SPEECH-SPECIFIC AUDITORY-EVOKED CORTICAL POTENTIALS - METHODS AND INITIAL CLINICAL-RESULTS

The purpose of this study was to find out whether specific cortical po tentials can be evoked and identified after word stimulation. The clin ical relevance was to be investigated in patients with aphasic syndrom es. Materials and methods: In 20 young adults with no signs of hearing impairment and in patients with manifest aphasic syndromes, word-evok ed cortical potentials were compared with those after an equivalent no ise stimulus. The test words were selected from the Freiburger Speech Comprehension Test. The duration of the words was between 450 and 640 ms. The stimulus was presented monaurally. The peak level was 70 dB HL . The noise stimulus was produced by modifying a low-band noise. Poten tials were measured between the ipsilateral mastoid and the contralate ral forehead. Data were analysed offline. Results: In healthy persons, the potentials after word and noise stimulation did not differ until 100 ms after the stimulus onset. After noise stimulation a negative ma ximum could be seen 100 ms after the stimulus onset, and a positive ma ximum 200 ms after the stimulus onset. After word stimulation, a posit ive maximum of higher amplitude than after noise stimulation was measu red 150 ms after the stimulus onset, and a negative maximum was measur ed 270 ms after the stimulus onset. In all test persons the difference curve of word- and noise-evoked potentials revealed a speech-specific component 170 ms (N 170) after the stimulus onset. The single-word an alysis showed that the potentials depend on the phonemes of the test w ord. The potentials do not alter when the stimulus side is changed. In patients with aphasia the potentials depend on the grade of the distu rbance of speech perception: global and Wernicke's aphasia show no sig nificant difference of speech- and noise-evoked potentials, whereas in Broca's aphasia a speech specific maximum is apparent. Conclusion: Th e speech-specific component may be regarded as a paradigm of cortical speech detection processes. Electrophysiological speech audiometry by means of word-evoked cortical potentials seems possible and may be use d for clinical purposes.