Brain-stem auditory-evoked potentials recorded directly from human brain-stem and thalamus.

Brain-stem auditory-evoked potentials were recorded in neurosurgical patients from surface electrodes applied to the VIIIth nerve, medulla, pons, midbrain and cortex; from depth electrodes in the thalamus; and from a movable electrode in the IVth, IIIrd, and lateral ventricles. The potentials recordable over the scalp within 10 ms after click stimulation are characterized by a slow positive wave (peaking at 5 to 6 ms) and a negative wave (8 to 10 ms) with 7 small positive wavelets superimposed upon them. The sources of these components have been identified by observing their increased amplitude in depth recordings, and by tracing the potentials from their intracranial maxima to the scalp. Wave I is generated within the most distal portion of the VIII nerve; Wave V in the midbrain (inferior colliculus); and Wave VI the medial geniculate body. Both low positive and negative components have their origins in the inferior colliculus. Intracranially-recorded brain-stem auditory-evoked potential showed very rapid changes in amplitude within the brain-stem but only slight changes in the more rostral regions, although their amplitude gradients varied in the different components. They also demonstrated minor but systematic shifts in latency with distance from the potential sources, reflecting a significant overlap of separate potentials. This effect must be taken into account in the interpretation of a 'concurrent' intracranial potential as the source of a far-field surface-recorded potential.