Imaging the neural correlates of tinnitus: a comparison between animal models and human studies

What's the buzz? The neuroscience and the treatment of tinnitus

Tinnitus is a pervasive public health issue that affects ∼15% of the United States population. Similar estimates have also been shown on a global scale, with similar prevalence found in Europe, Asia, and Africa. The severity of tinnitus is heterogeneous, ranging from mildly bothersome to extremely disruptive. In the United States, ∼10-20% of individuals who experience tinnitus report symptoms that severely reduce their quality of life. Due to the huge personal and societal burden, in the last 20 yr a concerted effort on basic and clinical research has significantly advanced our understanding and treatment of this disorder. Yet, neither full understanding, nor cure exists. We know that tinnitus is the persistent involuntary phantom percept of internally generated nonverbal indistinct noises and tones, which in most cases is initiated by acquired hearing loss and maintained only when this loss is coupled with distinct neuronal changes in auditory and extra-auditory brain networks. Yet, the exact mechanisms and patterns of neural activity that are necessary and sufficient for the perceptual generation and maintenance of tinnitus remain incompletely understood. Combinations of animal model and human research will be essential in filling these gaps. Nevertheless, the existing progress in investigating the neurophysiological mechanisms has improved current treatment and highlighted novel targets for drug development and clinical trials. The aim of this review is to thoroughly discuss the current state of human and animal tinnitus research, outline current challenges, and highlight new and exciting research opportunities.

Association of Tinnitus with Depression in a Normal Hearing Population

Background and Objectives: The relationship between depression in tinnitus patients without hearing loss remains elusive. This study aimed to investigate the association between tinnitus and normal hearing and depression. Materials and Methods: Participants aged ≥12 years with normal hearing levels were recruited from the Korea National Health and Nutrition Examination Survey (KNHANES), 2009-2012. Participants with normal hearing were divided into the tinnitus and non-tinnitus groups. The relationship between tinnitus with normal hearing and variables including age, sex, depression, ischemic heart diseases, stroke, diabetes, hypertension, dyslipidemia, chronic renal disease, noise exposure, and depression were analyzed. The odds of depression for tinnitus with normal hearing were estimated using multiple logistic regression tests with complex sampling. Results: The results showed that 4.9% (107/2221) and 2.8% (290/10,316) of participants in the tinnitus group and the non-tinnitus group, respectively, experienced depression (p < 0.001). Sex, ischemic heart disease, dyslipidemia, noise exposure, and depression were positively related to tinnitus with normal hearing. The odds ratio of depression for tinnitus with normal hearing were 1.89 (95% CI 1.37-2.60, p < 0.001). Conclusions: Tinnitus with normal hearing was related to the female sex, ischemic heart disease, dyslipidemia, noise exposure, and depression. Depression had the highest odds of tinnitus with normal hearing.

Hyperacusis in Children with Attention Deficit Hyperactivity Disorder: A Preliminary Study

The association between hyperacusis and developmental disorders such as autism spectrum disorders has been extensively reported in the literature; however, the specific prevalence of hyperacusis in attention deficit hyperactivity disorder (ADHD) has never been investigated. In this preliminary study, we evaluated the presence of hyperacusis in a small sample of children affected by ADHD compared to a control group of healthy children. Thirty normal hearing children with a diagnosis of ADHD and 30 children matched for sex and age were enrolled in the study. All children underwent audiological and multidisciplinary neuropsychiatric evaluation. Hearing was assessed using pure tone audiometry and immittance test; ADHD was diagnosed following the Diagnostic and Statistical Manual of Mental Disorder criteria. Hyperacusis was assessed through the administration of a questionnaire to parents and an interview with children. Hyperacusis was diagnosed in 11 children (36.7%) in the study group and in four children (13.3%) in the control group; this difference was statistically significant (p = 0.03). The preliminary results of this study suggest a higher presence of hyperacusis in children with attention deficit hyperactivity disorder compared to control children. More studies on larger samples are necessary to confirm these results.

Does Tinnitus Fill in the Gap Using Electrophysiology? A Scoping Review

The results showed a trend of increased post-gap amplitudes and reduced gap salience; however, the small number of articles yield and limited consensus prohibit any conclusions for clinical use. Nevertheless, gap-induced EPs may be further explored as a potential tool for tinnitus detection.

Regulation of auditory plasticity during critical periods and following hearing loss

Sensory input has profound effects on neuronal organization and sensory maps in the brain. The mechanisms regulating plasticity of the auditory pathway have been revealed by examining the consequences of altered auditory input during both developmental critical periods—when plasticity facilitates the optimization of neural circuits in concert with the external environment—and in adulthood—when hearing loss is linked to the generation of tinnitus. In this review, we summarize research identifying the molecular, cellular, and circuit-level mechanisms regulating neuronal organization and tonotopic map plasticity during developmental critical periods and in adulthood. These mechanisms are shared in both the juvenile and adult brain and along the length of the auditory pathway, where they serve to regulate disinhibitory networks, synaptic structure and function, as well as structural barriers to plasticity. Regulation of plasticity also involves both neuromodulatory circuits, which link plasticity with learning and attention, as well as ascending and descending auditory circuits, which link the auditory cortex and lower structures. Further work identifying the interplay of molecular and cellular mechanisms associating hearing loss-induced plasticity with tinnitus will continue to advance our understanding of this disorder and lead to new approaches to its treatment.

•

During CPs, brain plasticity is enhanced and sensitive to acoustic experience.

•

Enhanced plasticity can be reinstated in the adult brain following hearing loss.

•

Molecular, cellular, and circuit-level mechanisms regulate CP and adult plasticity.

•

Plasticity resulting from hearing loss may contribute to the emergence of tinnitus.

•

Modifying plasticity in the adult brain may offer new treatments for tinnitus.

Chronic Tinnitus Exhibits Bidirectional Functional Dysconnectivity in Frontostriatal Circuit

Purpose

The phantom sound of tinnitus is considered to be associated with abnormal functional coupling between the nucleus accumbens (NAc) and the prefrontal cortex, which may form a frontostriatal top-down gating system to evaluate and modulate sensory signals. Resting-state functional magnetic resonance imaging (fMRI) was used to recognize the aberrant directional connectivity of the NAc in chronic tinnitus and to ascertain the relationship between this connectivity and tinnitus characteristics.

Methods

Participants included chronic tinnitus patients (n = 50) and healthy controls (n = 55), matched for age, sex, education, and hearing thresholds. The hearing status of both groups was comparable. On the basis of the NAc as a seed region, a Granger causality analysis (GCA) study was conducted to investigate the directional connectivity and the relationship with tinnitus duration or distress.

Results

Compared with healthy controls, tinnitus patients exhibited abnormal directional connectivity between the NAc and the prefrontal cortex, principally the middle frontal gyrus (MFG), orbitofrontal cortex (OFC), and inferior frontal gyrus (IFG). Additionally, positive correlations between tinnitus handicap questionnaire (THQ) scores and increased directional connectivity from the right NAc to the left MFG (r = 0.357, p = 0.015) and from the right MFG to the left NAc (r = 0.626, p < 0.001) were observed. Furthermore, the enhanced directional connectivity from the right NAc to the right OFC was positively associated with the duration of tinnitus (r = 0.599, p < 0.001).

Conclusion

In concurrence with expectations, tinnitus distress was correlated with enhanced directional connectivity between the NAc and the prefrontal cortex. The current study not only helps illuminate the neural basis of the frontostriatal gating control of tinnitus sensation but also contributes to deciphering the neuropathological features of tinnitus.

The sound-sensitive tinnitus index: Psychometric properties of a scale to assess the impact of tinnitus exacerbated by sound

Context

Although studies of tinnitus exacerbated by sound exposure have indicated increased treatment challenges and intensified mental health and quality of life concerns, there is a lack of valid screening measures to differentiate or assess diagnostic factors and areas of impact unique to this specific symptom manifestation.

Aims

The purpose of this study was to design a self-rated measurement tool that can accurately assess the subjective impact of tinnitus negatively modulated by external sound.

Settings and Design

Based on review of established models of tinnitus and hyperacusis measurement and a two-part pilot study, the 20-item Sound-Sensitive Tinnitus Index (SSTI) was developed and administered in online survey format to 277 individuals worldwide.

Methods and Material

Cronbach's alpha was used to estimate reliability properties, and dimensional factor analysis was performed. To establish validity, statistical correlations of the SSTI were estimated with valid measures of related constructs including tinnitus, hyperacusis, depression, anxiety, and quality of life.

Results

Statistical analysis yielded high levels of internal consistency reliability, and convergent validity was demonstrated through significant correlations with all established measures of related constructs. Initial factor analysis indicated two components split between overall functional impact and coping factors, while rotated factor analyses revealed four distinct scale dimensions, labeled: functional challenges, relational and communication challenges, coping factors, and prevention and hearing protection.

Conclusions

As a valid and reliable measure, the SSTI fills an important gap as a clinical and research tool that can differentiate and assess severity and treatment progress in manifestations of combined tinnitus and auditory sensitivity symptoms.

Characteristics of somatic tinnitus patients with and without hyperacusis

Objective

Determine if somatic tinnitus patients with hyperacusis have different characteristics from those without hyperacusis.

Patients and methods

172 somatic tinnitus patients with (n = 82) and without (n = 90) hyperacusis referred to the Tinnitus Unit of Sapienza University of Rome between June 2012 and June 2016 were compared for demographic characteristics, tinnitus features, self-administered questionnaire scores, nature of somatic modulation and history.

Results

Compared to those without hyperacusis, patients with somatic tinnitus and hyperacusis: (a) were older (43.38 vs 39.12 years, p = 0.05), (b) were more likely to have bilateral tinnitus (67.08% vs 55.56%, p = 0.04), (c) had a higher prevalence of somatic modulation of tinnitus (53.65% vs 36.66%, p = 0.02) and (d) scored significantly worse on tinnitus annoyance (39.34 vs 22.81, p<0.001) and subjective hearing level (8.04 vs 1.83, p<0.001).

Conclusion

Our study shows significantly higher tinnitus modulation and worse self-rating of tinnitus and hearing ability in somatic tinnitus patients with hyperacusis versus somatic tinnitus patients without hyperacusis. These differences could prove useful in developing a better understanding of the pathophysiology and establishing a course of treatment for these two groups of patients.

Tinnitus: Prospects for Pharmacological Interventions With a Seesaw Model

Chronic tinnitus, the perception of lifelong constant ringing in ear, is one capital cause of disability in modern society. It is often present with various comorbid factors that severely affect quality of life, including insomnia, deficits in attention, anxiety, and depression. Currently, there are limited therapeutic treatments for alleviation of tinnitus. Tinnitus can involve a shift in neuronal excitation/inhibition (E/I) balance, which is largely modulated by ion channels and receptors. Thus, ongoing research is geared toward pharmaceutical approaches that modulate the function of ion channels and receptors. Here, we propose a seesaw model that delineates how tinnitus-related ion channels and receptors are involved in homeostatic E/I balance of neurons. This review provides a thorough account of our current mechanistic understanding of tinnitus and insight into future direction of drug development.

Pairing broadband noise with cortical stimulation induces extensive suppression of ascending sensory activity

OBJECTIVE

The corticofugal system can alter coding along the ascending sensory pathway. Within the auditory system, electrical stimulation of the auditory cortex (AC) paired with a pure tone can cause egocentric shifts in the tuning of auditory neurons, making them more sensitive to the pure tone frequency. Since tinnitus has been linked with hyperactivity across auditory neurons, we sought to develop a new neuromodulation approach that could suppress a wide range of neurons rather than enhance specific frequency-tuned neurons.

APPROACH

We performed experiments in the guinea pig to assess the effects of cortical stimulation paired with broadband noise (PN-Stim) on ascending auditory activity within the central nucleus of the inferior colliculus (CNIC), a widely studied region for AC stimulation paradigms.

MAIN RESULTS

All eight stimulated AC subregions induced extensive suppression of activity across the CNIC that was not possible with noise stimulation alone. This suppression built up over time and remained after the PN-Stim paradigm.

SIGNIFICANCE

We propose that the corticofugal system is designed to decrease the brain's input gain to irrelevant stimuli and PN-Stim is able to artificially amplify this effect to suppress neural firing across the auditory system. The PN-Stim concept may have potential for treating tinnitus and other neurological disorders.

Partial to complete suppression of unilateral noise-induced tinnitus in rats after cyclobenzaprine treatment

Some forms of tinnitus are believed to arise from abnormal central nervous system activity following a single or repeated noise exposure, for which there are no widely accepted pharmacological treatments. One central site that could be related to tinnitus awareness or modulation is the locus coeruleus, a brainstem structure associated with stress, arousal, and attention. In the present study, we evaluated the effects of cyclobenzaprine, a drug known to act on the rat locus coeruleus, on noise-induced tinnitus using Gap Prepulse Inhibition of the Acoustic Startle (GPIAS). In untreated rats, brief silent gaps presented prior to a 5-10-kHz bandpass startling stimulus produced robust GPIAS. Treatment with cyclobenzaprine alone had no effect on the ability of gaps to suppress the startle response. When animals were exposed to intense narrow-band (126 dB SPL, 16 kHz, 100 Hz BW) unilateral noise, GPIAS was significantly reduced, suggesting the presence of tinnitus. Following the noise exposure, a subset of rats that maintained a robust startle response continued to show GPIAS impairment at 6-20 kHz, 40 days post-noise, suggesting chronic tinnitus. When this subset of animals was treated with cyclobenzaprine, at a dose that had no significant effects on the startle response (0.5 mg/kg), GPIAS recovered partially or to near baseline levels at the affected frequencies. These results were consistent with the absence of tinnitus. By 48 h post-treatment, evidence of tinnitus re-emerged. Our results suggest that cyclobenzaprine was effective in transiently suppressing noise-induced tinnitus in rats.

Central gain control in tinnitus and hyperacusis

Sensorineural hearing loss induced by noise or ototoxic drug exposure reduces the neural activity transmitted from the cochlea to the central auditory system. Despite a reduced cochlear output, neural activity from more central auditory structures is paradoxically enhanced at suprathreshold intensities. This compensatory increase in the central auditory activity in response to the loss of sensory input is referred to as central gain enhancement. Enhanced central gain is hypothesized to be a potential mechanism that gives rise to hyperacusis and tinnitus, two debilitating auditory perceptual disorders that afflict millions of individuals. This review will examine the evidence for gain enhancement in the central auditory system in response to cochlear damage. Further, it will address the potential cellular and molecular mechanisms underlying this enhancement and discuss the contribution of central gain enhancement to tinnitus and hyperacusis. Current evidence suggests that multiple mechanisms with distinct temporal and spectral profiles are likely to contribute to central gain enhancement. Dissecting the contributions of these different mechanisms at different levels of the central auditory system is essential for elucidating the role of central gain enhancement in tinnitus and hyperacusis and, most importantly, the development of novel treatments for these disorders.

Neural changes accompanying tinnitus following unilateral acoustic trauma in the guinea pig

Animal models of tinnitus allow us to study the relationship between changes in neural activity and the tinnitus percept. Here, guinea pigs were subjected to unilateral noise trauma and tested behaviourally for tinnitus 8 weeks later. By comparing animals with tinnitus with those without, all of which were noise-exposed, we were able to identify changes unique to the tinnitus group. Three physiological markers known to change following noise exposure were examined: spontaneous firing rates (SFRs) and burst firing in the inferior colliculus (IC), evoked auditory brainstem responses (ABRs), and the number of neurons in the cochlear nucleus containing nitric oxide synthase (NOS). We obtained behavioural evidence of tinnitus in 12 of 16 (75%) animals. Both SFRs and incidences of burst firing were elevated in the IC of all noise-exposed animals, but there were no differences between tinnitus and no-tinnitus animals. There were significant decreases in ipsilateral ABR latencies in tinnitus animals, contrary to what might be expected with a small hearing loss. Furthermore, there was an ipsilateral-contralateral asymmetry in NOS staining in the ventral cochlear nucleus (VCN) that was only apparent in tinnitus animals. Tinnitus animals had a significantly greater number of NOS-containing neurons on the noise-exposed side, whereas no-tinnitus animals did not. These data suggest that measuring NOS in the VCN and recording ABRs supplement behavioural methods for confirming tinnitus in animals, and that nitric oxide is involved in plastic neural changes associated with tinnitus.

Underlying mechanisms of tinnitus: review and clinical implications

BACKGROUND

The study of tinnitus mechanisms has increased tenfold in the last decade. The common denominator for all of these studies is the goal of elucidating the underlying neural mechanisms of tinnitus with the ultimate purpose of finding a cure. While these basic science findings may not be immediately applicable to the clinician who works directly with patients to assist them in managing their reactions to tinnitus, a clear understanding of these findings is needed to develop the most effective procedures for alleviating tinnitus.

PURPOSE

The goal of this review is to provide audiologists and other health-care professionals with a basic understanding of the neurophysiological changes in the auditory system likely to be responsible for tinnitus.

RESULTS

It is increasingly clear that tinnitus is a pathology involving neuroplastic changes in central auditory structures that take place when the brain is deprived of its normal input by pathology in the cochlea. Cochlear pathology is not always expressed in the audiogram but may be detected by more sensitive measures. Neural changes can occur at the level of synapses between inner hair cells and the auditory nerve and within multiple levels of the central auditory pathway. Long-term maintenance of tinnitus is likely a function of a complex network of structures involving central auditory and nonauditory systems.

CONCLUSIONS

Patients often have expectations that a treatment exists to cure their tinnitus. They should be made aware that research is increasing to discover such a cure and that their reactions to tinnitus can be mitigated through the use of evidence-based behavioral interventions.

Suppression of noise-induced hyperactivity in the dorsal cochlear nucleus following application of the cholinergic agonist, carbachol

Increased spontaneous firing (hyperactivity) is induced in fusiform cells of the dorsal cochlear nucleus (DCN) following intense sound exposure and is implicated as a possible neural correlate of noise-induced tinnitus. Previous studies have shown that in normal hearing animals, fusiform cell activity can be modulated by activation of parallel fibers, which represent the axons of granule cells. The modulation consists of a transient excitation followed by a more prolonged period of inhibition, presumably reflecting direct excitatory inputs to fusiform cells and an indirect inhibitory input to fusiform cells from the granule cell-cartwheel cell system. We hypothesized that since granule cells can be activated by cholinergic inputs, it might be possible to suppress tinnitus-related hyperactivity of fusiform cells using the cholinergic agonist, carbachol. To test this hypothesis, we recorded multiunit spontaneous activity in the fusiform soma layer (FSL) of the DCN in control and tone-exposed hamsters (10 kHz, 115 dB SPL, 4h) before and after application of carbachol to the DCN surface. In both exposed and control animals, 100 μM carbachol had a transient excitatory effect on spontaneous activity followed by a rapid weakening of activity to near or below normal levels. In exposed animals, the weakening of activity was powerful enough to completely abolish the hyperactivity induced by intense sound exposure. This suppressive effect was partially reversed by application of atropine and was usually not associated with significant changes in neural best frequencies (BF) or BF thresholds. These findings demonstrate that noise-induced hyperactivity can be pharmacologically controlled and raise the possibility that attenuation of tinnitus may be achievable by using an agonist of the cholinergic system.

Behavioral evidence for possible simultaneous induction of hyperacusis and tinnitus following intense sound exposure

Many human subjects suffering from chronic tinnitus also suffer from hyperacusis, a heightened perception of loudness at moderate to intense sound levels. While numerous studies suggest that animals develop chronic tinnitus following intense noise exposure, it is not yet clear whether sound exposure also induces chronic hyperacusis-like responses in animals. We addressed this question by examining the chronic effects of intense sound exposure on the acoustic startle response (ASR) and its suppression by background noise containing brief gaps. We compared startle amplitudes in intense tone-exposed (10 kHz, 115 dB SPL, 4 h) and age-matched controls at 2-28 weeks post-exposure. While both groups showed similar startle thresholds, exposed animals showed a hyperacusis-like augmentation of ASR at high stimulus levels. Addition of background noise had little effect on ASR in controls but had a strong suppressive effect on startle in exposed animals, indicating a sensitization to background noise. When the background noise contained a gap preceding the startle stimulus, ASR was suppressed in control animals, but exposed animals showed a marked weakening of gap-induced suppression of ASR. This weakening of gap-induced startle suppression is consistent with the interpretation that the gap may have been masked by tinnitus. The associated hyper-responsiveness to startle stimuli presented alone and the sensitization to background noise suggest that hyperacusis may have also been induced. The results indicate that noise exposure leads to increases in the gain of auditory responsiveness and may offer a model of the association of hyperacusis with tinnitus.