Transcranial slow oscillatory stimulation drives consolidation of declarative memory by synchronization of the neocortex

Slow-wave sleep plays a crucial role in the consolidation of declarative memory. During this sleep stage the electroencephalogram predominantly shows slow oscillatory activity and slow sleep spindles. The latter are likely to be linked to hippocampal fast ripples and subsequent neuronal firing, presumably supporting the transformation of memory. Anodal transcranial direct-current stimulation generates a temporary negativity of the extracellular space in the underlying tissue, secondarily causing a shift of the neuronal membrane potential towards depolarization. A novel type of transcranial 'intermittent' slow oscillatory stimulation was applied over the frontal cortex during early stage-two sleep with the purpose of inducing endogenous slow oscillations and slow spindle activity. It is argued that this form of stimulation elicited a specific enhancement of the hippocampus-dependent declarative memory, while the nondeclarative memory remained unaffected. It is proposed that transcranial slow oscillatory stimulation, capable of modulating a wide thalamo-cortico-hippocampal network, could contribute to the treatment of memory and sleep disorders.