Differential Ear Effects of Profound Unilateral Deafness on the Adult Human Central Auditory System

Abstract

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 N 1b /P 2 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 ipsilateralcontralateral 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.

Effects of Sex, Age and Hearing Asymmetry on the Interaural Differences of Auditory Brainstem Responses

“…For unilaterally deaf subjects, analysis of the source dipole activity demonstrated that the reduced interhemispheric amplitude differences were ear dependent. Specifically, these changes were found only in subjects affected by profound left ear unilateral deafness [Khosla et al, 2003].…”

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

“…Khosla et al (2003) found that reduction in ipsilateral-contralateral amplitude differences for N1-P2 CAEP occurred in patients with profound left ear unilateral deafness. This fi nding suggests there is reorganisation in the auditory cortex in unilateral left deafness, with cortical activation increasing in the left hemisphere.…”

Functional-structural reorganisation of the neuronal network for auditory perception in subjects with unilateral hearing loss: Review of neuroimaging studies

“…Reorganization of the human auditory system after UHL has been shown using a range of different techniques both shortly after such a hearing loss occurs and several years later (Hanss et al 2009) in both developing and mature auditory systems (Khosla et al 2003). In adults with UHL, Vasama & Mäkelä (1995) found auditory evoked magnetic fields (AEFs) in auditory association areas not seen in normal hearing subjects following unilateral stimulation, suggesting that the effects of a UHL is not restricted only to the auditory cortex.…”