Central auditory impairment in unilateral diencephalic and telencephalic lesions

The extent of perceptual impairment following unilateral lesions in the auditory cortex, its thalamic or callosal afferents was studied with psychoacoustic tests. Thresholds for the discrimination of signal frequency, intensity and duration were acquired under three different conditions of headphone stimulation ('monaural', 'interaural', and 'dichotic signal/noise tests') using the three-alternative forced-choice procedure. The different test alternatives generated distinct auditory percepts, which is in accordance with the assumption of specific signal processing at the level of the auditory brainstem and at thalamocortical auditory areas. Twenty-one patients from neurology were studied who suffered from unilateral lesions in the auditory cortex, the auditory thalamus, or the acoustic radiation. Location and extent of the lesions were assessed by magnetic resonance imaging. Monaural tests of either ear revealed no deficits in auditory performance. The patients showed impaired discrimination of signal frequency, intensity and duration in the dichotic signal/noise tests, when the signals were presented to the ear contralateral and the noise ipsilateral to the lesion. With inverted signal and noise stimulation, however, the thresholds were in the range of age-matched controls. All patients were able to master the interaural tests, which indicates the preserved ability to lateralize sound sources to the left and to the right with either one of the auditory cortices left intact. Another 24 patients were studied who had lesions mostly close to but sparing the before-mentioned auditory structures. All of them showed unimpaired performance in all test alternatives. The results indicate the specificity of the dichotic signal/noise tests for the identification of unilateral lesions in thalamocortical auditory structures. In addition, the results also point to the capacity of each telencephalic hemisphere to process the full range of auditory lateralization from left to right.