Central Auditory Impairment in Unilateral Diencephalic and Telencephalic Lesions

Abstract

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.

Auditory Processing Disorders with and without Central Auditory Discrimination Deficits

“…Evidence of the specificity of the discrimination tests comes, inter alia, from data in patients with acquired brain lesions in auditory areas of the brainstem or in the respective subcortical and cortical areas. The different lesions were specifically associated with impaired performance in either the dichotic s/n tests or in the interaural tests (BungertKahl et al 2004;Biedermann et al 2008). The same tests were used in the present study to better understand how central auditory processing deficits and the diagnosis APD relate to each other.…”

“…All discrimination tests were based on a 3IFC paradigm. Participants were asked to differentiate between two reference signals and one test signal differing in one single acoustic feature with the position of the test signal randomly altered within the stimulus triplet (Biedermann et al 2008). Responses were given via pressing distinct buttons on a response box.…”

“…The different presentation conditions have been described earlier in detail (Bungert-Kahl et al 2004;Biedermann et al 2008;Freigang et al 2011). Briefly, discrimination thresholds were measured for the parameters frequency, level, signal duration, and for differences in amplitude modulation frequency.…”

“…The test results allowed differentiation between impairment of the auditory brainstem processing indicated by elevated thresholds in interaural frequency and duration discrimination and deficits of cortical/thalamic processing recognizable from alterations in dichotic s/n frequency, amplitude modulation, and duration discrimination (see Biedermann et al 2008 Some of the 6/7-year-old children were not able to accomplish the test of interaural duration discrimination. Values are given only for those children who successfully completed this test Median, 25th, and 75th percentile in the respective age groups are listed for the parameters frequency (Hz), level (dB), signal duration (ms), and amplitude modulation (Hz) in both conditions (interaural and dichotic signal/noise ).…”

“…Thus, in the dichotic s/n condition, the activation generated in the auditory cortex of either hemisphere differs-in the cortex contralateral to the ear where the signals are presented the representation of the signal input predominates the noise input, while in the other cortex the respective weighting is reversed. Thus, this test design allows for distinct assessment of auditory cortical domains in both hemispheres (Biedermann et al 2008). In the presently investigated children, there was no indication of a hemisphere-specific discrimination deficit in either auditory cortex.…”