Somatosensory evoked potentials from the human brain-stem: origins of short latency potentials

Short latency somatosensory evoked potentials (SEPs) were recorded in man from an array of electrodes within the IVth (the pons) and IIIrd ventricles (the thalamus) and the aqueduct of Sylvius (the midbrain). The slow negative (N15 and N20) and positive (P22) waves were preceded by 4 small positive (P9, P11, P13, and P14) waves by approximately 1 msec duration in scalp recordings. The sources of these waves have been differentiated on the basis of their timing and spatial gradients of corresponding intracranial potentials. While P11 is identified as volume conducted from below the brain-stem, P13 and P14 reflect synchronized volleys of the medial lemniscus and its branches to the various pontine and mesencephalic nuclei. In contrast to these earlier positive wavelets arising from fiber tracts, N15 may represent postsynaptic activities within these pontine and midbrain nuclei. N20 and P22 are the initial responses of the juxtarolandic cortex.