Glenis R. Long

New off-line method for detecting spontaneous otoacoustic emissions in human subjects

Spontaneous otoacoustic emissions were evaluated in 36 female and 40 male subjects. In agreement with the results of previous surveys, emissions were found to be more prevalent in female subjects and there was a tendency for the male subjects to have fewer emissions in their left ears. The digitization of five minute samples of ear canal signals, combined with sophisticated data analysis, produced a substantial reduction in the emission detection threshold. 588 emissions were detected in 72% of the subjects and 56% of the ears. Of the observed emissions, 18 could be identified with cubic distortion products of other emissions, and 11 could be identified as harmonic products (i.e., integral frequency multiples of other emissions). The large number of emissions detected (one subject had 32 in her right ear and 25 in her left) permitted evaluation of the pattern of separation of emissions. The average effective separation along the basilar membrane (according to the Greenwood frequency map) for adjacent emissions of all ears was 0.427 mm with interquartile values of 0.387 mm and 0.473 mm. The relationship between emission power, frequency, and full width at half maximum appears to be in agreement with the implications of a noise perturbed Van der Pol oscillator model of spontaneous emissions.

Investigations into the nature of the association between threshold microstructure and otoacoustic emissions

Three studies are described which investigate the nature of the association between threshold microstructure and otoacoustic emissions. In the first study, threshold dips (similar in shape to those seen in threshold microstructure) are produced by introducing a low-level masker. Threshold microstructure is not abolished when tonal probes are replaced by narrowband-noise probes, while dips induced by external tonal maskers are eliminated. These findings rule out a simple interpretation of the microstructure dips as an instance of masking by otoacoustic emissions. In the second study, ear-canal measurements of the interactions of external tones with spontaneous emissions indicate that, although beating is often detected near threshold maxima, stimuli close to threshold minima are perceived as tonal because the emission is frequency locked by the external tone. The last study shows that reduction of the levels of otoacoustic emissions by aspirin consumption is associated with an initial reduction of thresholds in regions of threshold microstructure, with the greatest reduction occurring at threshold maxima. This suggests that threshold maxima may be due, at least in part, to interference or masking by the nearby otoacoustic emissions. A simple analog (driven Van der Pol oscillator) of an external tone interacting with a spontaneous emission is used to interpret ear-canal pressure waveforms and associated psychophysical percepts (including threshold detection), for tones close in frequency to emissions.

Relaxation dynamics of spontaneous otoacoustic emissions perturbed by external tones. II. Suppression of interacting emissions

The level of a spontaneous otoacoustic emission (SOAE) during recovery from suppression by an external tone sometimes exhibits a prominent overshoot before reaching its normal level. At the onset of suppression, a less prominent undershoot is sometimes observed before the emission level stabilizes. The overshoot and undershoot are described in terms of the variable amount suppression produced by a neighboring higher-frequency SOAE which is responding more slowly to the modulation of the external tone. The variation of the SOAE amplitude during pulsed suppression is modeled by a pair of Van der Pol limit-cycle oscillators with the primary oscillator linearly coupled to the displacement of the secondary high-frequency one. We have found relaxation time constants for the onset of suppression of the order of 4.5 and 7.4 ms for the primary and secondary SOAEs, respectively, and for the recovery from suppression 4.8 and 10.48 ms for the primary and secondary SOAEs, respectively. The same model is also successful in describing the release from suppression of the primary SOAE by the secondary SOAE when the latter is partially suppressed by the external tone. Aspirin administration reduces the magnitude of the overshoot by reducing the level of the higher-frequency SOAE and thereby eliminating the suppression of the lower-frequency one.

Perceptual consequences of the interactions between spontaneous otoacoustic emissions and external tones. I. Monaural diplacusis and aftertones

Research into monaural diplacusis has led to the concept of idiotones (tone-like stimuli of cochlea origin). Spontaneous otoacoustic emissions (SOAEs) are tone-like stimuli generated by the cochlea and detected in the ear canal. In diplacusis, the existence of idiotones is inferred from disturbances of the perception of single tones. Spontaneous otoacoustic emissions are measured by placing a small microphone at the entrance to the ear canal. Many of the puzzling properties of the hypothesized idiotones are consistent with measurements of the interaction of SOAEs with external tones. The interactions of the SOAEs with external tones were analyzed acoustically. The perceptual properties evoked by 250 ms pulses (presented twice a second) of the acoustic stimuli used in the OAE experiments were systematically investigated. At some stimulus levels, all subjects reported the perception of a second tone alternating with the external tone. The relative pitch of this percept was consistent with the frequency of the SOAE. The frequency dependence of the signal levels needed for the percept had many aspects in common with the suppression tuning curves of the SOAEs. At lower levels of the external tone the subjects sometimes reported a perception of two simultaneous tones. This would be consistent with the subject detecting SOAEs when they are frequency shifted, but not suppressed. The consumption of aspirin by one subject reduced the SOAE into the noise floor and eliminated the monaural diplacusis.

Temperature dependence of spontaneous otoacoustic emissions in the edible frog (Rana esculenta)

The change in frequency of individual emissions in the European edible frog (Rana esculenta) when the temperature of the frog is modified, is part of a complex pattern of interaction between spontaneous otoacoustic emissions. At high temperatures (above 24 degrees C) two emissions are always detected (e.g., one near 800 Hz and one near 1200 Hz). The higher-frequency emission is lower in level and has a wider bandwidth than the lower-frequency emission. It is also often asymmetric and sometimes breaks into two emissions when an external suppressor tone is applied. When the temperature is decreased, these emissions are reduced in frequency at a rate of 0.04 octave/degree C. The higher-frequency emission becomes narrower and taller, and the lower-frequency emissions becomes broader and less intense. At approximately 18 degrees C the lowest of these emissions (now between 600 and 700 Hz) disappears and is replaced by a new emission approximately 100 Hz lower in frequency. When the temperature is carefully controlled the two emissions can exist simultaneously. The lowest-frequency emission changes 0.015 degree C/octave suggesting that the mechanisms controlling the frequency of this emission may be different than those determining the frequencies of the other emissions. All but the lowest-frequency emissions are maximal in level and have minimal bandwidth when the frequency is close to 700 Hz, which is interpreted as evidence that these emissions are filtered by a temperature-independent process.

Relaxation dynamics of spontaneous otoacoustic emissions perturbed by external tones. I. Response to pulsed single-tone suppressors.

The dynamic aspects of the suppression of spontaneous otoacoustic emissions by external tones are evaluated. A Van der Pol oscillator driven by an external tone is used as an interpretive model for data on the pulsed suppression of spontaneous emissions obtained from six female subjects. Typical results for both the onset of, and recovery from suppression yield 1/r1 (where -r1 is the negative linear component of the damping function) in the range of 2-25 ms. In accordance with the predictions of the model, (a) the relaxation time for the onset of suppression increases with the amount of suppression induced by the external tone, (b) the values of r1 and the amplitudes of the unsuppressed emissions exhibit an inverse correlation, (c) the values inferred for r1 are not significantly dependent on the frequency of the pulsed suppressor tone, and (d) the inferred r1 values are not significantly dependent upon the amount of suppression.

Changes in Spontaneous and Evoked Otoacoustic Emissions and Corresponding Psychoacoustic Threshold Microstructures Induced by Aspirin Consumption

Spontaneous otoacoustic emissions, delayed evoked emissions, synchronous evoked emissions and psychoacoustical threshold microstructure were monitored (in two subjects) before, during and after the consumption of 3.9g of aspirin per day for three consecutive days (12 doses of three 325mg tablets every 6 hours). Spontaneous emissions followed a pattern similar to that found by McFadden and Plattsmeir (1984). Evoked emissions were also reduced by aspirin consumption but persisted longer and recovered sooner. Reduction of psychoacoustic threshold microstructure associated with the emissions followed much the same time course as the evoked emissions. In most instances the reduction of threshold microstructure began with a lowering of threshold maxima (with threshold minima remaining relatively constant) and ended with all thresholds elevated.

Interactions Among Multiple Spontaneous Otoacoustic Emissions

Evidence has recently been obtained (Burns et al., 1984) for several interactions among spontaneous otoacoustic emissions (SOAEs) including intermodulation distortion products, mutual suppression, and noncontiguous-linked SOAEs which apparently share energy between two quasi-stable states. In this paper, we give an updated record of our findings on intermodulation distortion products and linked emissions, and give evidence that the former tend to occur when a distortion product frequency is close to that of a cochlear resonance. Computer simulations of the interactions among van der Pol oscillators, which represent nonlinear active elements in a simplified cochlear model, appear to qualitatively account for some of the observed features of SOAE interactions.

Quantitative Evaluation of Limit-Cycle Oscillator Models of Spontaneous Otoacoustic Emissions

Single limit-cycle-oscillator models of spontaneous otoacoustic emissions (SOAE’s) are based on the assumption that the pattern of interactions between spontaneous emissions and external tones in the ear canal may be partially described by the gross compaction of a full cochlear model to a single nonlinear differential equation such as that of a free (or driven) Van der Pol oscillator. Such an equation incorporates, in a highly idealized way, the type of nonlinear-active damping which, if assumed to be present over certain portions of the cochlear partition, would produce stabilized cochlear self-oscillations and lead to measurable spontaneous emissions in the ear-canal. These models have been used successfully by our group and by Wit and collaborators to account for a number of features of the emission data including: a) statistical properties of emissions (e.g. Bialek and Wit, 1984; Wit, 1986; van Dijk, 1990); b) suppression of emissions (Long and Tubis, 1990) and synchronization (phase locking) of emissions by external tones (e.g., van Dijk, 1990; Long, et al., 1990); and c) reduction of the level of emissions by aspirin consumption (e.g., Long and Tubis, 1988a,b).

Relaxation dynamics of spontaneous otoacoustic emissions perturbed by external tones. III. Response to a single tone at multiple suppression levels

The relaxation dynamics of spontaneous otoacoustic emissions (SOAEs) interacting with an external tone have been successfully described using a van der Pol limit cycle oscillator model [Murphy et al., J. Acoust. Soc. Am. 97, 3702-3710 (1995) and Murphy et al., J. Acoust. Soc. Am. 97, 3711-3720 (1995)]. Data were collected for an SOAE interacting with a single-frequency ipsilateral suppressor. Transitions between different suppressed states were achieved by adding or removing signal at the suppressor frequency. The relaxation dynamics of the van der Pol model provided a good fit to the data.