Effect of interstimulus interval on evoked otoacoustic emissions

Temporal suppression of the click-evoked otoacoustic emission level-curve

The click-evoked otoacoustic emission (CEOAE) level-curve grows linearly for clicks below 40-60 dB and saturates for higher inputs. This study investigates dynamic (i.e., time-dependent) features of the CEOAE level-curve by presenting a suppressor-click less than 8 ms before the test-click. An alteration of the CEOAE level-curve, designated here as temporal suppression, was observed within this time period, and was shown to depend on the levels and the temporal separation of the two clicks. Temporal suppression occurred for all four subjects tested, and resulted in a vertical offset from the unsuppressed level-curve for test-click levels greater than 50 dB peak-equivalent level (peSPL). Temporal suppression was greatest for suppressors presented 1-4 ms before the test click, and the magnitude and time scale of the effect were subject dependent. Temporal suppression was furthermore observed for the short- (i.e., 6-18 ms) and long-latency (i.e., 24-36 ms) regions of the CEOAE, indicating that temporal suppression similarly affects synchronized spontaneous otoacoustic emissions (SSOAEs) and purely evoked CEOAE components. Overall, this study demonstrates that temporal suppression of the CEOAE level-curve reflects a dynamic process in human cochlear processing that works on a time scale of 0-10 ms.

Temporal suppression and augmentation of click-evoked otoacoustic emissions

This study investigates temporal suppression of click-evoked otoacoustic emissions (CEOAEs), occurring when a suppressor-click is presented close in time to a test-click (e.g. 0-8ms). Various temporal suppression methods for examining temporal changes in cochlear compression were evaluated and measured here for seven subjects, both for short- and long-latency CEOAEs. Long-latency CEOAEs (duration >20ms) typically indicate the presence of synchronised spontaneous otoacoustic emissions (SSOAEs). Temporal suppression can only be linked to changes in CEOAE-compression if the suppressor-click affects the CEOAE magnitude. Phase changes induced by the suppressor-click were shown to bias suppression in two ways: (i) when a specific asymmetric measurement method was used and (ii) when synchronisation between the CEOAE and the click-stimuli was incomplete. When such biases were eliminated, temporal suppression and augmentation (the opposite effect) were observed and shown to be subject-dependent. This indicates that the nonlinearity underlying temporal suppression can work in a more (i.e., suppressed) or less (i.e., augmented) compressive state, depending on the inter-click interval and the subject under test. Temporal suppression was shown to be comparable for CEOAEs and SSOAEs, indicating similar underlying cochlear nonlinear mechanisms. This study contributes to a better understanding of the temporal properties of cochlear dynamics.

Dynamic nonlinear cochlear model predictions of click-evoked otoacoustic emission suppression

A comprehensive set of results from 2-click suppression experiments on otoacoustic emissions (OAEs) have been presented by Kapadia and Lutman [Kapadia, S., Lutman, M.E., 2000a. Nonlinear temporal interactions in click-evoked otoacoustic emissions. I. Assumed model and polarity-symmetry. Hear. Res. 146, 89-100]. They found that the degree of suppression of an OAE evoked by a test click varied systematically with the timing and the level of a suppressor click, being greatest for suppressor clicks occurring some time before the test click, particularly at lower levels of suppression. Kapadia and Lutman also showed that although the general shape of the graph of suppression against suppressor click timing could be predicted by a static power law model, this did not predict the asymmetry with respect to the timing of the suppressor click. A generalised automatic gain control (AGC) is presented as a simple example of a dynamic nonlinear system. Its steady state nonlinear behaviour, as quantified by its level curve, and its dynamic behaviour, as quantified by its transient response, can be independently set by the feedback gain law and detector time constant, respectively. The previously reported suppression results, with the asymmetry in the timing, are found to be predicted better by such an AGC having a level curve with a slope of about 0.5 dB/dB, and a detector time constant of about twice the period at the characteristic frequency. Although this gives adequate predictions for high suppression levels, it under predicts the suppression and the asymmetry for lower levels. Further research is required to establish whether simple peripheral feedback models can explain OAE suppression of this type.

Recovery of human compound action potential using a paired-click stimulation paradigm

The recovery process from adaptation of the compound action potential (CAP) was studied using an equilevel paired click stimulation paradigm in humans with normal hearing. The CAP amplitude to the second click of paired clicks was normalized to the amplitude to the first click. The second-click CAP amplitude recovered as a function of interclick interval (ICI) between the first and the second click of a pair. A regression line fitted to the recovered amplitude data demonstrated the logarithmic function of the ICI. Full recovery times changed from 118 to 278 ms with increasing click intensity. The regression lines for higher click intensities exhibited two different slopes in two ICI ranges: from 3 to 100 and 120 to 300 ms. We suppose that the CAP recovery for ICI <100 ms is attributable to both the relative refractoriness of auditory nerve and the short-term adaptation mechanisms, while, for ICI >100 ms chiefly to the short-term adaptation mechanisms. The recovery process of the second-click CAP slowed with increasing intensity, which is a similar result to that obtained in the animal experiments by Parham et al. The input-output (I-O) curve of the second-click CAP amplitudes exhibited a different slopes above and below 60 dB normal hearing level (nHL). We assume that the mechanisms underlying this characteristic curve pattern differ from those for the I-O curve of the CAP in response to single-click stimuli. We expect that investigating the CAP recovery in pathological ears will provide clinically useful information on cochlear synaptic function.

Temporal nonlinearity revealed by transient evoked otoacoustic emissions recorded to trains of multiple clicks

A series of detailed experiments is described that investigates how a transient evoked otoacoustic emission (TEOAE) recorded to one-click stimulus is affected by the presence of a variable number of preceding clicks presented over a range of interclick intervals (ICIs). Part of the rationale was to determine if the resulting nonlinear temporal interactions could help explain the amplitude reduction seen when TEOAEs are recorded at very high click rates, as when using maximum length sequence stimulation. Amongst the findings was that the presence of a preceding train of clicks could either suppress or enhance emission amplitude, depending on the number of clicks in the train and the ICI. Results also indicated that the duration of the click trains, rather than the ICI, was the important factor in yielding the most suppressed response and that this seemed to depend on stimulus level. The results recorded at two levels also suggested that the cochlear temporal nonlinearity being monitored was in part related to the nonlinear process that determines the compressive input/output function for stimulus level. It is hypothesised that nonlinear temporal overlap of vibration patterns on the basilar membrane may underlie much of the pattern of results.

Nonlinear temporal interactions in click-evoked otoacoustic emissions. I. Assumed model and polarity-symmetry

Click-evoked otoacoustic emissions (CEOAEs) are reduced in amplitude by the presentation of 'suppressor' clicks that either closely lead or follow the stimulus ('test') clicks. This suppression of the response represents nonlinear temporal interactions between the test and suppressor clicks and/or the CEOAEs they evoke. There are some discrepancies amongst previous reports of the phenomenon, and the underlying mechanisms are not understood. In particular, it is unclear whether the suppression reported simply reflects the compressive nonlinearity of the CEOAE input-output (I-O) function. This paper presents a simple model of the nonlinear interactions between CEOAEs evoked by two closely-spaced clicks. The model shows that suppression as reported may be entirely derived from CEOAE I-O nonlinearity, in combination with the extended duration of the cochlear responses to click stimuli. It is also shown experimentally that suppression is insensitive to the polarities of test and suppressor clicks, which is consistent with the model based on I-O nonlinearity. A companion paper (Kapadia and Lutman, Hear. Res. 146 (2000)) presents experimental findings from a detailed parametric study of nonlinear temporal interactions in CEOAEs in human subjects with normal hearing. The findings are compared with the pattern of results generated by the above model, in order to assess the role of I-O nonlinearity in these nonlinear interactions.

Nonlinear temporal interactions in click-evoked otoacoustic emissions. II. Experimental data

Click-evoked otoacoustic emissions (CEOAEs) are reduced in amplitude by the presentation of 'suppressor' clicks that either closely lead or follow the stimulus ('test') clicks. A model described in a companion paper (Kapadia and Lutman, Hear. Res. 146 (2000) 89-100) shows that such nonlinear temporal interactions, as previously reported, may be explained in terms of the compressive non-linearity of the CEOAE input-output (I-O) function. This paper presents the results of a detailed parametric investigation into such nonlinear interactions, studied in 12 normal adult ears over a wide range of test and suppressor click levels and inter-click intervals. The results differ from those generated by the model in a number of respects. Principally, maximum suppression is generally obtained for suppressors presented in advance of test clicks, rather than co-incident with the test clicks. The amount of advance depends systematically on the two click levels. The measured suppression can also exceed the theoretical maximum allowed by the model. It is concluded that the nonlinear temporal interactions measured do not simply reflect CEOAE I-O function non-linearity. They may, instead, arise from disturbance of the generator elements from their resting state prior to generation of the CEOAE. These results may also have general implications relating to cochlear responses to transient stimuli and indicate the potential of CEOAEs in probing aspects of cochlear mechanics.

Neonatal otoacoustic emissions recorded using maximum length sequence stimuli

OBJECTIVE

Maximum length sequence (MLS) stimulation allows transient evoked otoacoustic emissions (TEOAEs) to be recorded at very high stimulation rates. Previous work has focused on recording from normally hearing adult subjects; the aim of this study was to obtain information about emissions recorded using this technique from newborns and to compare these results with those obtained from adults. The feasibility of recording from newborns on the postnatal wards also was addressed.

DESIGN

The study comprised two parts. In the first, TEOAEs were collected at 13 stimulation rates from a selected group of babies. The second part of the study comprised only two stimulation rates, a conventional rate of 40 clicks/sec and the maximum MLS rate of 5000 clicks/sec.

RESULTS

The neonatal MLS TEOAEs behave in a similar manner to those obtained from adult subjects. The morphology of the waveforms was similar for the conventional and MLS TEOAEs. As the stimulus rate increases, the amplitude of the emission decreases, reaching an approximate plateau by 1000 to 2000 clicks/sec. The absolute reduction in amplitude seen at the high MLS rate is related to the amplitude of the conventional TEOAE but is always approximately the same when expressed as a percentage or proportion of that amplitude.

CONCLUSION

The theoretical advantages of speed and sensitivity seen for adult subjects also should hold true for the neonatal population. Although the system used to test was a prototype with none of the refinements found in commercial systems, it was possible to record adequate emissions from a ward-based population of newborns.

Effect of olivocochlear bundle section on evoked otoacoustic emissions recorded using maximum length sequences

Presenting clicks according to maximum length sequences (MLS) enables transient evoked otoacoustic emissions (TEOAE) to be recorded at very high stimulation rates. As the click rate is increased from 40 clicks/s up to a maximum rate of 5000 clicks/s there is a reduction in TEOAE amplitude that reaches an approximate asymptote at 1500 clicks/s. One hypothesis put forward to explain this MLS 'rate effect' is that ipsilateral efferent activity is involved. To test this hypothesis TEOAEs were recorded from both ears of five patients who had undergone a unilateral vestibular nerve section--a surgical procedure which also entails sectioning the olivocochlear bundle. TEOAEs were recorded conventionally at 40 clicks/s and using MLS stimulation at 5000 clicks/s. Increasing the rate from 40 to 5000 clicks/s was found to reduce the amplitude of the TEOAEs by equivalent amounts in ears ipsilateral and contralateral to a vestibular nerve section as well as in the ears of normal-hearing adults. Since an ear ipsilateral to a vestibular nerve section should have no efferent innervation the hypothesis that efferent activity is the major mechanism involved in the MLS rate effect is rejected. Instead, the possibility that intracochlear processes are the underlying mechanism will now be investigated.

Contralateral suppression and stimulus rate effects on evoked otoacoustic emissions

Transiently evoked otoacoustic emissions (EOAE) were recorded in normal-hearing humans using pseudorandom pulse trains. This allowed the effect of increasing stimulus rate to be studied on EOAE amplitudes, input-output (I/O) functions and suppression by contralateral stimulation. EOAEs are very probably due to micromechanical properties of cochlear outer hair cells, and contralateral suppression is considered to result from olivo-cochlear efferent activation. EOAEs at various stimulus rates showed excellent reproducibility. Total EOAE amplitude diminished as interstimulus interval (ISI) decreased from 20 to 3 ms, but not for ISIs under 3 ms. The amplitude reduction was significant only on EOAE spectrum bands below 3.4 kHz. I/O functions, which kept a linear pattern, were steeper, and contralateral suppression was lower, with the highest stimulus rate (1111 c/s) relative to other rates. The EOAE decline with increasing stimulus rate might be due to incomplete recovery after adaptation of outer hair cells. The lower contralateral suppression at high stimulus rates suggests that crossed olivo-cochlear bundle action is lessened when outer hair cells are responding to a high-rate stimulus. An explanation may be that contralateral stimulation and a high-rate ipsilateral stimulus act via the same mechanisms, i.e., that high-rate stimulation activates an ipsilateral efferent loop.

Transient evoked otoacoustic emissions recorder using maximum length sequences as a function of stimulus rate and level

OBJECTIVE

The recently developed technique of recording transient evoked otoacoustic emissions (TEOAEs) using clicks presented according to maximum length sequences (MLSs) enables very high stimulation rates to be used. The aim of this study was to provide normative data on the relationship between TEOAEs recorded conventionally (at 40 clicks/sec) and those recorded using the MLS technique (at 11 maximum rates between 100 and 5000 clicks/sec) to establish a baseline for future clinical studies.

DESIGN

TEOAEs were recorded at 12 rates from 12 normally hearing adult ears at click levels decreasing in 5 dB steps from 68 dB peSPL.

RESULTS

The morphology of the waveforms and the pattern of the input/output functions with latency were similar for conventional and MLS TEOAEs. The only major difference between TEOAEs recorded at the different rates was in their absolute amplitude. As the click rate was increased from 40 clicks/sec there was a reduction in amplitude that reached a near asymptote at approximately 1500 clicks/sec. When expressed as a percentage reduction in amplitude compared with that recorded at 40 clicks/sec, this MLS "rate effect" was independent of stimulus level over all but the lowest test level (38 dB peSPL SPL).

CONCLUSION

Over a wide range of amplitudes of conventionally recorded TEOAEs (21 to 450 microPa for the 9 to 13 msec section of the otoacoustic emission), the mechanism involved in the MLS rate effect seems to act in a way that reduces the amplitude by an almost constant proportion, whatever its original size.

Auditory evoked brain stem responses to trains of stimuli in human adults

OBJECTIVE

This study describes the effect of train length, interstimulus interval, intertrain interval (ITI), and stimulus duration on the transition from the unadapted to the adapted wave V auditory evoked brain stem response (ABR).

DESIGN

ABRs were recorded to stimuli presented at two different rates: a slow rate characterizing the unadapted response and a fast rate characterizing the adapted response. Trains of stimuli (a sequence of stimuli separated by intervals of silence) also were presented. Different stimulus parameters defining the trains were varied.

RESULTS

Given a sufficiently long ITI, the latency prolongation to the first three or four stimuli in a train was rapid. It was similar for trains differing in number of stimuli. After the first three or four stimuli, there was a more gradual latency prolongation as a function of stimulus number. Shorter ITIs had the effect of prolonging the latencies to all the stimuli in the trains, reducing the rate of latency prolongation over the first few stimuli, and causing responses to trains of different length to differ (e.g., two click train responses were shorter latency than four click train responses). An unexpected result was the latency prolongation of wave Vs after the presentation of the stimulus trains.

CONCLUSIONS

In response to a train of clicks, there seems to be a rapid increase in wave V latency to the first few clicks in the train followed by a more gradual latency prolongation to subsequent clicks in the train.