Neural mechanisms for the enhancing effects of sleep on memory consolidation.

Sleep subsequent to learning can facilitate memory consolidation. To elucidate neural mechanisms for this facilitating effects of sleep on memory consolidation, we recorded electroencephalogram from the hippocampus, a pivotal brain area for memory formation and storage, of monkeys while they were asleep. As a results we found that sharp-waves and associated ripples (SPW-ripple events) increased in a specific region of the hippocampus during non-REM sleep; a characteristic amplitude profile of SPW-ripple events appeared along the layer structure of this area. Because electroencephalography represents summation of electrical activities from ensemble of many neurons, it is difficult to clarify the contribution of each neural element, i.e., each neuron, to the mechanism of memory consolidation. Therefore, we also try to record single neuron activities both during awake and sleep conditions to determine functional roles of these neurons in memory consolidation.