S116 The heterogeneity of sleep slow waves

Abstract

NREM sleep slow waves reflect both the temporal dynamics of the synchronized onset and cessation of cortical neuron firing and the spatial extent of this synchronized firing pattern. This synchronization is crucially dependent on synaptic strength, and the renormalization of synaptic changes occurring in wakefulness is thought to be one of the main functions of NREM sleep. In line with this hypothesis, slow waves are more frequent, larger and steeper in cases when the synaptic load is high and the need for homeostatic synaptic renormalization is strong: early in the night, in younger subjects with greater neural plasticity and after prolonged or more intensive wakefulness. Slow waves exhibit substantial heterogeneity and may contribute to these processes to a differing degree. In NREM sleep, series of large-amplitude slow waves (Cyclic Alternating Pattern [CAP] A1) sometimes appear, either spontaneously with predictably cyclicity or after sensory stimulation. Slow waves in CAP A1 resemble slow waves normally recorded under high homeostatic sleep pressure: they are characterized by a greater spatial extent, larger amplitude, steeper slopes (even when correcting for amplitude) and higher EEG synchronization, even though their topography and propagation patterns are similar to ordinary (non-CAP) slow waves. These results indicate that CAP A1 represents a period of highly synchronous neuronal firing over large areas of the cortical mantle. This feature may contribute to the role CAP A1 plays in both normal synaptic homeostasis and – through greater synchronization – in the generation of epileptiform phenomena in epileptic patients.