The mismatch response in normal hearing adults: a performance comparison with stimuli relevant for objective validation of hearing aid fittings.

OBJECTIVE: A long-standing observation is that the Mismatch Response (MMR) has the potential to offer a clinically feasible index of sound discrimination. However, findings that positively identify MMRs at the individual level have been mixed, even for those who are normally hearing and who can discriminate sounds behaviourally. This complicates interpretation when an MMR is not observed. The objective of this study was to determine the reliability of the MMR using an optimised paradigm and a range of stimuli relevant to audiological applications in relation to objective verification of hearing aid fittings. DESIGN: MMRs were measured using an optimised 3-deviant paradigm in response to a range of sounds designed for aided and unaided sound field assessments, including complex tones (CTs) and speech-like signals. STUDY SAMPLE: Seventeen normally hearing adults (18-56 years). RESULTS: The most robust MMRs were recorded in response to CTs; responses were positively identified in 50 out of 51 instances (98%), assessed via objective Hotelling's T2 bias-free statistical analyses. CONCLUSIONS: The results indicate that CTs in conjunction with optimised recording and analysis parameters offer the potential to elicit robust MMRs, supporting future utilisation of MMRs for clinical audiological applications.

Emerging Topics in the Behavioral Neuroscience of Tinnitus.

This volume has highlighted the many recent advances in tinnitus theory, models, diagnostics, therapies, and therapeutics. But tinnitus knowledge is far from complete. In this chapter, contributors to the Behavioral Neuroscience of Tinnitus consider emerging topics and areas of research needed in light of recent findings. New research avenues and methods to explore are discussed. Issues pertaining to current assessment, treatment, and research methods are outlined, along with recommendations on new avenues to explore with research.

Principles and Methods for Psychoacoustic Evaluation of Tinnitus.

Tinnitus, the perception of sound in the absence of a physical sound in the environment, is highly heterogeneous. It varies in its etiology, characteristics, and impact on an individual's life. The sound is commonly described as "ringing," "buzzing," "crickets," "hissing," "humming." Tinnitus can be acute or chronic, mild or disabling. It can be perceived unilaterally or, more commonly, bilaterally. The sound and its location differ from person to person and fluctuate in the same individual over a certain period of time. This heterogeneity in characterization has important implications for research and clinical practice. Identifying patterns in how tinnitus sounds and its relationship to hearing may aid in identifying different forms of tinnitus and revealing their underlying mechanisms. However, the subjective nature of characterizing tinnitus makes it difficult to reliably define and measure. This chapter will focus on reviewing the psychoacoustic assessment of tinnitus, its relationship to cognitive and behavioral aspects of tinnitus, and its neuropathophysiology. In particular, it will describe the heterogeneity of tinnitus and tinnitus matching, and how individual variability in measures may be used to guide treatment and as a prognostic factor.

Increased auditory cortex neural response amplitude in adults with chronic unilateral conductive hearing impairment.

Animal studies have demonstrated that unilateral hearing loss can induce changes in neural response amplitude of the mature central auditory system (CAS). However, there is limited physiological evidence of these neural gain changes in the auditory cortex of human adults. The present study investigated the impact of chronic, unilateral conductive hearing impairment on cortical auditory evoked potentials (CAEPs) recorded from 15 adults (21-65 years old) in response to a 1 kHz tone (80 ms duration) presented to the impaired ear via a bone conduction transducer. The amplitude and latency of the main CAEP components were compared to those obtained from normal hearing age-matched control participants. Both P1-N1 and N1-P2 amplitudes were significantly larger in the hearing impaired relative to the control participants. Differences between groups in the mean latencies of P1, N1, and P2 were not statistically significant. These results are the first to provide direct evidence of increased neural response amplitude in the adult human auditory cortex in the presence of unilateral conductive hearing loss. Importantly, the study shows that central gain changes are a direct result of deprivation of sound rather than cochlear or neural pathology.

A review of plasticity induced by auditory and visual tetanic stimulation in humans.

Long-term potentiation is a form of synaptic plasticity thought to play an important role in learning and memory. Recently noninvasive methods have been developed to induce and measure activity similar to long-term potentiation in humans. Sensory tetani (trains of quickly repeating auditory or visual stimuli) alter the electroencephalogram in a manner similar to electrical stimulation that results in long-term potentiation. This review briefly covers the development of long-term potentiation research before focusing on in vivo human studies that produce long-term potentiation-like effects using auditory and visual stimulation. Similarities and differences between traditional (animal and brain tissue) long-term potentiation studies and human sensory tetanization studies will be discussed, as well as implications for perceptual learning. Although evidence for functional consequences of sensory tetanization remains scarce, studies involving clinical populations indicate that sensory induced plasticity paradigms may be developed into diagnostic and research tools in clinical settings. Individual differences in the effects of sensory tetanization are not well-understood and provide an interesting avenue for future research. Differences in effects found between research groups that have emerged as the field has progressed are also yet to be resolved.

Pump up the volume: could excessive neural gain explain tinnitus and hyperacusis?

Naturally occurring stimuli can vary over several orders of magnitude and may exceed the dynamic range of sensory neurons. As a result, sensory systems adapt their sensitivity by changing their responsiveness or 'gain'. While many peripheral adaptation processes are rapid, slow adaptation processes have been observed in response to sensory deprivation or elevated stimulation. This adaptation process alters neural gain in order to adjust the basic operating point of sensory processing. In the auditory system, abnormally high neural gain may result in higher spontaneous and/or stimulus-evoked neural firing rates, and this may have the unintended consequence of presenting as tinnitus and/or sound intolerance, respectively. Therefore, a better understanding of neural gain, in health and disease, may lead to more effective treatments for these aberrant auditory perceptions. This review provides a concise summary of (i) evidence for changes in neural gain in the auditory system of animals, (ii) physiological and perceptual changes in adult human listeners following an acute period of enhanced acoustic stimulation and/or deprivation, (iii) physiological evidence of excessive neural gain in tinnitus and hyperacusis patients, and (iv) the relevance of neural gain in the clinical treatment of tinnitus and hyperacusis.

Enhanced intensity discrimination in the intact ear of adults with unilateral deafness.

Physiological measures of neural activity in the auditory cortex have revealed plasticity following unilateral deafness. Central projections from the remaining ear reorganize to produce a stronger cortical response than normal. However, little is known about the perceptual consequences of this increase. One possibility is improved sound intensity discrimination. Intensity difference limens were measured in 11 individuals with unilateral deafness that were previously shown to exhibit increased cortical activity to sounds heard by the intact ear. Significantly smaller mean difference limens were observed compared with controls. These results provide evidence of the perceptual consequences of plasticity in humans following unilateral deafness.

'Getting used to' hearing aids from the perspective of adult hearing-aid users.

OBJECTIVE: To describe getting used to hearing aids from the perspective of adult hearing-aid users. DESIGN: Three focus group discussions were carried out. A topic guide and discussion exercises were used to elicit views on getting used to hearing aids. Discussion was audio recorded, transcribed verbatim, and subjected to qualitative content analysis. STUDY SAMPLE: Adult hearing aid users (n = 16). RESULTS: Participants described getting used to hearing aids as a multi-factorial process which included adjusting to altered sensory input, practical matters such as cleaning and maintenance, and managing the psychosocial impact of hearing-aid use, such as on self-image. Users reported a process of discovering benefits and limitations of hearing aids leading to individual patterns of use that was relatively independent of input from audiologists. CONCLUSIONS: Getting used to hearing aids is a challenging multi-factorial process with both psychosocial and practical difficulties besides demands of adjusting to hearing-aid input.

Evidence for multiple mechanisms of cortical plasticity: a study of humans with late-onset profound unilateral deafness.

OBJECTIVE: To investigate 1: plasticity due to partial unilateral deafness of slow onset and 2: the time course of plasticity following abrupt, profound unilateral deafness in adult humans using cortical auditory evoked potentials. METHODS: Baseline data were measured from six participants with partial unilateral deafness due to an acoustic neuroma and compared with data from six controls. Further measurements were made in the unilaterally deaf group at 1-, 3- and 6-months post surgery for acoustic neuroma removal and consequent profound unilateral deafness. Data were recorded from 30 channels in response to pure tones presented to the intact ear. RESULTS: Baseline data revealed statistically higher amplitudes in unilaterally deaf participants but with normal hemispheric asymmetry. Longitudinal data revealed further increases in P1 amplitudes by 1-month post-surgery, and in N1 and P2 amplitudes by 6-months post-surgery, with statistically different scalp field topographies indicating reduced hemispheric asymmetries. CONCLUSION: Different patterns of plasticity occur following partial and profound unilateral deafness. Plasticity occurs both relatively rapidly and more gradually over at least 6-months post-surgery. SIGNIFICANCE: The different patterns of change over time are consistent with multiple physiological mechanisms of plasticity. Unravelling these mechanisms and their time course in humans may be relevant in understanding and, ultimately, influencing plasticity for therapeutic gain.

Investigation of cortical and subcortical plasticity following short-term unilateral auditory deprivation in normal hearing adults.

There is growing evidence that auditory stimulation or deprivation can induce physiological and perceptual changes in the auditory system of normal hearing adults. The present study investigated cortical (hemispheric asymmetry) and subcortical (acoustic reflex threshold) changes in 11 normal hearing adults after 7 days of continuous unilateral earplug use (around 30 dB of attenuation at the high frequencies). The results revealed: (a) a decrease in high frequency acoustic reflex thresholds of around 7 dB in the ear that had been plugged and (b) no change in hemispheric asymmetry. The change in acoustic reflex is consistent with subcortical plasticity. It is unclear if homoeostatic plasticity preserved the normal hemispheric asymmetry or if this is the result of the experimental paradigm.

Source analysis reveals plasticity in the auditory cortex: evidence for reduced hemispheric asymmetries following unilateral deafness.

OBJECTIVE: To investigate the effect of acquired unilateral deafness on hemispheric asymmetries in adult humans using cortical auditory evoked potentials. METHODS: N1 cortical auditory evoked potentials were measured from 30 channels in 18 unilateral profoundly-deaf participants (6 right-sided and 12 left-sided deafness) and 18 audiogram-matched controls. Stimuli were 0.5-kHz and 4-kHz tones presented monaurally, and the data were analysed using global field power and dipole source analysis. RESULTS: There was a statistically significant difference in dipole source strength and orientation between the two groups. Similar changes (increased dipole strength and more medial orientation) were apparent after profound unilateral deafness of either ear and for both stimuli. CONCLUSIONS: The results reveal evidence of central auditory system plasticity that is consistent with animal models having experimentally induced unilateral deafness. SIGNIFICANCE: The trend towards reduced hemispheric asymmetries was reflected in the dipole source model by changes in dipole strength, location and orientation. These findings may explain the inconsistencies reported in previous studies involving dipole source analysis where location and orientation have not always been considered.