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

The role of the Ventral Nucleus of the Trapezoid Body in the auditory prepulse inhibition of the acoustic startle reflex

Cholinergic signaling is essential to mediate the auditory prepulse inhibition (PPI), an operational measure of sensorimotor gating, that refers to the reduction of the acoustic startle reflex (ASR) when a low-intensity, non-startling acoustic stimulus (the prepulse) is presented just before the onset of the acoustic startle stimulus. The cochlear root neurons (CRNs) are the first cells of the ASR circuit to receive cholinergic inputs from non-olivocochlear neurons of the ventral nucleus of the trapezoid body (VNTB) and subsequently decrease their neuronal activity in response to auditory prepulses. Yet, the contribution of the VNTB-CRNs pathway to the mediation of PPI has not been fully elucidated. In this study, we used the immunotoxin anti-choline acetyltransferase (ChAT)-saporin as well as electrolytic lesions of the medial olivocochlear bundle to selectively eliminate cholinergic VNTB neurons, and then assessed the ASR and PPI paradigms. Retrograde track-tracing experiments were conducted to precisely determine the site of lesioning VNTB neurons projecting to the CRNs. Additionally, the effects of VNTB lesions and the integrity of the auditory pathway were evaluated via auditory brain responses tests, ChAT- and FOS-immunohistochemistry. Consequently, we established three experimental groups: 1) intact control rats (non-lesioned), 2) rats with bilateral lesions of the olivocochlear bundle (OCB-lesioned), and 3) rats with bilateral immunolesions affecting both the olivocochlear bundle and the VNTB (OCB/VNTB-lesioned). All experimental groups underwent ASR and PPI tests at several interstimulus intervals before the lesion and 7, 14, and 21 days after it. Our results show that the ASR amplitude remained unaffected both before and after the lesion across all experimental groups, suggesting that the VNTB does not contribute to the ASR. The%PPI increased across the time points of evaluation in the control and OCB-lesioned groups but not in the OCB/VNTB-lesioned group. At the ISI of 50 ms, the OCB-lesioned group exhibited a significant increase in%PPI (p < 0.01), which did not occur in the OCB/VNTB-lesioned group. Therefore, the ablation of cholinergic non-olivocochlear neurons in the OCB/VNTB-lesioned group suggests that these neurons contribute to the mediation of auditory PPI at the 50 ms ISI through their cholinergic projections to CRNs. Our study strongly reinforces the notion that auditory PPI encompasses a complex mechanism of top-down cholinergic modulation, effectively attenuating the ASR across different interstimulus intervals within multiple pathways.

Neural correlates of perceptual learning in the auditory brainstem: efferent activity predicts and reflects improvement at a speech-in-noise discrimination task

An extensive corticofugal system extends from the auditory cortex toward subcortical nuclei along the auditory pathway. Corticofugal influences reach even into the inner ear via the efferents of the olivocochlear bundle, the medial branch of which modulates preneural sound amplification gain. This corticofugal system is thought to contribute to neuroplasticity underlying auditory perceptual learning. In the present study, we investigated the involvement of the medial olivocochlear bundle (MOCB) in perceptual learning as a result of auditory training. MOCB activity was monitored in normal-hearing adult listeners during a 5 d training regimen on a consonant-vowel phoneme-in-noise discrimination task. The results show significant group learning, with great inter-individual variability in initial performance and improvement. As observed in previous auditory training studies, poor initial performers tended to show greater learning. Strikingly, MOCB activity measured on the first training day strongly predicted the subsequent amount of improvement, such that weaker initial MOCB activity was associated with greater improvement. Moreover, in listeners that improved significantly, an increase in MOCB activity was observed after training. Thus, as discrimination thresholds of listeners converged over the course of training, differences in MOCB activity between listeners decreased. Additional analysis showed that MOCB activity did not explain variation in performance between listeners on any training day but rather reflected an individual listener's performance relative to their personal optimal range. The findings suggest an MOCB-mediated listening strategy that facilitates speech-in-noise perception. The operation of this strategy is flexible and susceptible to training, presumably because of task-related adaptation of descending control from the cortex.

An investigation into the relationship between input–output nonlinearities and rate-induced nonlinearities of click-evoked otoacoustic emissions recorded using maximum length sequences

The maximum length sequence (MLS) technique allows otoacoustic emissions (OAEs) to be recorded using clicks presented at very high presentation rates. It has previously been found that increasing the click presentation rate leads to increasing suppression (termed "rate-suppression") of the MLS evoked OAE (Hine, J.E., Thornton, A.R.D., 1997. Transient evoked otoacoustic emissions recorded using maximum length sequences as a function of stimulus rate and level. Ear Hear. 18, 121-128). It has been suggested that the source of rate-suppression arises from the same nonlinear processes that give rise to the well-known nonlinear growth of OAEs. Based on this assumption, a simple model of rate-suppression (Kapadia, S., Lutman, M.E., 2001. Static input-output nonlinearity as the source of nonlinear effects in maximum length sequence click-evoked OAEs. Br. J. Audiol. 35, 103-112) predicts that both input-output (I/O) nonlinearity and rate-suppression can be unified by characterising the stimulus in terms of its acoustic power which, at high rates, is proportional to the click presentation rate. The objective of this study was to test this simple model by recording MLS OAEs from a group of normally hearing adults over a range of stimulus rates from 40 to 5000 clicks/s, and of stimulus levels from 45 to 70dB peSPL. The results are broadly in agreement with the predictions from the model, though there appears to be some tendency for the model to slightly overestimate the degree of rate-suppression for a given degree of I/O nonlinearity. It is also suggested that the model may break down more significantly in the presence of spontaneous OAEs.

Medial olivocochlear efferent reflex in humans: otoacoustic emission (OAE) measurement issues and the advantages of stimulus frequency OAEs

Otoacoustic emissions (OAEs) are useful for studying medial olivocochlear (MOC) efferents, but several unresolved methodological issues cloud the interpretation of the data they produce. Most efferent assays use a "probe stimulus" to produce an OAE and an "elicitor stimulus" to evoke efferent activity and thereby change the OAE. However, little attention has been given to whether the probe stimulus itself elicits efferent activity. In addition, most studies use only contralateral ( re the probe) elicitors and do not include measurements to rule out middle-ear muscle (MEM) contractions. Here we describe methods to deal with these problems and present a new efferent assay based on stimulus frequency OAEs (SFOAEs) that incorporates these methods. By using a postelicitor window, we make measurements in individual subjects of efferent effects from contralateral, ipsilateral, and bilateral elicitors. Using our SFOAE assay, we demonstrate that commonly used probe sounds (clicks, tone pips, and tone pairs) elicit efferent activity, by themselves. Thus, results of efferent assays using these probe stimuli can be confounded by unwanted efferent activation. In contrast, the single 40 dB SPL tone used as the probe sound for SFOAE-based measurements evoked little or no efferent activity. Since they evoke efferent activation, clicks, tone pips, and tone pairs can be used in an adaptation efferent assay, but such paradigms are limited in measurement scope compared to paradigms that separate probe and elicitor stimuli. Finally, we describe tests to distinguish middle-ear muscle (MEM) effects from MOC effects for a number of OAE assays and show results from SFOAE-based tests. The SFOAE assay used in this study provides a sensitive, flexible, frequency-specific assay of medial efferent activation that uses a low-level probe sound that elicits little or no efferent activity, and thus provides results that can be interpreted without the confound of unintended efferent activation.

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.

Temporal non-linearities of the cochlear amplifier revealed by maximum length sequence stimulation

OBJECTIVES

The objective of this study was to examine whether temporal non-linearities of the cochlear amplifier, as reflected by otoacoustic emissions (OAEs), exist and are distinct from any recording system non-linearities.

METHODS

Maximum length sequence stimulation, at various stimulus rates, was used to evoke OAEs from normally hearing subjects. Recordings from a 2cc cavity were also made. The data were analyzed to obtain the linear response and estimates of the slices of the 2nd and 3rd order Volterra kernels. This provided a measure of two and 3 click non-linear temporal interactions, respectively.

RESULTS

The results showed that temporal non-linearities of OAEs do exist, are stable and repeatable within individuals and have properties that differ from those shown by the conventional linear response. Whilst some of the non-linear response properties conformed to the expected pattern, of increasing amplitude with increase in stimulus rate, there are some areas in which they show an unpredicted complexity.

CONCLUSIONS

Whilst system non-linearities could be found, there was no difficulty in distinguishing between the physiological and system non-linear components. New areas of research and application may result from the use of these new OAE responses.

Static input-output non-linearity as the source of non-linear effects in maximum length sequence click-evoked OAEs

The application of the maximum length sequence (MLS) technique to the recording of click-evoked otoacoustic emissions (CEOAEs) allows for a reduction in test time by one to two orders of magnitude. This is because the technique permits the use of extremely high click rates, as inter-click intervals are not constrained to be greater than the duration of the response. However, increasing the click rate also causes a progressive reduction in amplitude, or 'suppression', of the CEOAE. The origin of this suppression is unclear, with diverse suggestions in the literature as to its nature and mechanism. This paper presents a simple model of the well-known compressive non-linearity of the CEOAE level function, based on a static amplitude non-linearity within each of a number of narrowband frequency channels. The response of the model to MLS stimulation demonstrates suppression broadly of the form and magnitude previously reported in experimental studies. Furthermore, the model exhibits the generation of additional non-linear components that have been speculated on in connection with CEOAE recordings using the MLS technique. It is concluded that the MLS suppression phenomenon is derived largely, if not entirely, from the static non-linearity of the CEOAE level function. The approach to modelling the phenomenon as described here also bears promise for understanding various aspects of non-linearity in MLS-based CEOAE recordings.

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.