Functional reorganization of the auditory pathways (or lack thereof) in callosal agenesis is predicted by monaural sound localization performance.

Neuroimaging studies show that permanent peripheral lesions such as unilateral deafness cause functional reorganization in the auditory pathways. However, functional reorganization of the auditory pathways as a result of higher-level damage or abnormalities remains poorly investigated. A relatively recent behavioural study points to functional changes in the auditory pathways in some, but interestingly not in all, of the acallosal individuals that were tested. The present study uses fMRI to investigate auditory activities in both cerebral hemispheres in those same acallosal subjects in order to directly investigate the contributions of ipsilateral and contralateral functional pathways reorganization. Predictions were made that functional reorganization could be predicted from behavioural performance. As reported previously in a number of neuroimaging studies, results showed that in neurologically intact subjects, binaural stimulation induced balanced activities between both hemispheres, while monaural stimulation induced strong contralateral activities and weak ipsilateral activities. In accordance with behavioural predictions, some acallosal subjects showed patterns of auditory cortical activities that were similar to those observed in neurologically intact subjects while others showed functional reorganization of the auditory pathways. Essentially they showed a significant increase and a significant decrease of neural activities in the contralateral and/or ipsilateral pathways, respectively. These findings indicate that at least in some acallosal subjects, functional reorganization inside the auditory pathways does contribute to compensate for the absence of the corpus callosum.

Auditory pathways fail to re-establish normal cortical activation patterns in response to binaural stimulation following a unilateral lesion of the inferior colliculus.

The aim of this study was to investigate cortical activation in response to binaural stimulus presentations in an individual (FX) with a circumscribed traumatic hemorrhagic lesion of the right inferior colliculus. FX and control subjects were exposed to complex sounds while undergoing a functional magnetic resonance imaging assessment. Whereas normally-hearing individuals show well-balanced bilateral activation patterns in response to binaural auditory stimulation, the same stimuli produced stronger activation in the left hemisphere in FX. Combined with previous data, these findings reinforce the notion that the inferior colliculus is an essential auditory relay and that its loss cannot be significantly compensated.

Auditory scene analysis following unilateral inferior colliculus infarct.

Event-related potentials in the form of mismatch negativity were recorded to investigate auditory scene analysis capabilities in a person with a very circumscribed haemorrhagic lesion at the level of the right inferior colliculus. The results provide the first objective evidence that processing at the level of the inferior colliculus plays an important role in human auditory frequency discrimination. Moreover, the electrophysiological data suggest that following this unilateral lesion, the auditory pathways fail to reorganize efficiently.

A physiological correlate for the intolerance to both internal and external sounds.

The notion that stress can induce chronic tinnitus, or increase its intensity, is predominant in the literature on tinnitus. However, there is little empirical support for this claim, since previous studies rely merely on subjective reports. In the present study, we used an objective physiological measure of stress (i.e., basal secretion of the stress hormone cortisol), in order to assess the relationship between tinnitus and stress. We hypothesized that tinnitus, a permanent internal sound, should behave as a stressor and should be accompanied by chronically elevated cortisol levels in individuals with severe tinnitus. In addition, we hypothesized that suffering from severe tinnitus should generalize to an intolerance towards external sounds. Two groups each comprised of 18 participants (with and without tinnitus) matched on education and health status participated in the study. Tinnitus severity, as assessed by tinnitus-related distress, was high in half of the tinnitus participants, and low in the other half. Basal cortisol levels were measured using saliva samples (five saliva samples per day for 3 days within a week) taken in the participant's natural environment. Intolerance to external sounds was assessed psychometrically. The high tinnitus-related distress group had chronic cortisol levels greater than both the low tinnitus-related distress and control groups, and also displayed greater intolerance to external sounds. The low tinnitus-related distress and control groups did not differ from each other on either of these measures. Our study thus provides the first physiological and empirical evidence of a link between intolerance to both internal (tinnitus) and external sounds in persons with tinnitus, and is compatible with the clinical observation that severe tinnitus is associated with high stress levels.