Speech recognition in noise: estimating effects of compressive nonlinearities in the basilar-membrane response

OBJECTIVES: This experiment was designed to estimate effects of cochlear nonlinearities on tonal and speech masking for individuals with normal hearing who have a range of quiet thresholds. Physiological and psychophysical evidence indicates that for signals close to the characteristic frequency (CF) of a place on the basilar membrane, the normal growth of response of the basilar membrane is linear at lower stimulus levels and compressed at medium to higher stimulus levels. In contrast, at moderate to high CFs, the basilar membrane responds more linearly to stimuli at frequencies well below the CF regardless of input level. Thus, the hypothesis tested was that masker effectiveness would change as a function of stimulus level consistent with the underlying basilar membrane response. Specifically, with a fixed-level speech signal and a speech-shaped masker that ranges from low to higher levels, the resulting response of the basilar membrane to the masker would be linear at lower levels and compressed at medium to higher levels. This would result in relatively less effective masking at higher masker levels. It was further hypothesized that the transition from linear to compressed responses to both tones and maskers would occur at higher levels for listeners with higher quiet thresholds than for listeners with lower quiet thresholds. DESIGN: Tonal thresholds and speech recognition in noise were measured as a function of masker level. A 10-msec, 2.0-kHz tone was presented in a lower frequency masker ranging from 40 to 85 dB SPL. Moderate-level speech was presented in interrupted noise at six levels ranging from 47 to 77 dB SPL. To minimize differences in speech audibility that could arise during the "off" periods of the interrupted noise, a low-level steady-state "threshold-matching noise" was also present during measurement of speech recognition. Subjects were 30 adults with normal hearing with a 20-dB range of average quiet thresholds. RESULTS: Tonal breakpoints (i.e., the levels corresponding to the transitions from linear to nonlinear responses) were significantly correlated with quiet thresholds, whereas slopes measured above the breakpoints were not. Speech recognition in noise was consistent with the hypothesis that the response of the basilar membrane to the masker was linear at lower levels and compressed at medium to higher levels, resulting in less effective masking at higher masker levels. That is, at lower masker levels, as masker level increased, mean observed speech scores declined as predicted using the articulation index, an audibility-based model. With further increases in masker level, mean scores declined less than predicted. Moreover, for subjects with higher quiet thresholds, masker effectiveness remained constant for a wider range of masker levels than for subjects with lower quiet thresholds, consistent with the hypothesis that the transition from linear to compressed responses occurred at higher levels. Finally, significant negative correlations were obtained between individual subjects' tonal and speech measures. CONCLUSIONS: Results from tonal and speech tasks were consistent with basilar membrane nonlinearities and consistent with changes in nonlinearities with minor threshold elevations, providing support for their role in the understanding of speech in noise with increases in noise level.     

Effect of click intensity on click-evoked otoacoustic emission waveforms: implications for the origin of emissions

The rather shallow growth of click-evoked otoacoustic emissions (CEOAE) with click intensity, namely <1 dB/dB, distinguishes genuine CEOAEs from stimulus artifacts, thereby providing the rationale for the popular 'derived nonlinear recording' method. However, other CEOAE nonlinearities regarding phase or envelope dependence on stimulus intensity have been barely acknowledged so far. The present work used CEOAEs from 20 normal ears recorded in response to 50-86 dB peak equivalent SPL clicks. The phases of CEOAE spectral components varied considerably with click intensity (sometimes more than 120 degrees ), mostly in a monotonic manner and in such a way that in the majority of ears, phase lagged with increasing intensity. When present, synchronized spontaneous otoacoustic emissions exhibited the same behavior. In a few instances, conspicuous frequency shifts of CEOAE spectral peaks were seen. In contrast to CEOAE phases, envelopes were almost intensity-invariant. This behavior contrasts with that of basilar membrane motion at the place tuned to the stimulus frequency, as consistently disclosed by several recent publications, i.e., no phase shift and large envelope shift with stimulus intensity. It is thought that the phase invariance of basilar membrane motion implies that whatever they do, outer hair cells cannot alter the resonance frequency of the cochlear partition. If one elaborates along this line of reasoning, the large phase shift of CEOAEs with click intensity implies that CEOAEs at frequency f cannot come from the place tuned to f and that instead, they may be intermodulation distortion products produced by nonlinear interactions between spectral components of the click stimulus over a significant length of the basilar membrane.     

Modeling rapid waveform compression on the basilar membrane as multiple-bandpass-nonlinearity filtering

Evidence has accumulated from experimental intracochlear studies that nonlinear mechanical response of the basilar membrane is responsible for cochlear frequency tuning and is the major source of extracochlear nonlinear phenomena in cochlear sound analysis. Known basilar-membrane data provide a basis for synthesizing and quantifying conceptions of cochlear signal processing derived earlier from extracochlear studies that indicated the existence of rapid waveform compression and dual signal processing. The multiple-bandpass-nonlinearity (MBNL) model represents and generalizes available measurements of basilar-membrane mechanical responses in terms of a rapid nonlinear mixing at each place of an insensitive, linearlike lowpass filter with a sensitive, compressive bandpass filter. The dual filters are associated with the tails and tips of cochlear frequency tuning curves. Simulations of published nonlinear mechanical responses of the basilar membrane and predicted correlations with auditory-nerve responses are systematically explored. Correlations between model and biophysical data suggest that the model represents a nonlinear mixing by outer hair cells of hydromechanical and electromechanical signals, and thus provides a quantitative tool for biophysical study of cochlear mechanisms.     

The Spatial Buildup of Compression and Suppression in the Mammalian Cochlea

We recorded responses of the gerbil basilar membrane (BM) to wideband tone complexes. The intensity of one component was varied and the effects on the amplitude and phase of the others were assessed. This suppression paradigm enabled us to vary probe frequency and suppressor frequency independently, allowing the use of simple scaling arguments to analyze the spatial buildup of the nonlinear interaction between traveling waves. Most suppressors had the same effects on probe amplitude and phase as did wideband intensity increments. The main exception were suppressors above the characteristic frequency (CF) of the recording location, for which the frequency range of most affected probes was not constant, but shifted upward with suppressor frequency. BM displacement reliably predicted the effectiveness of low-side suppressors, but not high-side suppressors. We found “anti-suppression” of probes well below CF, i.e., suppressor-induced enhancement of probe response amplitude. Large (>1 cycle) phase effects occurred for above-CF probes. Phase shifts varied nonmonotonically, but systematically, with suppressor level, probe frequency, and suppressor frequency, reconciling apparent discrepancies in the literature. The analysis of spatial buildup revealed an accumulation of local effects on the propagation of the traveling wave, with larger BM displacement reducing the local forward gain. The propagation speed of the wave was also affected. With larger BM displacement, the basal portion of the wave slowed down, while the apical part sped up. This framework of spatial buildup of local effects unifies the widely different effects of overall intensity, low-side suppressors, and high-side suppressors on BM responses.     

Estimates of basilar-membrane nonlinearity effects on masking of tones and speech

OBJECTIVE: The aim of this experiment was to assess the contribution of cochlear nonlinearities to speech recognition in noise for individuals with normal hearing and a range of quiet thresholds. For signals close to the characteristic frequency (CF) of a place on the basilar membrane, the normal growth of response of the basilar membrane is linear at lower signal levels and compressed at medium to higher signal levels. In contrast, at moderate to high CFs, the basilar membrane responds more linearly to stimuli at frequencies well below the CF regardless of input level. Thus, for moderate-level speech and a lower frequency masker, the response to the masker grows linearly whereas the response to the speech is compressed, which may result in changes in the effectiveness of the masker on speech recognition with increases in masker level. To test this hypothesis, observed speech-recognition scores were compared with scores predicted using an audibility-based model, which did not include nonlinear effects that may influence masker effectiveness. DESIGN: Growth of simultaneous masking was measured for moderate-level bandpass-filtered nonsense syllables and for 350-msec pure tones at frequencies within the speech passband. Masker frequencies were within (on-frequency) or below (off-frequency) the speech passband. Estimates of basilar-membrane nonlinearities were derived from growth-of-masking functions for 10-msec, 2.0- and 4.0-kHz tones in narrowband, off-frequency maskers presented simultaneously. Subjects were 26 adults with normal hearing with approximately a 20-dB range of average quiet thresholds. RESULTS: Breakpoints (i.e., the levels corresponding to the transitions from linear to nonlinear responses) were strongly associated with quiet thresholds but slopes measured above the breakpoints were independent of quiet thresholds. Individual differences were substantially larger for off-frequency masking of pure tones and speech than for on-frequency masking of pure tones and speech. Using an audibility-based predictive model, the change in speech audibility resulting from the compressed response to speech with increasing off-frequency masker level (and the resulting decline in scores) was well predicted from nonlinear growth of masking for pure tones measured in the same off-frequency masker. However, absolute speech-recognition predictions were generally inaccurate and were a function of how well pure-tone signal levels at masked threshold estimated masker effectiveness for speech. That is, subjects with lower off-frequency masked thresholds had less accurate predictions of speech recognition in off-frequency maskers. CONCLUSIONS: Large individual differences in off-frequency masking of pure tones and speech are consistent with the assumption that small changes in the shape of the basilar-membrane input-output function result in large changes in the amount of off-frequency masking but small (if any) changes in on-frequency masking where the signal and masker are subject to a similar compression. Growth of off-frequency masking of pure tones and speech were correlated with each other, consistent with the underlying basilar-membrane response, and consistent with changes in breakpoints for subjects with normal hearing and a range of quiet thresholds. These results provide support for a role of nonlinear effects in the understanding of speech in noise.     

Energy Flux in the Cochlea: Evidence Against Power Amplification of the Traveling Wave

Traveling waves in the inner ear exhibit an amplitude peak that shifts with frequency. The peaking is commonly believed to rely on motile processes that amplify the wave by inserting energy. We recorded the vibrations at adjacent positions on the basilar membrane in sensitive gerbil cochleae and tested the putative power amplification in two ways. First, we determined the energy flux of the traveling wave at its peak and compared it to the acoustic power entering the ear, thereby obtaining the net cochlear power gain. For soft sounds, the energy flux at the peak was 1 ± 0.6 dB less than the middle ear input power. For more intense sounds, increasingly smaller fractions of the acoustic power actually reached the peak region. Thus, we found no net power amplification of soft sounds and a strong net attenuation of intense sounds. Second, we analyzed local wave propagation on the basilar membrane. We found that the waves slowed down abruptly when approaching their peak, causing an energy densification that quantitatively matched the amplitude peaking, similar to the growth of sea waves approaching the beach. Thus, we found no local power amplification of soft sounds and strong local attenuation of intense sounds. The most parsimonious interpretation of these findings is that cochlear sensitivity is not realized by amplifying acoustic energy, but by spatially focusing it, and that dynamic compression is realized by adjusting the amount of dissipation to sound intensity.     

Basilar membrane mechanics in the hook region of cat and guinea-pig cochleae: sharp tuning and nonlinearity in the absence of baseline position shifts.

A heterodyne laser interferometer was used to observe the movements of small (approximately 20 microns) stainless-steel beads placed on the basilar membrane in the hook region of cat and guinea-pig cochleae. In several preparations, the displacement patterns observed exhibited sharp nonlinear tuning; in one cat this tuning was comparable to that commonly observed in single auditory-nerve fibers. The most sensitive frequencies of the preparations ranged from 31-40 kHz in the cat, and 28-32 kHz in the guinea-pig. The sharp tuning and nonlinearity of the basilar membrane responses was not apparent in surgically or acoustically traumatized preparations. The response nonlinearities were susceptible to temporary threshold shifts and disappeared within a few minutes post-mortem. Stimulus-related shifts in the baseline position of the basilar membrane were not apparent at low stimulus levels. Such shifts were occasionally observed at higher stimulus levels (e.g., > 90 dB SPL), but never approached the fundamental (oscillatory) component of basilar membrane vibration in magnitude. These findings are discussed in relation to previous observations by other workers.     

A three-dimensional nonlinear active cochlear model analyzed by the WKB-numeric method

A physiologically based nonlinear active cochlear model is presented. The model includes the three-dimensional viscous fluid effects, an orthotropic cochlear partition with dimensional and material property variation along its length, and a nonlinear active feed-forward mechanism of the organ of Corti. A hybrid asymptotic and numerical method combined with Fourier series expansions is used to provide a fast and efficient iterative procedure for modeling and simulation of the nonlinear responses in the active cochlea. The simulation results for the chinchilla cochlea compare very well with experimental measurements, capturing several nonlinear features observed in basilar membrane responses. These include compression of response with stimulus level, two-tone suppressions, and generation of harmonic distortion and distortion products.     

Dynamic properties of the responses of single neurons in the inferior colliculus of the rat

The responses of single cells in the central nucleus of the inferior colliculus of the rat were studied with characteristic frequency tones amplitude modulated by pseudorandom noise or sinusoidal waveforms, in order to investigate the degree to which these responses can be described by a linear model. When pseudorandom noise was used as the modulating waveform, period histograms of the response locked to the periodicity of the noise were cross-correlated with a single period of the noise. The response of a model, having this cross-correlogram as its impulse response and the pseudorandom noise sequence as the input waveform, differed in appearance from the corresponding period histogram of the neural discharges, indicating that the latter contained a non-negligible, nonlinear component. Further manipulation of the data showed that the most significant nonlinearities were of even order, which indicates that the changes in neural discharge rate to increments and decrements in stimulus intensity are asymmetrical. In some units, particularly at low mean stimulus intensities, this was clearly evident as a halfwave rectification of the period histogram. The magnitude of the modulation of the period histograms increased as a function of the sound intensity for some units, while in others it decreased; in still other units the magnitude of the modulation of the neural discharges was relatively constant over a large range of stimulus intensities. When the modulation transfer functions were estimated from the responses to noise-modulated sounds they were found to be very similar to those obtained using sinusoidally modulated sound, despite large degrees of nonlinearity being present in the responses to both types of sound.     

Spatial Tuning Curves Along the Chick Basilar Papilla in Normal and Sound-Exposed Ears

Intense sound exposure destroys chick short hair cells and damages the tectorial membrane. Within a few days postexposure, signs of repair appear resulting in nearly complete structural recovery of the inner ear. Tectorial membrane repair, however, is incomplete, leaving a permanent defect on the sensory surface. The consequences of this defect on cochlear function, and particularly frequency analysis, are unclear. The present study organizes the sound-induced discharge activity of cochlear nerve units to describe the distribution of neural activity along the tonotopic axis of the basilar papilla. The distribution of this activity is compared in 12-day postexposed and age-matched control groups. Spontaneous activity, tuning curves, and rate–intensity functions were measured in each unit. Discharge activity at 60 frequency and intensity combinations was identified in the tuning curves of hundreds of units. Activity at each of these criterion frequency/intensity combinations was plotted against the units characteristic frequency to construct spatial tuning curves (STCs). The STCs depict tone-driven cochlear nerve activity along the length of the papilla. Tuning sharpness, low- and high- frequency slopes, and the maximum response were quantified for each STC. The sharpness of tuning increased with increasing criterion frequency. However, within a frequency, increasing sound intensity yielded more broadly tuned STCs. Also, the high-frequency slope was consistently steeper than the low-frequency slope. The STCs of exposed ears exhibited slightly less frequency selectivity than control ears across all frequencies and larger maximum responses for STCs with criterion frequencies spanning the tectorial membrane defect. When rate–intensity types were segregated, differences were observed in the STCs between saturating and sloping-up units. We propose that STC shape may be determined by global mechanical events, as well as localized tuning and nonlinear processes associated with individual hair cells. The results indicated that 12 days after intense sound exposure, global and local contributions to spatially distributed neural activity are restored.     

Basilar Membrane Responses to Noise at a Basal Site of the Chinchilla Cochlea: Quasi-Linear Filtering

Basilar membrane responses to clicks and to white noise were recorded using laser velocimetry at basal sites of the chinchilla cochlea with characteristic frequencies near 10 kHz. Responses to noise grew at compressive rates and their instantaneous frequencies decreased with increasing stimulus level. First-order Wiener kernels were computed by cross-correlation of the noise stimuli and the responses. For linear systems, first-order Wiener kernels are identical to unit impulse responses. In the case of basilar membrane responses, first-order Wiener kernels and responses to clicks measured at the same sites were similar but not identical. Both consisted of transient oscillations with onset frequencies which increased rapidly, over about 0.5 ms, from 4–5 kHz to the characteristic frequency. Both first-order Wiener kernels and responses to clicks were more highly damped, exhibited slower frequency modulation, and grew at compressive rates with increasing stimulus levels. Responses to clicks had longer durations than the Wiener kernels. The statistical distribution of basilar membrane responses to Gaussian white noise is also Gaussian and the envelopes of the responses are Rayleigh distributed, as they should be for Gaussian noise passing through a linear band-pass filter. Accordingly, basilar membrane responses were accurately predicted by linear filters specified by the first-order Wiener kernels of responses to noise presented at the same level. Overall, the results indicate that cochlear nonlinearity is not instantaneous and resembles automatic gain control.     

Level-Dependent Changes in Perception of Speech Envelope Cues

Level-dependent changes in temporal envelope fluctuations in speech and related changes in speech recognition may reveal effects of basilar-membrane nonlinearities. As a result of compression in the basilar-membrane response, the “effective” magnitude of envelope fluctuations may be reduced as speech level increases from lower level (more linear) to mid-level (more compressive) regions. With further increases to a more linear region, speech envelope fluctuations may become more pronounced. To assess these effects, recognition of consonants and key words in sentences was measured as a function of speech level for younger adults with normal hearing. Consonant–vowel syllables and sentences were spectrally degraded using “noise vocoder” processing to maximize perceptual effects of changes to the speech envelope. Broadband noise at a fixed signal-to-noise ratio maintained constant audibility as speech level increased. Results revealed significant increases in scores and envelope-dependent feature transmission from 45 to 60 dB SPL and decreasing scores and feature transmission from 60 to 85 dB SPL. This quadratic pattern, with speech recognition maximized at mid levels and poorer at lower and higher levels, is consistent with a role of cochlear nonlinearities in perception of speech envelope cues.     

Stimulus Frequency Otoacoustic Emission Delays and Generating Mechanisms in Guinea Pigs,Chinchillas, and Simulations

According to coherent reflection theory (CRT), stimulus frequency otoacoustic emissions (SFOAEs) arise from cochlear irregularities coherently reflecting energy from basilar membrane motion within the traveling-wave peak. This reflected energy arrives in the ear canal predominantly with a single delay at each frequency. However, data from humans and animals indicate that (1) SFOAEs can have multiple delay components, (2) low-frequency SFOAE delays are too short to be accounted for by CRT, and (3) “SFOAEs” obtained with a 2nd (“suppressor”) tone ≥2 octaves above the probe tone have been interpreted as arising from the area basal to the region of cochlear amplification. To explore these issues, we collected SFOAEs by the suppression method in guinea pigs and time-frequency analyzed these data, simulated SFOAEs, and published chinchilla SFOAEs. Time-frequency analysis revealed that most frequencies showed only one SFOAE delay component while other frequencies had multiple components including some with short delays. We found no systematic patterns in the occurrence of multiple delay components. Using a cochlear model that had significant basilar membrane motion only in the peak region of the traveling wave, simulated SFOAEs had single and multiple delay components similar to the animal SFOAEs. This result indicates that multiple components (including ones with short delays) can originate from cochlear mechanical irregularities in the SFOAE peak region and are not necessarily indicative of SFOAE sources in regions ≥2 octaves basal of the SFOAE peak region. We conclude that SFOAEs obtained with suppressors close to the probe frequency provide information primarily about the mechanical response in the region that receives amplification, and we attribute the too-short SFOAE delays at low frequencies to distortion-source SFOAEs and coherent reflection from multiple cochlear motions. Our findings suggest that CRT needs revision to include reflections from multiple motions in the cochlear apex.     

Cochlear nonlinearities inferred from two-tone distortion products in the ear canal of the alligator lizard

Distortion products ( DPs ) evoked by two-tone stimuli at frequencies F1 and F2 were measured in the ear-canal sound pressure of the alligator lizard. The largest sound pressures measured, other than those at F1 and F2, where at the cubic difference frequencies 2F1-F2 and 2F2-F1. All cubic DPs were greatly reduced by destruction of the basilar membrane, which suggests that its nonlinear properties are the source of the DPs . Measurements following acoustic overstimulation show a complex relationship between the magnitude of DPs and cochlear state, as assessed by measurements of cochlear potential, and indicate the existence of multiple nonlinear sources within the inner ear. Relative magnitudes of the DPs and their dependence on stimulus level suggest that the inner-ear DP sources are cubic nonlinearities. The DPs are not highly sensitive to either average stimulus frequency or stimulus frequency separation, suggesting that the nonlinear processes are within the macromechanical processes of the inner ear. Contrary to some interpretations of ear-canal DP measurements in mammals, we conclude that DPs need not be associated with hair-cell processes and are not particularly useful indicators of cochlear health.     

Measurement of basilar membrane vibrations and evaluation of the cochlear condition

The experimental procedure for measuring basilar membrane responses to acoustic signals is described. The surgical procedure developed for opening the cochlea with minimal trauma is presented. Each experiment included sound pressure level measurements to define the input signal, cochlear microphonic (CM) measurements to monitor the cochlear condition, interferometric measurements and histological evaluation of the cochleas. The characteristic frequency (CF) and the sensitivity at CF for the basilar membrane response is correlated with the change of CM response observed in six animals. It is demonstrated that both tuning characteristics are extremely sensitive to cochlear trauma as evidenced by changes of CM.     

Basilar Membrane Vibrations Near the Round Window of the Gerbil Cochlea

Using a laser velocimeter, responses to tones were measured at a basilar membrane site located about 1.2 mm from the extreme basal end of the gerbil cochlea. In two exceptional cochleae in which function was only moderately disrupted by surgical preparations, basilar membrane responses had characteristic frequencies (CFs) of 34–37 kHz and exhibited a CF-specific compressive nonlinearity: Sensitivity near the CF decreased systematically and the response peaks shifted toward lower frequencies with increasing stimulus level. Response phases also changed with increases in stimulus level, exhibiting small relative lags and leads at frequencies just lower and higher than CF, respectively. Basilar membrane responses to low-level CF tones exceeded the magnitude of stapes vibrations by 54–56 dB. Response phases led stapes vibrations by about 90° at low stimulus frequencies; at higher frequencies, basilar membrane responses increasingly lagged stapes vibration, accumulating 1.5 periods of phase lag at CF. Postmortem, nonlinearities were abolished and responses peaked at ~0.5 octave below CF, with phases which lagged and led in vivo responses at frequencies lower and higher than CF, respectively. In conclusion, basilar membrane responses near the round window of the gerbil cochlea closely resemble those for other basal cochlear sites in gerbil and other species.     

Variation in the Phase of Response to Low-Frequency Pure Tones in the Guinea Pig Auditory Nerve as Functions of Stimulus Level and Frequency

The directionality of hair cell stimulation combined with the vibration of the basilar membrane causes the auditory nerve fiber action potentials, in response to low-frequency stimuli, to occur at a particular phase of the stimulus waveform. Because direct mechanical measurements at the cochlear apex are difficult, such phase locking has often been used to indirectly infer the basilar membrane motion. Here, we confirm and extend earlier data from mammals using sine wave stimulation over a wide range of sound levels (up to 90 dB sound pressure level). We recorded phase-locked responses to pure tones over a wide range of frequencies and sound levels of a large population of auditory nerve fibers in the anesthetized guinea pig. The results indicate that, for a constant frequency of stimulation, the phase lag decreases with increases in the characteristic frequency (CF) of the nerve fiber. The phase lag decreases up to a CF above the stimulation frequency, beyond which it decreases at a much slower rate. Such phase changes are consistent with known basal cochlear mechanics. Measurements from individual fibers showed smaller but systematic variations in phase with sound level, confirming previous reports. We found a “null” stimulation frequency at which little variation in phase occurred with sound level. This null frequency was often not at the CF. At stimulation frequencies below the null, there was a progressive lag with sound level and a progressive lead for stimulation frequencies above the null. This was maximally 0.2 cycles.     

Stimulus Frequency Otoacoustic Emissions Provide No Evidence for the Role of Efferents in the Enhancement Effect

Auditory enhancement refers to the perceptual phenomenon that a target sound is heard out more readily from a background sound if the background is presented alone first. Here we used stimulus-frequency otoacoustic emissions (SFOAEs) to test the hypothesis that activation of the medial olivocochlear efferent system contributes to auditory enhancement effects. The SFOAEs were used as a tool to measure changes in cochlear responses to a target component and the neighboring components of a multitone background between conditions producing enhancement and conditions producing no enhancement. In the “enhancement” condition, the target and multitone background were preceded by a precursor stimulus with a spectral notch around the signal frequency; in the control (no-enhancement) condition, the target and multitone background were presented without the precursor. In an experiment using a wideband multitone stimulus known to produce significant psychophysical enhancement effects, SFOAEs showed no changes consistent with enhancement, but some aspects of the results indicated possible contamination of the SFOAE magnitudes by the activation of the middle-ear-muscle reflex. The same SFOAE measurements performed using narrower-band stimuli at lower sound levels also showed no SFOAE changes consistent with either absolute or relative enhancement despite robust psychophysical enhancement effects observed in the same listeners with the same stimuli. The results suggest that cochlear efferent control does not play a significant role in auditory enhancement effects.     

Synchronized responses of primary auditory fibre-populations in Caiman crocodilus (L.) to single tones and clicks

Measurements of the responses to tones and clicks were made from single primary auditory fibres of the caiman. The distribution of the amplitude and phase of the fundamental component of the response rate modulation over the best frequencies of the fibres is comparable to that reported in the cat, despite the fact that the basilar membrane in caiman is only 4.5 mm long. However, much higher intensities are needed in the caiman (75-85 dB SPL) than reported in the cat (20 dB SPL) to obtain systematic distributions of the phase of the responses, probably due to the larger scatter of the phase responses in the caiman. The slopes of the phase distributions are very similar to those in cat. Single unit phase responses as a function of stimulus frequency at 85 dB SPL can be approximated by one, or in fibres with low best frequency, two straight lines. At lower intensities the deviation of the phase-frequency responses from a straight line increases as the group delay at the best frequency becomes larger. The shortest latencies of click responses are obtained with rarefaction clicks. Group delay estimates obtained from the responses to clicks and from the straight line approximations of the phase-frequency responses are related in a way expected for linear filter systems and accurately predict the measured distributions of the phase of the responses over the neural best frequency. The obtained group delays and click latencies in the caiman are very similar to those reported by other workers in the cat, the squirrel monkey and the treefrog, despite large morphological and probably functional differences of their inner ears. The click latencies are also very similar to those in the pigeon. The results are consistent with the existence of a mechanical travelling wave reported previously on the basilar membrane of the caiman, but at the same stimulus level the phase characteristic of the present single unit responses is steeper and the wave length estimates from the neural population phase distributions are shorter than those observed directly in the motion of the basilar membrane. Since the neural responses are an indirect estimate of the basilar membrane motion it cannot be decided whether the difference between neural and mechanical data is due to deterioration of the basilar membrane responses during the direct measurements or whether the basilar membrane response is sharpened by additional tuning mechanisms.     

Response from the exposed intracranial human auditory nerve to low-frequency tones: basic characteristics

The responses recorded from the exposed intracranial portion of the eighth nerve in man with normal hearing to short bursts of low-frequency tones (500, 1000, and 1500 Hz) consist of two components; these two components can be separated by adding and subtracting, respectively, the responses to tonebursts of opposite polarity. Subtracting the responses to tones of opposite polarity reveals a waveform that resembles the sinusoidal waveform of the stimulus (frequency-following response = FFR), while adding the responses to tones of opposite polarity reveals a slow component, the waveform of which is more variable than the frequency-following component. The initial deflection of the slow component of the response to 1000 Hz and to 1500 Hz is a positive peak followed by a slow, negative deflection, and the response to 1500-Hz tonebursts often shows a clear off-response. The slow component of the response to 500-Hz tones often has an initial negative peak followed by a slow, positive or negative wave. The temporal relationship between the stimulus tone and the frequency-following component changes only slightly when the intensity of the sound is changed, whereas the latency of the slow potential decreases with increasing stimulus intensity. The FFR can be masked by noise, and the results of masking with highpass-filtered noise indicate that the frequency-following response may be generated at a location on the basilar membrane that is tuned to a frequency that is higher than that of the stimulus tone.