Thalamic and cortical high-frequency (600 Hz) somatosensory-evoked potential (SEP) components are modulated by slight arousal changes in awake subjects.

Human somatosensory-evoked potentials (SEP) recorded at the scalp after conventional electrical median-nerve stimulation contain a low-amplitude (<500 nV), high-frequency (approximately 600 Hz) burst of repetitive wavelets, which are superimposed onto the primary cortical response N20. Previous electroencephalographic (EEG) studies have shown: (1) that these wavelets are generated near the hand area of the primary somatosensory cortex and in deep fibers of thalamocortical afferences; and (2) that only the 600-Hz burst, but not the N20 is decreased during sleep. Since the thalamus is involved in regulating both, selective attention and arousal, the present study aimed at characterizing the effects of focused attention and slight arousal changes on the 600-Hz oscillations. A dipole-source analysis of 64-channel SEP recordings after electric right-median-nerve stimulation allowed the comparison of brainstem, thalamic, and two cortical (one tangential, one radial) source activities in ten awake human subjects under two slightly different arousal states (eyes open vs. eyes closed), each tested for three conditions of focused attention (directed towards rare acoustic and right- or left-hand somatosensory target stimuli). While the N20 was not modified at all, the source strength of the high-frequency wavelet burst was significantly increased for eyes opened versus eyes closed, at the thalamic source site as well as for the tangentially oriented cortical source. In contrast, there were no significant differences between conditions with different attentional targets. This evidence for modulatory effects of increased arousal (eyes open) on both thalamic and cortically generated high-frequency SEP activity fits the hypothesis that the 600-Hz SEP burst at least partially represents an arousal-dependent signal generated at the thalamic level and transmitted to the primary somatosensory cortex.