BRAIN-STEM AUDITORY-EVOKED POTENTIALS (BAEPS) FROM BASAL SURFACE OF TEMPORAL-LOBE RECORDED FROM CHRONIC SUBDURAL ELECTRODES

BAEPs were recorded from the basal surface of the temporal lobe by sub dural electrodes chronically implanted in 6 patients who were evaluate d for surgical management of intractable partial seizures. Near-field recordings were obtained by recording between the subdural electrode c losest and most distant to the brain-stem. Far-field recordings were o btained by recording between the subdural electrodes and an indifferen t electrode over the spinal process of the seventh cervical vertebrae. The recordings were compared with standard ear-vertex recordings. Aft er ipsilateral ear stimulation, the subdural electrode closest to the brain-stem recorded large amplitude waves I and II, followed by less w ell-defined waves of longer latencies. Recordings to contralateral sti mulation showed no clearly defined waves I and II and a large amplitud e wave Vn. Waves III, IV, V, Vn and VI were of opposite polarity after ipsi- and contralateral stimulation. These findings indicate that wav es I and II are generated ipsilaterally to the stimulation side, where as wave Vn has a contralateral origin. Wave Vn may be generated in the brachium of the inferior colliculus, as suggested from latency and fr om dipole configuration studies. This conclusion is consistent with th e classical anatomical observations that the supracollicular auditory pathways are predominantly crossed.