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Abstract

The comprehension of spoken language is based on th e central auditory analysis of complex acoustic signals. Many psychophysical studies have reported a direct rela tion between gradual, spectrotemporal modifications of s peech sounds and the impairment of the comprehensio n of such altered sounds. Thus, it is reasonable to a ssume that deficits in the understanding of speech seen in patients with acquired brain lesions may, to a cert ain degree, be due to the impaired central processi ng of acoustic signals. In this study we report on the de sign of psychoacoustic tests for evaluating signal processing at different levels of the central auditory system. These tests allow a differentiation between signal processing in (i) the auditory periphery, (ii) the auditory br ainstem and (iii) diencephalic or telencephalic aud itory areas. Specific care was taken to design reliable tests th at can be administered with a minimal amount of ins truction and stress to the subjects. This is of special impo rtance for the examination of patients with acquire d brain lesions who might suffer from cognitive deficits an d/or have language comprehension problems that coul d make understanding complex psychophysical test inst ructions difficult. The main body of this report provides normative dat a on age-depend- ent alterations in auditory processing gathered fro m one hundred normal-hearing subjects between 20 an d 70 years of age. Monaural pure tone thresholds were us ed to evaluate the performance of the peripheral he aring system and served as the basis for the subsequent t ests. The integrity of auditory brainstem processin g was evaluated by quantifying interaural frequency, inte nsity and duration discrimination. All the respecti ve inter- aural differences induce lateralized auditory perce ptions that are critically dependent on binaural pr ocessing. Diencephalic/ telencephalic processing was tested b y varying the same acoustic parameters (frequency, inten- sity and tone duration) as well as measuring discri mination limens for successively presented signals. Mon- aural signal presentation permits the measurement o f the respective processing in general, while prese nting these signals together with noise pulses to the oth er ear (i.e. dichotic signal/noise stimulation) specifically measures the contribution of the cortical hemisphere contral ateral to the side where the signals were presented . In nor- mal-hearing subjects, the difference limens of the monaural and dichotic signal/noise tests did not differ systemati- cally, while the interaural thresholds were signifi cantly lower at low frequencies. Both the pure tone threshold and all discrimination limens deteriorated with age . Taking age dependency into consideration is indis pens- able to any neurological evaluation of central audi tory performance. Data on patients with acquired le sions in the brainstem, lesions in the left or right superio r temporal plane or subcortical lesions are present ed in this report as an indication for the specificity of the tests.