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 con ventional electrical median-nerve stimulation contain a low-amplitude (<500 nV), high-frequency (similar to 600 Hz) burst of repetitive wavelets, whic h are superimposed onto the primary cortical response N20. Previous electro encephalographic (EEG) studies have shown: (1) that these wavelets are gene rated near the hand area of the primary somatosensory cortex and in deep fi bers 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 r egulating 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 re cordings after electric right-median-nerve stimulation allowed the comparis on of brainstem, thalamic, and two cortical (one tangential, one radial) so urce activities in ten awake human subjects under two slightly different ar ousal states (eyes open vs. eyes closed), each tested for three conditions of focused attention (directed towards rare acoustic and right- or left-han d somatosensory target stimuli). While the N20 was not modified at all, the source strength of the high-frequency wavelet burst was significantly incr eased for eyes opened versus eyes closed. at the thalamic source site as we ll as for the tangentially oriented cortical source. In contrast, there wer e no significant differences between conditions with different attentional targets. This evidence for modulatory effects of increased arousal (eyes op en) on both thalamic and cortically generated high-frequency SEP activity f its 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.