Somatosensory system evoked potentials during waking behavior and sleep in the cat

Abstract

During suppression of phasic motor activity in cats, a 12–16 c/sec EEG pattern has been described and termed the sensorimotor rhythm (SMR). This EEG activity has been found primarily in the somatosensory system. Other related studies have demonstrated that during SMR conditioning and associated motor suppression, changes occurred in several physiological functions to resemble more closely those seen in slow wave or quiet sleep. The purpose of this study was to examine evoked potential changes in the somatosensory system during suppression of motor activity in both waking and sleep states. All data were collected from chronic freely moving cats. Evoked potentials were tabled according to stimulus and recording sites, wave components, and behavioral states. Nine behavioral states were defined using a simple technique employing EEG patterns from sensorimotor and visual cortices along with EOG activity. The evoked potential data provided a quantitative measurement for comparing behavioral states. During absence of phasic motor activity in awake and active sleep states, evoked response amplitudes at all three levels in the somatosensory system tended to change in the direction of quiet sleep. This observation supports the hypothesis that neural mechanisms associated with the suppression of CNS functions during quiet sleep are also operative in awake and active sleep states. The marked evoked potential changes observed in waking and active sleep suggest that a similar functional modulation of somatosensory afferent activity is present during these states and that the somatosensory system is actively participating in sensory information processing. The marked decrease in the thalamic evoked response observed during the SMR supports the hypothesis that this rhythmic EEG activity is generated, at least in part, by recurrent inhibitory processes located at the thalamic level. In contrast to this, no evoked potential changes occurred at any level within the somatosensory system during spindle activity in quiet sleep. This is in concurrence with the absence of sleep spindle activity previously noted in this system during quiet sleep.