Dynamic changes of gamma activities of somatic cortical evoked potentials during wake-sleep states in rats.

Somatic evoked potentials (SEPs) from three brain sites elicited by electrical stimulation in 10 rats were recorded throughout wake-sleep states with intrinsic changes in temporal architectures under different vigilance states. Based on the patterns of spontaneous brain and muscle activities, three characteristic vigilance states could be classified: awake, slow-wave sleep (SWS), and paradoxical sleep (PS). Spontaneous gamma activities prominently appeared under awake and PS states, but less under SWS. SEP was filtered out via a zero-phase highpass filter (20 Hz) to extract the gamma activity of the SEP (gammaSEP). Gamma oscillations of SEPs were clearly observed and were reset by extrinsic electrical stimulation under awake and PS, but not under SWS state. Dynamic changes of gammaSEPs during wake-sleep states were also confirmed by multiple single-trial spectral analyses. Moreover, gamma oscillations were initiated at the parietal site, and the speed of its propagation in both frontal and occipital directions was significantly different. In addition, a clear two-component architecture of SEPs was observed under awake and PS states, and the gamma rhythmic activity was associated with the second component. Because gamma oscillations are related to feature binding in the waking state, evoked gammaSEPs in PS may be related to sensory integration analogous to the awake ones. By contrast, a long-lasting biphasic component of SEPs, which might be associated with augmenting response, was observed during SWS. Based on these results, the sleeping brain continuously monitors and selectively processes incoming flow. Our results also strongly support a two-stage information processing taking place in the cortex during sleep.