Brain stem acoustic evoked potentials recorded from the human mesencephalon.

Near field brain stem evoked potentials (NF-BAEPs) were recorded by means of an intraparenchymatous electrode placed in the right mesencephalon, during a stereotactic-guided biopsy of a tumor. The traces were compared with far field BAEPs (FF-BAEPs) obtained intraoperatively with scalp electrodes. The findings suggest a peripheral origin of wave II, a bilateral source of wave III and a main contralateral source of wave IV, while wave V seems to take origin mainly from the homolateral upper brain stem. Some intraparenchymatous activities do not project to the scalp. The possible impairments of NF-BAEP due to the presence of the tumor and the challenges of unidimensional single electrode recording are discussed.