Changes of recovery function in median nerve somatosensory evoked potentials from awake to sleep

To investigate how the sleep affects the recovery functions of somatosensory evoked potential, we studied somatosensory evoked potential recovery functions of the median nerve during awake to sleep in eight healthy adults, using the paired conditioning and test stimulus paradigm. The most notable difference between wakefulness and sleep was the much greater enhancement of parietal P26 and frontal P22 in sleep than in awake state at interstimulus intervals of 20 to 100 milliseconds of paired stimulus paradigm. Although the enhanced P26 and P22 were observed in control somatosensory evoked potential during sleep, the augmentation was much greater when conditioned by preceding stimulus. In contrast, the parietal P40-N60 and frontal fP40-fN60 of test response were more depressed in sleep than in wakefulness. As a whole, the recovery curves in sleep appeared to be shifted to the right as compared with those in the awake, suggesting delayed recovery function in sleep. The augmentation of frontal P22 and parietal P26 in sleep, contrasting depressed other components, most notably fP40-fN60 and parietal P40-N60, imply that the former components reflect inhibitory and the latter to the excitatory process. This is the first study that demonstrates the somatosensory evoked potential recovery functions are different from those of awake state.