Ear-specific neuroplasticity for sound localization in individuals with single-sided deafness.

Spatial hearing, which largely relies on binaural time/level cues, is a challenge for patients with asymmetric hearing. The degree of the deficit is largely variable, and better sound localization performance is frequently reported. Studies on the compensatory mechanism revealed that monaural level cues and monoaural spectral cues contribute to variable behavior in those patients who lack binaural spatial cues. However, changes in the monaural level cues have not yet been separately investigated. In this study, the use of the level cue in sound localization was measured using stimuli of 1 kHz at a fixed level in patients with single-sided deafness (SSD), the most severe form of asymmetric hearing. The mean absolute error (MAE) was calculated and related to the duration/age onset of SSD. To elucidate the biological correlate of this variable behavior, sound localization ability was compared with the cortical volume of the parcellated auditory cortex. In both SSD patients (n = 26) and normal controls with one ear acutely plugged (n = 23), localization performance was best on the intact ear side; otherwise, there was wide interindividual variability. In the SSD group, the MAE on the intact ear side was worse than that of the acutely plugged controls, and it deteriorated with longer duration/younger age at SSD onset. On the impaired ear side, MAE improved with longer duration/younger age at SSD onset. Performance asymmetry across lateral hemifields decreased in the SSD group, and the maximum decrease was observed with the most extended duration/youngest age at SSD onset. The decreased functional asymmetry in patients with right SSD was related to greater cortical volumes in the right posterior superior temporal gyrus and the left planum temporale, which are typically involved in auditory spatial processing. The study results suggest that structural plasticity in the auditory cortex is related to behavioral changes in sound localization when utilizing monaural level cues in patients with SSD.

Single-sided deafness (SSD) induces cortical neural plastic changes according to duration of deafness. However, it is still unclear how the auditory cortical changes accompany the subcortical neural changes. The present study aimed to find the neural plastic changes in the cortical and subcortical auditory system following adult-onset single-sided deafness (SSD) using Mn-enhanced magnetic resonance imaging (MEMRI). B57BL/6 mice (postnatal 8-week-old) were divided into three groups: the SSD-4-week group (postnatal 12-week-old, n = 11), the SSD-8-week group (postnatal 16-week-old, n = 11), and a normal-hearing control group (postnatal 8-week-old, n = 9). The left cochlea was ablated in the SSD groups. White Gaussian noise was delivered for 24 h before MEMRI acquisition. T1-weighted MRI data were analyzed from the cochlear nucleus (CN), superior olivary complex (SOC), lateral lemniscus (LL), inferior colliculus (IC), medial geniculate body (MG), and auditory cortex (AC). The differences in relative Mn2+-enhanced signal intensities (Mn2+SI) and laterality were analyzed between the groups. Four weeks after the SSD procedure, the ipsilateral side of the SSD showed significantly lower Mn2+SI in the CN than the control group. On the other hand, the contralateral side of the SSD demonstrated significantly lower Mn2+SI in the SOC, LL, and IC. These decreased Mn2+SI values were partially recovered at 8 weeks after the SSD procedure. The interaural Mn2+SI differences representing the interaural dominance were highest in CN and then became less prominently higher in the auditory neural system. The SSD-8-week group still showed interaural differences in the CN, LL, and IC. In contrast, the MG and AC did not show any significant intergroup or interaural differences in Mn2+SI. In conclusion, subcortical auditory neural activities were decreased after SSD, and the interaural differences were diluted in the higher auditory nervous system. These findings were attenuated with time. Subcortical auditory neural changes after SSD may contribute to the change in tinnitus severity and the outcomes of cochlear implantation in SSD patients.

Objective: We wished to investigate central auditory processing in unilateral hearing loss (UHL), as little is discussed regarding its impact on auditory processing. Study design: Central auditory processing was evaluated in children with UHL using the P300 component of event related potentials. Speech and tone stimuli were used for eliciting P300 in three groups of children: group A (bilateral normal hearing children); group B (children with right UHL); and group C (children with left UHL). Results: In children with a right UHL, P300 amplitudes were significantly reduced in response to speech when compared to a tone stimulus. Comparing the speech-evoked P300 response revealed a significantly delayed latency in children with UHL compared to controls. Conclusion: Children with UHL showed central auditory processing defects that are more pronounced in children with right UHL. Speech processing is more vulnerable to the effect of UHL than simple stimuli.

Disrupted binaural hearing is thought to contribute in part to the academic, social, and communication problems often associated with unilateral hearing loss (UHL) in childhood. It is not known, however, if putting a hearing aid in the impaired ear of a child with UHL will lead to bilateral or binaural benefit. This study seeks to utilize sound localization acuity measurements to assess hearing aid amplification efficacy in children with UHL. To measure sound localization ability in children with UHL who use a hearing aid in the impaired ear to determine the extent to which amplification, age, early intervention, and degree of hearing loss affects localization acuity. A within-subjects experimental design using repeated measures is used to determine the effect of amplification on localization acuity in children with UHL. A between-subjects experimental design is used to compare localization acuity between children with UHL and age-matched controls with normal hearing. Twelve children with UHL who used a hearing aid in the impaired ear and 12 age-matched controls with normal hearing. Children with UHL were divided into two groups based on degree of hearing loss. Children in both groups were divided into two age groups: older children (10-14 yr) and younger children (6-9 yr). All testing was done in a sound-treated booth with a horizontal array of 15 loudspeakers (radius of 1 m). The stimulus was a spondee word, "baseball": the level averaged 60 dB SPL and randomly roved (±8 dB). Each child was asked to identify the location of a sound source. In the experimental study a significant interaction was found between hearing aid amplification and child age. A hearing aid significantly improved localization acuity in younger children with UHL and significantly impaired localization acuity in older children. A significant correlation was found between age at intervention and localization bilateral benefit. Children who were fit earlier showed bilateral benefit whereas children who were fit later showed bilateral interference. Development, however, may play a role in sound localization acuity. When unaided, older children had significantly better localization acuity than younger children with UHL. A hearing aid can provide bilateral localization benefit to some children with UHL. Early intervention may increase the likelihood of bilateral benefit. However, developmental factors appear to play a role in improving localization abilities over time for children with UHL. Nonetheless, without a means of establishing bilateral benefit with hearing aid amplification, localization performance in children with UHL will rarely equal that of peers.

Objectives. To investigate the activation pattern of the human auditory cortex to monaural sound stimuli using functional magnetic resonance imaging (fMRI) in volunteers with normal hearing or unilateral conductive hearing loss. Method. Ten normal hearing volunteers and seven volunteers with unilateral conductive hearing loss were subjected to a random sequence of monaural sound stimuli and silence, whilst undergoing fMRI in a 3 Tesla scanner. The sound stimuli consisted of narrowband noise (250 Hz and 4000 Hz), as well as wideband noise (250–4000 Hz). A signal averaging algorithm was used to detect the response in the primary auditory cortex. A laterality index was calculated and compared between the study groups. Results. In the normal hearing group the auditory areas of both hemispheres showed activity in response to monaural stimuli, but the responses were consistently greater in the contralateral primary auditory cortex. Wideband stimuli produced the greatest degree of contralaterality and the largest response across the auditory cortex. The responses to high and low frequency stimuli were found to be conform with established tonotopic organization of the primary auditory cortex. There was no difference in the laterality index between the study groups. Conclusions. We have developed a non‐invasive way to investigate the response pattern of the primary auditory cortex to sound stimuli using fMRI. A similar activation pattern of the auditory cortex was found in both study groups suggesting a preservation of the organisation of the ascending auditory pathways and auditory cortex in unilateral conductive hearing loss.

Accurate sound localization requires normal binaural input and precise auditory neuronal representation of sound spatial locations. Previous studies showed that unilateral hearing loss profoundly impaired the sound localization abilities. However, the underlying neural mechanism is not fully understood. Here, we investigated how chronic unilateral conductive hearing loss (UCHL) affected the neural tuning to sound source azimuth in the primary auditory cortex (AI). The UCHL was manipulated by the removal of tympanic membrane and malleus in the right ear of young (P14) rats and adult (P57) rats. We recorded the azimuth tuning of neurons in the left AI contralateral to the operated ear in the two groups of rats that experienced 2 months of UCHL, and in the left AI of age-matched control rats. We found that AI neurons in control rats showed predominant preference to sound from contralateral azimuths. However, UCHL weakened the cortical neuronal representation of contralateral azimuths on the operated ear side and strengthened the cortical neuronal representation of ipsilateral azimuths on the intact ear side. This effect was stronger in rats with UCHL at young age than in rats with UCHL in adulthood. Moreover, UCHL degraded the azimuth selectivity and azimuth sensitivity of AI neurons, and this effect was stronger in rats with UCHL in adulthood than in rats with UCHL at young age. These findings highlight a remarkable age-related experience-dependent plasticity of neural tuning to sound source azimuth in AI, and imply a neural mechanism for the impacts of chronic UCHL on sound localization abilities.

An fMRI-study on single-sided deafness: Spectral-temporal properties and side of stimulation modulates hemispheric dominance

Topological reorganization after partial auditory deprivation—a structural connectivity study in single-sided deafness

Developing an International Core Outcome Set for SSD Interventions

Neuroimaging studies have identified alterations in functional connectivity between specific brain regions in patients with unilateral hearing loss (UHL) and different influence of the side of UHL on neural plasticity. However, little is known about changes of whole-brain functional networks in patients with UHL and whether differences exist in topological organization between right-sided UHL (RUHL) and left-sided UHL (LUHL). To address this issue, we employed resting-state fMRI (rs-fMRI) and graph-theoretical approaches to investigate the topological alterations of brain functional connectomes in patients with RUHL and LUHL. Data from 44 patients with UHL (including 22 RUHL patients and 22 LUHL patients) and 37 healthy control subjects (HCs) were collected. Functional brain networks were constructed for each participant, following by graph-theoretical network analyses at connectional and global (e.g., small-worldness) levels. The correlations between brain network topologies and clinical variables were further studied. Using network-based analysis, we found a subnetwork in the visual cortex which had significantly lower connectivity strength in patients with RUHL as compared to HCs. At global level, all participants showed small-world architecture in functional brain networks, however, significantly lower normalized clustering coefficient and small-worldness were observed in patients with RUHL than in HCs. Moreover, these abnormal network metrics were demonstrated to be correlated with the clinical variables and cognitive performance of patients with RUHL. Notably, no significant alterations in the functional brain networks were found in patients with LUHL. Our findings demonstrate that RUHL (rather than LUHL) is accompanied with aberrant topological organization of the functional brain connectome, indicating different pathophysiological mechanisms between RUHL and LUHL from a viewpoint of network topology.

Attentional modulation of auditory cortical activity in individuals with single-sided deafness

Characteristics of neurons in auditory cortex of monkeys performing a simple auditory task.

Redefining Auditory Processing Disorder: A Speech-Language Pathologist’s Perspective

Rats are born deaf and start hearing at the end of the second postnatal week, when the ear canals open and low-intensity sounds start to evoke responses in the auditory cortex. Here, using μECoG electrode arrays and intracortical silicon probe recordings, we found that bone-conducted (BC) sounds evoked biphasic responses in the auditory cortex starting from postnatal day (P) 8. The initial phase of these responses, generated by thalamocortical input, was followed by intracortical propagation within supragranular layers. BC-evoked responses co-localized with the responses evoked by electrical stimulation of the cochlea and the deepest layers of the inferior colliculus prior to onset of low-threshold hearing (P13), as well as with the responses evoked by high-frequency (30 kHz) low-intensity (70 dB) air-conducted sounds after that. Thus, BC signals reach high-frequency processing regions of the auditory cortex well before the onset of low-threshold hearing, reflecting early integrity of the auditory system.

Multisensory connections of monkey auditory cerebral cortex