Karolina Kluk

Can ECAP Measures Be Used for Totally Objective Programming of Cochlear Implants

ABSTRACTAn experiment was conducted with eight cochlear implant subjects to investigate the feasibility of using electrically evoked compound action potential (ECAP) measures other than ECAP thresholds to predict the way that behavioral thresholds change with rate of stimulation, and hence, whether they can be used without combination with behavioral measures to determine program stimulus levels for cochlear implants. Loudness models indicate that two peripheral neural response characteristics contribute to the slope of the threshold versus rate function: the way that neural activity to each stimulus pulse decreases as rate increases and the slope of the neural response versus stimulus current function. ECAP measures related to these two characteristics were measured: the way that ECAP amplitude decreases with stimulus rate and the ECAP amplitude growth function, respectively. A loudness model (incorporating temporal integration and the two neural response characteristics) and regression analyses were used to evaluate whether the ECAP measures could predict the average slope of the behavioral threshold versus current function and whether individual variation in the measures could predict individual variation in the slope of the threshold function. The average change of behavioral threshold with increasing rate was well predicted by the model when using the average ECAP data. However, the individual variations in the slope of the thresholds versus rate functions were not well predicted by individual variations in ECAP data. It was concluded that these ECAP measures are not useful for fully objective programming, possibly because they do not accurately reflect the neural response characteristics assumed by the model, or are measured at current levels much higher than threshold currents.

Effects of noise exposure on young adults with normal audiograms I: Electrophysiology

&NA; Noise‐induced cochlear synaptopathy has been demonstrated in numerous rodent studies. In these animal models, the disorder is characterized by a reduction in amplitude of wave I of the auditory brainstem response (ABR) to high‐level stimuli, whereas the response at threshold is unaffected. The aim of the present study was to determine if this disorder is prevalent in young adult humans with normal audiometric hearing. One hundred and twenty six participants (75 females) aged 18–36 were tested. Participants had a wide range of lifetime noise exposures as estimated by a structured interview. Audiometric thresholds did not differ across noise exposures up to 8 kHz, although 16‐kHz audiometric thresholds were elevated with increasing noise exposure for females but not for males. ABRs were measured in response to high‐pass (1.5 kHz) filtered clicks of 80 and 100 dB peSPL. Frequency‐following responses (FFRs) were measured to 80 dB SPL pure tones from 240 to 285 Hz, and to 80 dB SPL 4 kHz pure tones amplitude modulated at frequencies from 240 to 285 Hz (transposed tones). The bandwidth of the ABR stimuli and the carrier frequency of the transposed tones were chosen to target the 3–6 kHz characteristic frequency region which is usually associated with noise damage in humans. The results indicate no relation between noise exposure and the amplitude of the ABR. In particular, wave I of the ABR did not decrease with increasing noise exposure as predicted. ABR wave V latency increased with increasing noise exposure for the 80 dB peSPL click. High carrier‐frequency (envelope) FFR signal‐to‐noise ratios decreased as a function of noise exposure in males but not females. However, these correlations were not significant after the effects of age were controlled. The results suggest either that noise‐induced cochlear synaptopathy is not a significant problem in young, audiometrically normal adults, or that the ABR and FFR are relatively insensitive to this disorder in young humans, although it is possible that the effects become more pronounced with age. HighlightsLarge study on the effects of lifetime noise exposure in normal‐hearing young adults.No clear evidence for noise‐induced cochlear synaptopathy in ABR or FFR measures.Noise exposure associated with elevated 16‐kHz audiometric thresholds in females.

Vestibular receptors contribute to cortical auditory evoked potentials.

Acoustic sensitivity of the vestibular apparatus is well-established, but the contribution of vestibular receptors to the late auditory evoked potentials of cortical origin is unknown. Evoked potentials from 500 Hz tone pips were recorded using 70 channel EEG at several intensities below and above the vestibular acoustic threshold, as determined by vestibular evoked myogenic potentials (VEMPs). In healthy subjects both auditory mid- and long-latency auditory evoked potentials (AEPs), consisting of Na, Pa, N1 and P2 waves, were observed in the sub-threshold conditions. However, in passing through the vestibular threshold, systematic changes were observed in the morphology of the potentials and in the intensity dependence of their amplitude and latency. These changes were absent in a patient without functioning vestibular receptors. In particular, for the healthy subjects there was a fronto-central negativity, which appeared at about 42 ms, referred to as an N42, prior to the AEP N1. Source analysis of both the N42 and N1 indicated involvement of cingulate cortex, as well as bilateral superior temporal cortex. Our findings are best explained by vestibular receptors contributing to what were hitherto considered as purely auditory evoked potentials and in addition tentatively identify a new component that appears to be primarily of vestibular origin.

Place specificity of monopolar and tripolar stimuli in cochlear implants: The influence of residual masking

This experiment investigated whether place specificity of neural activity evoked by cochlear implant stimulation is improved in tripolar compared to monopolar mode using a forward masking protocol addressing some limitations of previous methods of measurement and analysis. The amount of residual masking (masking remaining at long masker-probe delays) was also measured, and its potential influence on the specificity measures was evaluated. The masker stimulus comprised equally loud interleaved mono- or tripolar stimulation on two electrodes equidistant from a central probe electrode in an apical and basal direction, reducing the influence of off-site listening. The effect of masker-probe distance on the threshold shift of the tripolar probe was analyzed to derive a measure of place specificity. On average, tripolar maskers were more place specific than monopolar maskers, although the mean effect was small. There was no significant effect of masker level on specificity or on the differences observed between modes. The mean influence of residual masking on normalized masking functions was similar for the two modes and, therefore, did not influence the comparison of specificity between the modes. However, variability in amount of residual masking was observed between subjects, and therefore should be considered in forward masking studies that compare place specificity across subjects.

Toward a Diagnostic Test for Hidden Hearing Loss

Cochlear synaptopathy (or hidden hearing loss), due to noise exposure or aging, has been demonstrated in animal models using histological techniques. However, diagnosis of the condition in individual humans is problematic because of (a) test reliability and (b) lack of a gold standard validation measure. Wave I of the transient-evoked auditory brainstem response is a noninvasive electrophysiological measure of auditory nerve function and has been validated in the animal models. However, in humans, Wave I amplitude shows high variability both between and within individuals. The frequency-following response, a sustained evoked potential reflecting synchronous neural activity in the rostral brainstem, is potentially more robust than auditory brainstem response Wave I. However, the frequency-following response is a measure of central activity and may be dependent on individual differences in central processing. Psychophysical measures are also affected by intersubject variability in central processing. Differential measures may help to reduce intersubject variability due to unrelated factors. A measure can be compared, within an individual, between conditions that are affected differently by cochlear synaptopathy. Validation of the metrics is also an issue. Comparisons with animal models, computational modeling, auditory nerve imaging, and human temporal bone histology are all potential options for validation, but there are technical and practical hurdles and difficulties in interpretation. Despite the obstacles, a diagnostic test for hidden hearing loss is a worthwhile goal, with important implications for clinical practice and health surveillance.

Source analysis of short and long latency vestibular-evoked potentials (VsEPs) produced by left vs. right ear air-conducted 500 Hz tone pips.

Todd et al. (2014) have recently demonstrated the presence of vestibular dependent changes both in the morphology and in the intensity dependence of auditory evoked potentials (AEPs) when passing through the vestibular threshold as determined by vestibular evoked myogenic potentials (VEMPs). In this paper we extend this work by comparing left vs. right ear stimulation and by conducting a source analysis of the resulting evoked potentials of short and long latency. Ten healthy, right-handed subjects were recruited and evoked potentials were recorded to both left- and right-ear sound stimulation, above and below vestibular threshold. Below VEMP threshold, typical AEPs were recorded, consisting of mid-latency (MLR) waves Na and Pa followed by long latency AEPs (LAEPs) N1 and P2. In the supra-threshold condition, the expected changes in morphology were observed, consisting of: (1) short-latency vestibular evoked potentials (VsEPs) which have no auditory correlate, i.e. the ocular VEMP (OVEMP) and inion response related potentials; (2) a later deflection, labelled N42/P52, followed by the LAEPs N1 and P2. Statistical analysis of the vestibular dependent responses indicated a contralateral effect for inion related short-latency responses and a left-ear/right-hemisphere advantage for the long-latency responses. Source analysis indicated that the short-latency effects may be mediated by a contralateral projection to left cerebellum, while the long-latency effects were mediated by a contralateral projection to right cingulate cortex. In addition we found evidence of a possible vestibular contribution to the auditory T-complex in radial temporal lobe sources. These last results raise the possibility that acoustic activation of the otolith organs could potentially contribute to auditory processing.

Prevalence of cochlear dead regions in new referrals and existing adult hearing aid users

Objectives: The primary aim of this study was to identify the prevalence of dead regions (DRs) in new adult hearing aid referrals and existing adult hearing aid users. Secondary aims included determining the effect of hearing threshold levels and slope, age, and sex on the presence of DRs. Design: Three hundred and seventy-six adults were recruited from a U.K. National Health Service audiology clinic. Three hundred and forty-three participants (674 ears) with a sensorineural hearing impairment were assessed for the presence of a DR at audiometric frequencies from 0.5 to 4 kHz using the Threshold Equalizing Noise test. Results: The overall prevalence of DRs was 36% (95% confidence interval 31–41). The prevalence in new referrals, and in new and existing hearing aid users was 31% (25–37), 33% (26–40), and 43% (35–51), respectively. The overall prevalence of extensive DRs, defined as spanning ≥3 consecutive frequencies, was 3% (1–5). Conclusions: On the basis of the findings from the Threshold Equalizing Noise test, prevalence of DRs was relatively high in adult hearing aid users with a sensorineural hearing impairment. However, in most cases, the DR was limited to a small frequency region. This suggests that, in most cases, the presence of a DR may not be clinically significant. The difference in DR prevalence between new referrals and existing hearing aid users was not statistically significant. Hearing threshold levels, slope of hearing impairment, age, and sex could not be used to reliably identify DRs.

Auditory steady state responses in normal-hearing and hearing-impaired adults: an analysis of between-session amplitude and latency repeatability, test time, and F ratio detection paradigms.

Objectives: The aim of this study was to assess the between-session repeatability of auditory steady state response (ASSR) amplitudes and to examine F ratio response detection parameters. Design: Suprathreshold ASSRs were recorded from 20 normal-hearing and 10 hearing-impaired subjects. Amplitudes and latencies were recorded in two test sessions conducted on separate days. Analysis: The repeatability coefficients (limits of expected variation in repeat measurements) for amplitude and latency of ASSRs were calculated. The test time required for the responses to reach significance at 1%, 2%, and 5% F ratios was analyzed. The percentage false response detection rate was calculated to determine the suitability of current ASSR threshold estimation protocols for use in audiology clinics. Results: The repeatability coefficients for the amplitude of ASSRs were 29 nV for the normal-hearing subjects and 57 nV for the hearing-impaired subjects. The repeatability coefficients for the latency of ASSR were 1.10 msec for the normal-hearing subjects and 1.19 msec for the hearing-impaired subjects. High false-positive detection rates were found for detection procedures that used variable test time (“stop when significance reached” methods). Conclusions: The results of this study showed that ASSR amplitudes are highly variable between test sessions with an average estimated variability in response amplitude of ±40% for normal-hearing participants and ±97% for hearing-impaired participants. This could be a possible cause of test–retest differences in ASSR threshold measurements, as it could potentially lead to thresholds that were above the EEG noise level and significant in one test session subsequently falling below the EEG noise level in the repeat test session leading to insignificant response and thus poorer ASSR threshold.

Effects of noise exposure on young adults with normal audiograms II: Behavioral measures

ABSTRACT An estimate of lifetime noise exposure was used as the primary predictor of performance on a range of behavioral tasks: frequency and intensity difference limens, amplitude modulation detection, interaural phase discrimination, the digit triplet speech test, the co‐ordinate response speech measure, an auditory localization task, a musical consonance task and a subjective report of hearing ability. One hundred and thirty‐eight participants (81 females) aged 18–36 years were tested, with a wide range of self‐reported noise exposure. All had normal pure‐tone audiograms up to 8 kHz. It was predicted that increased lifetime noise exposure, which we assume to be concordant with noise‐induced cochlear synaptopathy, would elevate behavioral thresholds, in particular for stimuli with high levels in a high spectral region. However, the results showed little effect of noise exposure on performance. There were a number of weak relations with noise exposure across the test battery, although many of these were in the opposite direction to the predictions, and none were statistically significant after correction for multiple comparisons. There were also no strong correlations between electrophysiological measures of synaptopathy published previously and the behavioral measures reported here. Consistent with our previous electrophysiological results, the present results provide no evidence that noise exposure is related to significant perceptual deficits in young listeners with normal audiometric hearing. It is possible that the effects of noise‐induced cochlear synaptopathy are only measurable in humans with extreme noise exposures, and that these effects always co‐occur with a loss of audiometric sensitivity. HIGHLIGHTSLarge study on the effects of lifetime noise exposure in normal‐hearing young adults.Performance on a range of behavioral tasks unrelated to noise exposure history.Effects of cochlear synaptopathy not evident in young audiometrically normal cohort.

Interpulse interval discrimination within and across channels: Comparison of monopolar and tripolar mode of stimulation

Perception of temporal patterns is crucial to speech understanding and music perception in normal hearing, and is fundamental in the design and implementation of processing strategies for cochlear implants. Two experiments described here investigated the effect of stimulation mode (monopolar versus tripolar) on interpulse interval discrimination using single-electrode stimulation (experiment 1) and dual-electrode stimulation (experiment 2). Experiment 1 required participants to discriminate stimuli containing different interpulse intervals and experiment 2 required listeners to discriminate between two dual-electrode stimuli that had the same temporal pattern on each electrode, but differed in inter-electrode timing. The hypotheses were that (i) stimulation mode would affect the ability to distinguish interpulse interval patterns on a single electrode and (ii) the electrode separation range in which subjects were sensitive to inter-electrode timing would be more restricted in tripolar than in monopolar stimulation. Results in nine cochlear implant users showed that mode did not have a significant mean effect on either the ability to discriminate interpulse intervals in single-electrode stimulation or the range of electrode separation in dual-electrode stimulation in which participants were sensitive to inter-electrode timing. In conclusion, tripolar stimulation did not show any advantage in delivering temporal information within or across channels in this group.