Profound Unilateral Deafness Research Articles

Ear-Specific Hemispheric Asymmetry in Unilateral Deafness Revealed by Auditory Cortical Activity.

Profound unilateral deafness reduces the ability to localize sounds achieved via binaural hearing. Furthermore, unilateral deafness promotes a substantial change in cortical processing to binaural stimulation, thereby leading to reorganization over the whole brain. Although distinct patterns in the hemispheric laterality depending on the side and duration of deafness have been suggested, the neurological mechanisms underlying the difference in relation to behavioral performance when detecting spatially varied cues remain unknown. To elucidate the mechanism, we compared N1/P2 auditory cortical activities and the pattern of hemispheric asymmetry of normal hearing, unilaterally deaf (UD), and simulated acute unilateral hearing loss groups while passively listening to speech sounds delivered from different locations under open free field condition. The behavioral performances of the participants concerning sound localization were measured by detecting sound sources in the azimuth plane. The results reveal a delayed reaction time in the right-sided UD (RUD) group for the sound localization task and prolonged P2 latency compared to the left-sided UD (LUD) group. Moreover, the RUD group showed adaptive cortical reorganization evidenced by increased responses in the hemisphere ipsilateral to the intact ear for individuals with better sound localization whereas left-sided unilateral deafness caused contralateral dominance in activity from the hearing ear. The brain dynamics of right-sided unilateral deafness indicate greater capability of adaptive change to compensate for impairment in spatial hearing. In addition, cortical N1 responses to spatially varied speech sounds in unilateral deaf people were inversely related to the duration of deafness in the area encompassing the right auditory cortex, indicating that early intervention would be needed to protect from maladaptation of the central auditory system following unilateral deafness.

UNILATERAL PROFOUND DEAFNESS – A POSSIBLE MECHANISM

Purpose: To explain the clinical findings of patients with profound unilateral deafness who have recovered the hearing function after a combined treatment of hyperbaric oxygenation and dehydrant therapy. Material and method: 11 patients with profound sudden unilateral hearing loss above 90 dB SPL, intolerance to sounds similar to recruitment, at levels 10-20 dB above the hearing threshold (105-120 dB SPL), positive glycerol test and unexpected good recovery of hearing within 1-2 months are described. Results: All patients have perfect outcome results without the use of glucocorticoids. Conclusions: Based on articles about experiments with genetically modified animals and acquired deafness, we propose a possible mechanism of damage explaining the clinical findings and the favorable outcome of the disease – a disconnection between the tectorial membrane and stereocilia. Thus mechano-electrical transducer (MET) of hair cells cannot be activated. The combined use of hyperbaric oxygenation and dehydrant medications can lead to recovery of the hearing function.

Evaluation of Patients with Delayed Endolymphatic Hydrops by Videonystagmography, Vestibular-Evoked Myogenic Potentials, and Electrocochleography

Introduction: The categorization of delayed endolymphatic hydrops (DEH) based on the ear which produces vertigo may sometimes cause misdiagnosis. Objectives: The aim of this study was investigating the vestibular-evoked myogenic potentials (VEMPs), electrocochleography (ECoG), and videonystagmography (VNG) in cases with DEH to determine the ear that originates symptoms. Methods: In this cross-sectional study, 34 patients – 20 males and 14 females – with profound unilateral sensorineural deafness and vertigo attacks were recruited and evaluated by the ECoG, VNG, and VEMPs tests. Results: The average age was around 43; the summating potential/action potential was abnormal in 29.4% of patients in their normal auditive ear. In 32.4, 17.6, and 50% of cases with a deaf ear, absent, normal, and abnormal VEMPs results were sequentially observed, respectively. In normal-hearing ears, absent, normal, and abnormal VEMPs were observed in 23.5, 50, and 26.5%, respectively. In the normal-hearing ear, the distribution of abnormal VEMPs was 26.5%, and in the deaf ear, this parameter was abnormal in 50% of the opposite ear (p value = 0.00021). In the VNG test, among patients with a normal-hearing ear, results in 27 and 7 patients were sequentially normal and hypofunction. Conclusion and Significance: The probability of a hypofunction VNG test in a normal-hearing ear might be greater when the VEMPs results of the contralateral deaf ear are normal. In patients with a normal-hearing ear, the distribution of abnormal VEMPs in the contralateral deaf ear is greater, although the intact side may also manifest abnormality in VEMPs tests. The initial evaluation should begin in a deaf ear as well as for the normal-hearing ear ere utilizing ablation surgery.

Preceding profound deafness and co-factors promote development of endolymphatic hydrops in preliminary patients with delayed endolymphatic hydrops

Conclusion: Profound deafness (PD) is unlikely to be the only cause of endolymphatic hydrops (EH) in patients with delayed endolymphatic hydrops (DEH). Further studies in large patient cohorts are required to determine the etiologic co-factors involved in the development of EH in patients with PD and DEH.Objectives: DEH usually occurs in patients with sustained unilateral PD who then develop episodic vertigo or fluctuating hearing loss. The latency can vary markedly between PD and development of DEH, but the factors affecting inner ear homeostasis and causing EH during this latency period remain unclear. The study aimed to assess the differences in EH and functional status of the inner ear between patients with PD only and those with DEH.Method: Two PD and 10 DEH patients who had developed PD in early childhood were enrolled. Gadolinium-enhanced magnetic resonance imaging of the inner ear was performed in all patients to assess the degree of EH in the vestibule and cochlea.Results: There was no evidence of EH in the affected ears of patients with PD alone. DEH patients, however, showed significant EH in either the cochlea or vestibule, suggesting that EH development and DEH onset were strongly correlated.

Multidimensional scaling between acoustic and electric stimuli in cochlear implant users with contralateral hearing

This study investigated the perceptual relationship between acoustic and electric stimuli presented to CI users with functional contralateral hearing.Fourteen subjects with unilateral profound deafness implanted with a MED-EL CI scaled the perceptual differences between pure tones presented to the acoustic hearing ear and electric biphasic pulse trains presented to the implanted ear. The differences were analyzed with a multidimensional scaling (MDS) analysis. Additionally, speech performance in noise was tested using sentence material presented in different spatial configurations while patients listened with both their acoustic hearing and implanted ears.Results of alternating least squares scaling (ALSCAL) analysis consistently demonstrate that a change in place of stimulation is in the same perceptual dimension as a change in acoustic frequency. However, the relative perceptual differences between the acoustic and the electric stimuli varied greatly across subjects. A degree of perceptual separation between acoustic and electric stimulation (quantified by relative dimensional weightings from an INDSCAL analysis) was hypothesized that would indicate a change in perceptual quality, but also be predictive of performance with combined acoustic and electric hearing. Perceptual separation between acoustic and electric stimuli was observed for some subjects. However, no relationship between the degree of perceptual separation and performance was found.

Congenital unilateral deafness affects cerebral organization of reading.

It is known that early sensory deprivation modifies brain functional structure and connectivity. The aim of the present study was to investigate the neuro-functional organization of reading in a patient with profound congenital unilateral deafness. Using event-related potentials (ERPs), we compared cortical networks supporting the processing of written words in patient RA (completely deaf in the right ear since birth) and in a group of control volunteers. We found that congenital unilateral hearing deprivation modifies neural mechanisms of word reading. Indeed, while written word processing was left-lateralized in controls, we found a strong right lateralization of the fusiform and inferior occipital gyri activation in RA. This finding goes in the same direction of recent proposals that the ventral occipito-temporal activity in word reading seem to lateralize to the same hemisphere as the one involved in spoken language processing.

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

ObjectiveTo 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. MethodsBaseline 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. ResultsBaseline 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. ConclusionDifferent 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. SignificanceThe 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.

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

ObjectiveTo investigate the effect of acquired unilateral deafness on hemispheric asymmetries in adult humans using cortical auditory evoked potentials. MethodsN1 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. ResultsThere 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. ConclusionsThe results reveal evidence of central auditory system plasticity that is consistent with animal models having experimentally induced unilateral deafness. SignificanceThe 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.

Cochlear Implantation in Unilateral Deaf Subjects Associated With Ipsilateral Tinnitus

In subjects who are deaf and who also have tinnitus in the affected ear, tinnitus treatments based on acoustic input are impossible. On the other hand, tinnitus suppression using electric stimulation has been reported to be successful. Therefore, a study was initiated to investigate the potential of cochlear implantation (CI) in unilateral deaf subjects regarding tinnitus suppression, device acceptance, and restoration of spatial hearing. Five subjects with severe to profound unilateral deafness having also ipsilateral tinnitus were enrolled. In monthly visits, the speech processor program was optimized, and the hearing performance as well as tinnitus were monitored. In addition, it was investigated whether the CI improves hearing in adverse listening situations when combined with the normal hearing side. In 3 participants, the tinnitus was significantly suppressed while wearing the device. In the other 2 participants, the tinnitus could be reduced in certain situations. Speech perception tests revealed a significant benefit with the CI in combination with the normal-hearing side for 3 participants. All participants accepted the device in a clinical setting; adaptation of the frequency allocation was not required. Improvements were found regarding the hearing and the tinnitus. Not all participants benefit from the CI to the same degree and in the same situations.The results indicate that cochlear implantation in subjects with unilateral severe to profound hearing loss and ipsilateral tinnitus may be beneficial on a case-to-case basis. Further work needs to be performed to define the appropriate indication criteria.

Bilateral sequential cochlear implantation in the congenitally deaf child: Evidence to support the concept of a ‘Critical Age’ after which the second ear is less likely to provide an adequate level of speech perception on its own

This study attempts to answer the question of whether there is a ‘critical age’ after which a second contralateral cochlear implant is less likely to provide enough speech perception to be of practical use. The study was not designed to predict factors that determine successful binaural implant use, but to see if there was evidence to help determine the latest age at which the second ear can usefully be implanted, should the first side fail and become unusable.Outcome data, in the form of speech perception test results, were collected from 11 cochlear implant programmes in the UK and one centre in Australia. Forty-seven congenitally bilaterally deaf subjects who received bilateral sequential implants were recruited to the study. The study also included four subjects with congenital unilateral profound deafness who had lost all hearing in their only hearing ear and received a cochlear implant in their unilaterally congenitally deaf ear. Of those 34 subjects for whom complete sets of data were available, the majority (72%) of those receiving their second (or unilateral) implant up to the age of 13 years scored 60 per cent or above in the Bamford Kowal Bench (BKB) sentence test, or equivalent. In contrast, of those nine receiving their second or unilateral implant at the age of 15 or above, none achieved adequate levels of speech perception on formal testing: two scored 29 per cent and 30 per cent, respectively, and the rest seven per cent or less.A discriminant function analysis performed on the data suggests that it is unlikely that a second contralateral implant received after the age of 16 to 18 years will, on its own, provide adequate levels of speech perception. As more children receive sequential bilateral cochlear implants and the pool of data enlarges the situation is likely to become clearer.The results provide support for the concept of a ‘critical age’ for implanting the second ear in successful congenitally deaf unilateral cochlear implant users. This would argue against ‘preserving’ the second ear beyond a certain age, in order to use newer models of cochlear implant or for the purpose of hair cell regeneration and similar procedures in the future. The results suggest a new and more absolute reason for bilateral implantation of congenitally deaf children at an early age. Copyright © 2009 John Wiley & Sons, Ltd.

Bilateral sequential cochlear implantation in the congenitally deaf child: evidence to support the concept of a ‘critical age’ after which the second ear is less likely to provide an adequate level of speech perception on its own

This study attempts to answer the question of whether there is a 'critical age' after which a second contralateral cochlear implant is less likely to provide enough speech perception to be of practical use. The study was not designed to predict factors that determine successful binaural implant use, but to see if there was evidence to help determine the latest age at which the second ear can usefully be implanted, should the first side fail and become unusable.Outcome data, in the form of speech perception test results, were collected from 11 cochlear implant programmes in the UK and one centre in Australia. Forty-seven congenitally bilaterally deaf subjects who received bilateral sequential implants were recruited to the study. The study also included four subjects with congenital unilateral profound deafness who had lost all hearing in their only hearing ear and received a cochlear implant in their unilaterally congenitally deaf ear. Of those 34 subjects for whom complete sets of data were available, the majority (72%) of those receiving their second (or unilateral) implant up to the age of 13 years scored 60 per cent or above in the Bamford Kowal Bench (BKB) sentence test, or equivalent. In contrast, of those nine receiving their second or unilateral implant at the age of 15 or above, none achieved adequate levels of speech perception on formal testing: two scored 29 per cent and 30 per cent, respectively, and the rest seven per cent or less.A discriminant function analysis performed on the data suggests that it is unlikely that a second contralateral implant received after the age of 16 to 18 years will, on its own, provide adequate levels of speech perception. As more children receive sequential bilateral cochlear implants and the pool of data enlarges the situation is likely to become clearer.The results provide support for the concept of a 'critical age' for implanting the second ear in successful congenitally deaf unilateral cochlear implant users. This would argue against 'preserving' the second ear beyond a certain age, in order to use newer models of cochlear implant or for the purpose of hair cell regeneration and similar procedures in the future. The results suggest a new and more absolute reason for bilateral implantation of congenitally deaf children at an early age.

遅発性内リンパ水腫について―ことにメニエール病との比較において―

Delayed endolymphatic hydrops (DEH) is a Meniere’s syndrome that typically occurs in patients suffering from longstanding unilateral profound inner-ear hearing loss. Approximately 55 to 65% of the hearing loss preceding DEH is due to juvenile unilateral profound deafness (early childhood unilateral profound sensorineural hearing loss of unknown etiology). The incidence of ipsilateral DEH (approximately 60%), in which the ear with profound hearing loss suffers progressive endolymphatic hydrops, is higher than that of the contralateral type of DEH (about 40%), in which the formation of progressive endolymphatic hydrops takes place in the ear opposite to the previously deafened ear. The prevalence of DEH in the general population (23 or less per 100,000) is similar to that of definitive Meniere’s disease, and the distribution of age at onset of DEH closely resembles that of Meniere’s disease except for some patients with childhood onset of DEH. Further, the rate of patients with the contralateral type of DEH (about 40%, although the rate is about 15% if the contralateral type is restricted to patients showing typical Meniere’s symptoms) is similar to that of patients with bilateral Meniere’s disease (about 40%, with a rate of about 14% if Meniere’s disease in the second ear is restricted to definitive Meniere’s disease). Since DEH is closely similar to Meniere’s disease not only in symptoms, but also in many clinical-statistical characteristics, the diseases may be identical in pathogenesis and the pathophysiology of progressive endolymphatic hydrops, although the two conditions have different etiologies.

Plasma antidiuretic hormone in cases with the early onset of profound unilateral deafness

Objective The p-ADH level in cases of juvenile unilateral profound deafness (JUPD) and the timecourse of the level were examined to investigate whether or not an increase of p-ADH is involved in the development of delayed endolymphatic hydrops (DEH) in JUPD. Materials and methods In 90 consecutive patients with unilateral profound or total sensorineural deafness with the onset in early childhood, pure-tone audiometric examination and the measurement of p-ADH and plasma osmolality (p-OSM) were followed up once or twice a year as far as possible. At every testing, we performed careful history-taking about episodic vertigo/dizziness, fluctuant hearing loss, and tinnitus in order to find out whether patients had experienced these clinical signs of the development of DEH. Results Means and standard deviation (S.D.) of p-ADH level and osmolality in all samples tested ( n = 368) were 7.3 ± 7.0 pg/mL (0.7–52.0 pg/mL), and 288.6 ± 4.4 mOsm/L (273–306 mOsm/L), respectively. The mean of p-ADH level was much higher than those previously reported in children and adolescents. High levels of p-ADH (over 5.0 pg/mL) were often observed in subjects between 6 and 19 years of age, but not so frequently in subjects of 20 years of age or older. Long-term follow-up of p-ADH levels revealed that DEH frequently developed in cases with persistent elevation of p-ADH. Conclusions The elevation of p-ADH is likely to promote the development of DEH in cases of JUPD, although the underlying mechanism remains to be elucidated.

Does long-term unilateral deafness change auditory evoked potential asymmetries?

ObjectiveTo investigate the long-term cortical changes in auditory evoked potential (AEP) asymmetries associated with profound unilateral deafness. MethodsElectroencephalographic (EEG) recordings from 68 channels were used to measure auditory cortex responses to monaural stimulation from 7 unilaterally deaf patients and 7 audiogram-matched controls. Source localization of the AEP N100 response was carried out and regional source waveform amplitude and latency asymmetries were analysed for activity in the N100 latency range and for the middle latency response (MLR) range. ResultsAsymmetry indices (contralateral−ipsilateral)/(contralateral+ipsilateral) showed that matched control subjects, like normally hearing participants, produced activity in the N100 latency range that was more contralaterally dominant for left compared to right ear stimulation. Contrary to expectation, source waveforms and asymmetry indices in the MLR and N100 latency range were similar for unilaterally deaf patients, their matched controls and a group of normally hearing participants. ConclusionsRegional source waveform analysis revealed no evidence of systematic cortical changes in hemispheric asymmetries associated with long-term unilateral deafness. It is possible that a reorganization of cortical asymmetries to a ‘normal’ pattern had taken place in the years between deafness and testing. SignificanceElectrophysiological measures of auditory hemispheric asymmetries do not suggest long-term cortical reorganisation as a result of profound unilateral deafness.

Cortical auditory evoked responses from an implanted ear after 50 years of profound unilateral deafness

A male with unilateral deafness in the right ear since 8 years of age developed a sudden hearing loss in the left ear at age 63. A hearing aid was fitted in the left ear with limited benefit. The right ear received a cochlear implant (CI) 20 months later. Cortical auditory evoked potentials (CAEPs) and speech recognition scores (SRS) were measured in free-field three, six and nine months after implantation with the hearing aid alone, CI alone and bimodal condition (hearing aid and CI together). Three months after implantation the cortical responses for the two ears were similar, despite more than 50 years of unilateral auditory deprivation. CAEPs measured over time show evidence of binaural interaction and improvements in SRS. Copyright © 2007 John Wiley & Sons, Ltd.

Isolated Cochlear Nerve Hypoplasia with Various Internal Auditory Meatus Deformities in Children

We report neuro-otological findings in isolated congenital cochlear nerve hypoplasia with various bony deformities and evaluate relationships between functional impairment and the radiologic dimensions of the internal auditory meatus (IAM). We performed imaging and functional analyses on consecutive juvenile or adolescent patients between 2001 and 2005 with "isolated" unilateral hypoplasia of the cochlear nerve, without inner ear anomaly or other deformities. Among 20 patients with unilateral profound deafness who underwent imaging studies, 10 (50%) passed the inclusion criteria. In all affected ears, auditory brain stem responses were absent and the speech discrimination score was very poor (0% to 5%). Distortion product otoacoustic emissions were good in 2 ears, fair in 1 ear, and poor in 7 ears. Caloric responses were absent in 2 ears, reduced in 3 ears, and normal in 5 ears. Inferior vestibular nerve function and facial nerve function were normal in all ears. Distortion product otoacoustic emissions and caloric responses tended to be better in ears with less severe narrowing of the IAM. The risk of co-involvement of the inner ear and superior vestibular nerve functions is higher in the presence of a narrower bony IAM. Cochlear nerve hypoplasia is proposed as one of the most important causes of juvenile unilateral deafness because of its unexpectedly high incidence.

Cortical auditory evoked responses from an implanted ear after 50 years of profound unilateral deafness

A male with unilateral deafness in the right ear since 8 years of age developed a sudden hearing loss in the left ear at age 63. A hearing aid was fitted in the left ear with limited benefit. The right ear received a cochlear implant (CI) 20 months later. Cortical auditory evoked potentials (CAEPs) and speech recognition scores (SRS) were measured in free-field three, six and nine months after implantation with the hearing aid alone, CI alone and bimodal condition (hearing aid and CI together). Three months after implantation the cortical responses for the two ears were similar, despite more than 50 years of unilateral auditory deprivation. CAEPs measured over time show evidence of binaural interaction and improvements in SRS. Copyright © 2007 John Wiley & Sons, Ltd.

Nonsyndromic Isolated Unilateral Cochlear Nerve Aplasia without Narrow Internal Auditory Meatus: A Previously Overlooked Cause of Unilateral Profound Deafness in Childhood

Juvenile or adolescent unilateral profound sensorineural deafness (worldwide prevalence, 0.1% to 0.2%) has been attributed to postnatal viral infection, sudden deafness, prenatal and perinatal problems including maternal rubella and viral infections, congenital innerear anomalies, and other factors. Herein, 2 cases are reported and another potentially important cause of unilateral profound hearing loss is proposed. Two nonsyndromic cases of a presently "very rare" cause of pediatric unilateral deafness are presented as a retrospective case study. The 2 patients showed isolated aplasia of the cochlear nerve; other branches of the eighth cranial nerve, the seventh nerve, and the inner ear were spared,and there was no anomaly of the internal auditory meatus. Both functional and imaging studies confirmed the isolated lesion (absence) of the cochlear nerve. Because of the absence of bony abnormalities, such cases may have been overlooked. The authors would like to advocate this isolated anomaly of thecochlear nerve as an important cause of juvenile or adolescent unilateral profound deafness.

Auditory cortical response to monaural stimulation detected by functional magnetic resonance imaging

Few reports have characterized auditory processing in monaural listening, which is important for the understanding of auditory brain activity in subjects with hearing loss. We measure regional brain activity in response to monaural audition of 95 dB sound pressure level (SPL) monosyllables using functional magnetic resonance imaging (fMRI) in the 10 subjects with normal hearing and four subjects with unilateral deafness as controls for “cross-hearing”. Images were analyzed by statistical parametric mapping software. In subjects without hearing loss, the stimuli elicited cortical activation in the primary auditory and auditory association regions, particularly contralaterally where extent of activation was approximately 2.5 times the ipsilateral extent. All patients with profound unilateral deafness showed no statistically apparent response in the primary auditory and auditory association regions, ruling out an important influence from cross-hearing. We found fMRI to be a useful technique for analysis of auditory processing that should be applicable to patients with various hearing abnormalities.

Differential ear effects of profound unilateral deafness on the adult human central auditory system.

This study investigates the effects of profound acquired unilateral deafness on the adult human central auditory system by analyzing long-latency auditory evoked potentials (AEPs) with dipole source modeling methods. AEPs, elicited by clicks presented to the intact ear in 19 adult subjects with profound unilateral deafness and monaurally to each ear in eight adult normal-hearing controls, were recorded with a 31-channel system. The responses in the 70-210 ms time window, encompassing the N1b/P2 and Ta/Tb components of the AEPs, were modeled by a vertically and a laterally oriented dipole source in each hemisphere. Peak latencies and amplitudes of the major components of the dipole waveforms were measured in the hemispheres ipsilateral and contralateral to the stimulated ear. The normal-hearing subjects showed significant ipsilateral-contralateral latency and amplitude differences, with contralateral source activities that were typically larger and peaked earlier than the ipsilateral activities. In addition, the ipsilateral-contralateral amplitude differences from monaural presentation were similar for left and for right ear stimulation. For unilaterally deaf subjects, the previously reported reduction in ipsilateral-contralateral amplitude differences based on scalp waveforms was also observed in the dipole source waveforms. However, analysis of the source dipole activity demonstrated that the reduced inter-hemispheric amplitude differences were ear dependent. Specifically, these changes were found only in those subjects affected by profound left ear unilateral deafness.