Auditory evoked potentials recorded directly from the human VIIIth nerve and brain stem: origins of their fast and slow components.

Brain stem auditory evoked potentials in humans are composed of fast pseudorhythmic positive waves and slow positive and negative waves. The sources of these components have been identified on the basis of the observation of amplitude increase in depth records during surgery and by tracing these potentials from their intracranial maximum to the scalp by using movable electrodes within the IVth, IIIrd and lateral ventricles. The first wave I represents postsynaptic activities of the first auditory neurones; axonal volleys travelling along the auditory nerve do not contribute to this wave, which is obviously discrepant with the currently accepted view. Waves II and III are primarily generated within the pons, with possible contributions from the auditory nerve. Waves IV and VI originate from the pons and the medial geniculate body respectively. Both slow positive and negative waves have their origin in the inferior colliculus (IC). The slow positive waves probably represent incoming axonal volley and the negative waves, the postsynaptic potentials in the IC.