High density EEG studies of local sleep changes following synaptic plasticity in humans

Recent experiments showed that sleep SWA can be regulated locally in the cerebral cortex, pointing to a link between SWA regulation and synaptic plasticity, and, more specifically, that local SWA may increase after manipulations that favor local synaptic potentiation and decrease after those that promote local synaptic depression. To further investigate the connection between cortical plasticity and sleep SWA, in the first study presented on this dissertation we employed hd-EEG recordings together with a paired associative stimulation TMS protocol. As expected, such a protocol lead to a sustained increase (LTP-like) or decrease (LTD-like) of cortical excitability as measured by both motor evoked potentials and TMS-evoked cortical responses over sensorimotor cortex. During subsequent sleep, SWA increased locally in subjects whose TMS-evoked cortical responses had increased after PAS, and decreased in subjects whose cortical responses had decreased. Changes in TMS-evoked cortical EEG response and change in sleep SWA were localized to similar cortical regions and were positively correlated. In the second study, a whole night hd-EEG recording was adopted in order to assess the effects of an implicit visuomotor learning task -performed 12 hours before sleep time- over sleep SWA. The predicted increase in SWA was found over a cluster of five electrodes projecting over the right parietal cortex -a region whose circuits are specifically involved during task execution- thus confirming a close relationship between learning processes, synaptic plasticity and local sleep regulation. Together, these results suggest that changes in cortical excitability lead to corresponding changes in local sleep regulation, as reflected by SWA, thus providing evidence for a tight relationship between cortical plasticity and sleep intensity and suggesting a role for sleep in regulating cortical connection strength at synapses as proposed by the Synaptic Homeostasis Hypothesis (Tononi and Cirelli, 2003).