). EFRs were recorded from Mongolian gerbils using amplitude-modulated stimuli comprised of from 1 to 4 tone pairs. Five stimulus tone pairs were used having center frequencies of approximately 0.3, 1, 2, 3, and 5 kHz; their corresponding envelope frequencies were approximately 38, 55, 66, 85, and 142 Hz. The magnitude of the EFR to each tone pair was measured separately and in combination with other simultaneously presented tone pairs. Small decreases (1–3 dB) in response magnitude, relative to the single tone pair condition, resulted from the addition of 1–2 tone pairs of higher frequencies; this effect was more pronounced with the addition of a third and fourth tone pair. However, the addition of the 300 Hz tone pair resulted in the significant enhancement of the response to higher frequency tone pairs. These data indicate that multiple tone pair EFRs may be a useful technique for the rapid acquisition of frequency-specific audiometric information.     

Sound calibration and distortion product otoacoustic emissions at high frequencies

Distortion product otoacoustic emissions offer the potential for assessing inner ear function at high frequencies. However, commonly employed methods for calibrating the acoustic system used in these studies can lead to errors of ± 20 dB or more in the estimate of eardrum sound pressure levels above 2–3 kHz [Siegel, J. Acoust. Soc. Am. 95, 2589–2597 (1994)]. We assessed the magnitude of these errors by measuring the distortion product emission 2f₁–f₂ (f₁ < f₂) in human subjects using either of two microphone locations to calibrate the stimulus. Either the emission probe microphone itself or a probe tube positioned near the eardrum were used in calibrations. The emissions collected with f₁ in the vicinity of 5–7 kHz showed a pronounced peak in level relative to other stimulus frequencies when the emission probe was used for calibration. The peak at 5–7 kHz disappeared when the probe tube near the eardrum was used for calibration. The discrepancy in emission levels between the two calibration procedures was as large as 20 dB. The difference is attributed to the presence of standing waves in the ear canal. The systematic errors in estimating eardrum sound pressure level using the emission probe microphone undoubtedly contribute to the variability of emission measurements for high-frequency stimuli.     

An evoked response study of the first-order difference tone in man.

The amplitude of the N1-P2 component of a response evoked by a 250 Hz test stimulus presented once each 5 sec was measured. Interposed at a rate of one per sec were either 1) a 4000 Hz tone, 2) a 4000 Hz and a 4250 Hz tone presented simultaneously which generated within the ear an audible 250 Hz difference tone, or 3) a 250 Hz tone. The results indicated that the N1-P2 component when preceded by the 4000 Hz plus 4250 Hz stimulus pair was larger than when preceded by a 250 Hz tone, but smaller than when preceded by a 4000 Hz tome. Apparently the processing of the difference tone is unlike that of a sinusoid of the same frequency or that of a high frequency sinusoid which participated in the generation of the difference tone. Parallel electrophysiological data from cochlear recordings are discussed.     

Action Potentials in the Cochlea: Masking,Adaptation and Recruitment

Action potentials were picked up from the round window of guinea pigs. By averaging the cochlear responses to tone bursts, the sine wave polarity of which was reversed in half of the presentations, the CM was cancelled out. During steady tones of various frequencies and various intensities, the AP thresholds for short tone bursts of different frequencies were determined. In some experiments the short tone burst was presented shortly after a long tone burst: forward masking.

The masking curves of the steady pure tones are compared with the psycho-acoustic masking curves as given in the literature. Another curve is the frequency response curve of a certain location of the cochlea measured by de Boer with the method of reverse correlation. The frequency response curve gives the ability of the cochlea to initiate nerve impulses for various frequencies. The shapes of all these curves are similar and they point to a rather high frequency-selectivity in the cochlea. Tuning curves of single nerve fibers have the steepest slopes compared with the other curves.

The masking curves measured during forward masking have the same shape but the thresholds are less elevated for the same intensity of the masking tone.

Above threshold AP responses were measured with the method of forward masking. They are reduced when compared with the responses in the no-masking situation. The reduction has the highest value when the frequency of the test stimulus is near the frequency of the masking tone. At a constant level of the masking tone the reduction diminishes with increasing level of the test stimulus showing a kind of recruitment phenomenon. This is valid for all frequencies of the test stimulus and at high levels of the test stimulus the reduction has almost disappeared.     

Dual response audiometry: a time-saving technique for enhanced objective auditory assessment.

The effectiveness of objective audiometric assessment can be improved by simultaneously recording transient evoked otoacoustic emissions (TEOAE) and auditory brainstem responses (ABR). Using a stimulation paradigm based on sequences of linearly balanced click stimuli (as described by Kemp et al.) acoustical and electrical responses of the auditory system can be obtained in one single run (dual response audiometry, DRA). The click stimuli are presented via an ear canal probe containing a speaker and a miniature microphone. EEG activity is recorded from surface electrodes fixed at the vertex and the mastoid ipsilateral to stimulus presentation. Microphone output and voltage difference between electrodes are fed into a dual-channel data acquisition system, where they are separately amplified and filtered into appropriate frequency ranges. After each stimulus, sweeps of 256 samples within a time window of 17 ms are taken of both signals. They are subject to artefact rejection and averaging of amplitude and polarity. The electrical responses to low and high level clicks within one stimulus sequence are processed separately, whereas the acoustical responses are summated across levels in order to eliminate stimulus-related contamination. As the result of one single run, ABR at two levels and non-linear TEOAEs are obtained within approximately 1 min. The signal quality is estimated by correlation analysis and binomial statistics. Among various features of DRA, the most important advantage is the improvement of the success rate. The influence of perturbations is limited since muscle artefacts due to motor activity affect only the ABR, whereas noise contamination affects only the TEOAE. The accuracy of threshold determination is better than with conventional ABR since the stimulus level is measured in situ. One DRA examination provides complete information about the functional integrity of the cochlea and neural pathways without additional time. It appears ideal for the application as a second stage infant screen.     

MEASUREMENT OF AMPLITUDE AND DELAY OF STIMULUS FREQUENCY OTOACOUSTIC EMISSIONS

Although stimulus frequency otoacoustic emissions (SFOAEs) have been used as a non-invasive measure of cochlear mechanics, clinical and experimental application of SFOAEs has been limited by difficulties in accurately deriving quantitative information from sound pressure measured in the ear canal. In this study, a novel signal processing method for multicomponent analysis (MCA) was used to measure the amplitude and delay of the SFOAE. This report shows the delay-frequency distribution of the SFOAE measured from the human ear. A low level acoustical suppressor near the probe tone significantly suppressed the SFOAE, strongly indicating that the SFOAE was generated at characteristic frequency locations. Information derived from this method may reveal more details of cochlear mechanics in the human ear.     

Cochlear nerve acoustic envelope response detection is improved by the addition of random-phased tonal stimuli

We test Lowenstein's dc bias hypothesis as an alternative mechanism for the phenomenon sometimes called 'stochastic resonance'. Probe stimuli consisting of paired phase-locked tones at frequencies f(1) and f(2) (where f(2)-f(1)=800 Hz, f(1)>4.5 kHz) and at equal intensity were used to generate synchronous 800 Hz cochlear nerve activity (envelope responses). When a background tone of the same intensity, with a frequency halfway between f(1) and f(2), is presented simultaneously with the probe stimulus, the envelope response amplitude typically decreases. Consistent with Lowenstein's hypothesis, however, when the intensities of the probe and background tone are near the detection threshold of the envelope response (approximately 0-20 dB sound pressure level), the simultaneous presence of the background tone often increases the amplitude of the envelope response. At these same intensity levels, when the background tone precedes the probe stimulus, it decreases the amplitude of the response to the probe stimulus. The effects of simultaneous presentation of the probe and the background tone are frequency-dependent, becoming less pronounced or reversing as the frequency of the background tone departs from those of the probe stimuli.     

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.     

Acoustic modulation of electrically evoked otoacoustic emission in chickens

Electrically evoked otoacoustic emissions (EEOAEs) can be elicited from the chicken inner ear. Since lesion studies implicate hair cells are the source of EEOAEs, we hypothesized that acoustic stimuli would modulate EEOAE amplitude at cochlear locations where the acoustic and electrical stimuli overlap. To assess this interaction, EEOAEs were measured as the frequency and amplitude of the acoustic stimuli were varied. EEOAEs, evoked by AC current (3-250 microA rms) delivered to the round window had a broad band pass response (1-6 kHz) with a peak between 3 and 4 kHz and maximum amplitude of 27 dB SPL. EEOAE suppression/enhancement tuning curves were measured at 2, 3, 4 and 6 kHz by varying the frequency of a 70 dB SPL tone and measuring the change in EEOAE amplitude. EEOAE tuning curves were characterized by a tip; a narrow range of frequencies where EEOAE amplitude was suppressed by as much as 5 dB, and by sidebands, a range of frequencies above and below the tip where EEOAE amplitude was enhanced by as much as 1.5 dB. The best suppression frequency, or characteristic frequency, was close to the frequency of the EEOAE elicited by the 3- or 4-kHz electric stimulus. However, the characteristic frequency was displaced towards higher frequencies for the 2-kHz electric stimulus, and towards lower frequencies for the 6-kHz electric stimulus. EEOAE suppression increased approximately linearly with acoustic level. These results suggest that EEOAEs evoked by round window stimulation are predominantly generated by hair cells near the 3- to 4-kHz region of the cochlea.     

The latency of auditory nerve brainstem evoked responses to air- and bone-conducted stimuli

The auditory nerve brainstem evoked responses (ABRs) to bone conduction (BC) stimuli are longer in latency than those to air conduction (AC). In order to study the mechanism of this difference, ABR wave I was recorded in experimental animals in response to low intensity (0–20 dB above their threshold) logon stimuli delivered by BC and by using the same bone vibrator to generate the air-conducted stimulus. The BC stimuli were delivered to skull bone, and directly to the contents of the cranial cavity (brain and cerebrospinal fluid) through a craniotomy. ABR wave I in response to BC stimuli delivered to skull bone was significantly longer in latency than that to BC delivered on the brain, while there was no latency difference between AC stimuli and BC to the brain. Furthermore, the vibration (measured with an accelerometer) recorded on the brain during BC stimulation of skull bone was always delayed compared to that measured on the skull. Thus there is a delay in the transfer of vibratory energy from the skull bone to the underlying contents of the cranial cavity. From there, the delayed vibrations of the contents of the cranial cavity are transmitted to the inner ear. This is probably the mechanism of the longer latency BC response compared to the AC response.     

Acoustic Reflex Amplitude and Noise-Induced Hearing Loss

Acoustic reflex maximum amplitude measurements elicited both contralaterally and ipsilaterally were obtained from subjects with noise-induced hearing loss and compared with those obtained from normal-hearing subjects. The eliciting signal was a pure tone of 1 kHz presented for 1 000 ms. The groups were matched on age, sex, static immittance and ear canal volume. Acoustic reflex amplitudes were clearly reduced in noise-impaired subjects compared with normal-hearing subjects at a frequency where their hearing thresholds were normal.     

Acoustic reflex amplitude and noise-induced hearing loss

Acoustic reflex maximum amplitude measurements elicited both contralaterally and ipsilaterally were obtained from subjects with noise-induced hearing loss and compared with those obtained from normal-hearing subjects. The eliciting signal was a pure tone of 1 kHz presented for 1,000 ms. The groups were matched on age, sex, static immittance and ear canal volume. Acoustic reflex amplitudes were clearly reduced in noise-impaired subjects compared with normal-hearing subjects at a frequency where their hearing thresholds were normal.     

The effects of maturation and stimulus parameters on the optimal f(2)/f(1) ratio of the 2f(1)-f(2) distortion product otoacoustic emission in neonates(1)

Distortion product otoacoustic emission (DPOAE) measurements are becoming popular in the clinical realm because they have been shown to reflect cochlear function. The primary tones used to evoke the DPOAE are important in determining the amplitude of the emission recorded in the ear canal. This study examined the ratio of the primaries necessary to determine the maximum amplitude emission as a function of development, stimulus level and frequency. Optimum f(2)/f(1) ratios were measured utilizing the f(1)-sweep technique from 105 neonates between 30-42 weeks conceptional age (CA) and 40 adults. No significant difference for optimum ratio was shown between the neonatal and the adult groups. Primary tone frequency had a significant effect on optimum ratio for both neonates and adults. Low f(2) frequencies (<4 kHz) were associated with higher optimum ratios than high f(2) frequencies (>4 kHz). The adult group was used to investigate the effect of stimulus level on the optimum f(2)/f(1) ratio for f(2) frequencies from 1.7 to 10 kHz. Regression analysis showed significant differences across levels of the primaries at all frequencies except for f(2)=3.4 and 7.0 kHz. These differences in f(2)/f(1) ratio across stimulus frequency and level may be attributed to the change in the shape of the excitation profiles along the basilar membrane.     

Effects of level on nonspectral frequency difference limens for electrical and acoustic stimuli

The purpose of this experiment was to study the effects of stimulus level on discrimination of frequency as represented in the temporal waveforms of acoustic and electrical signals. The subjects were four nonhuman primates in which one ear had been deafened and implanted with an electrode array and the other ear was untreated. Frequency difference limens for 100 Hz electrical sinusoidal stimulation via a cochlear implant in the deafened ear were compared to those for 100 Hz sinusoidally amplitude-modulated white noise (SAM noise) acoustic stimuli to the normal-hearing contralateral ear. To correct for loudness cues, levels of the test stimuli were varied relative to the reference-stimulus level. The test-stimulus levels at which the percent responses were minimum were determined. These levels were used to measure the frequency difference limens. Frequency difference limens for the electrical stimuli decreased as a function of reference-stimulus level through most of the dynamic range, while those for the acoustic stimuli reached a minimum at 20 dB to 40 dB above threshold. For the electrical stimuli the slopes and relative positions of the frequency difference limen vs. level functions varied from subject to subject and with changes in electrode configuration within a subject. These differences were related to threshold level and dynamic range. At higher levels of stimulation, frequency difference limens for acoustic and electrical stimuli fell in the same range. The slopes and relative positions of the frequency difference limen vs. level functions for electrical stimuli did not parallel those of level difference limen vs. level functions collected simultaneously from the same ears. The data suggest that nonspectral frequency discrimination may depend on the number of nerve fibers stimulated. With prostheses in cochleas with less than a full complement of auditory nerve fibers, the data suggest that stimulation level is an important variable influencing discriminability.     

Suppression of the acoustically evoked auditory-nerve response by electrical stimulation in the cochlea of the guinea pig

There is increasing interest in the use of electro-acoustical stimulation in people with a cochlear implant that have residual low-frequency hearing in the implanted ear. This raises the issue of how electrical and acoustical stimulation interact in the cochlea. We have investigated the effect of electrical stimulation on the acoustically evoked compound action potential (CAP) in normal-hearing guinea pigs. CAPs were evoked by tone bursts, and electric stimuli were delivered at the base of the cochlea using extracochlear electrodes. CAPs could be suppressed by electrical stimulation under various conditions. The dependence of CAP suppression on several parameters was investigated, including frequency and level of the acoustic stimulus, current level of the electric stimulus and the interval between electric and acoustic stimulus (EAI). Most pronounced suppression was observed when CAPs were evoked with high-frequency tones of low level. Suppression increased with current level and at high currents low-frequency evoked CAPs could also be suppressed. Suppression was typically absent several milliseconds after the electric stimulus. Suppression mediated by direct neural responses and hair cell mediated (electrophonic) responses is discussed. We conclude that the high-frequency part of the cochlea can be stimulated electrically with little detrimental effects on CAPs evoked by low-frequency tones.     

Temporal synchronization in the primary auditory response in the pigeon

Spike potentials were recorded from single fibres in the auditory nerve of the pigeon. In responses elicited by tonal stimuli, the timing of each spike relative to stimulus waveform was measured and period histograms were constructed. Phase locking of spikes was estimated in terms of a synchronicity index obtained by vector addition within the period histogram. A second measure of synchrony in the spike responses was obtained, that of temporal dispersion. For a population of fibres, vector strength of phase locking decreased for frequencies above 1 kHz, as reported for several other species. Temporal dispersion, however, also decreased with frequency, indicating enhanced temporal synchrony as frequency increased within the bandwidth of phase locking. The upper frequency limit of phase locking appears to depend on irreducible jitter of biological origin in the timing of spikes. For individual fibres, the bandwidth of synchronization of spikes consistently exceeds the response area, covering in addition the areas of suppression adjacent to the response area. Spike trains suppressed by a tonal stimulus become synchronized to that stimulus. Phase angles of synchronized responses systematically change as a function of tone level, when tone frequency is above or below CF, as reported for other avian species. Synchronicity and phase angle intensity functions are quite independent of spike rate intensity functions.