Distinction between slow waves and delta waves sheds light to sleep homeostasis and their association to hippocampal sharp waves ripples

Cortical slow waves and delta waves are hallmarks of NREM sleep and key elements in physiological processes such as memory consolidation and sleep homeostasis. However, no clear distinction has been made between the two types of electrophysiological events. We introduce a new distinction between slow waves, large amplitude waves on filtered LFP cortical signals, and delta waves, characterized by an inversion between deep and superficial layers and the co-occurrence with a cortical down state. Detection of slow waves, as large amplitude waves during NREM sleep, reveals that a large portion of them does not share the properties of delta waves and does not co-occur with cortical down states. Therefore, we distinguish type-1 slow waves, that are associated to a down state, from type-2 slow waves. We show that 1) only type-1 slow waves undergo strong homeostatic regulations and 2) type-2 slow waves create confusion about the temporal link with hippocampal sharp-wave ripples. Thus, we confirm that delta waves detected with our new method can be used as a proxy for down state. Altogether, this confirms the importance of a robust and accurate definition of delta waves to understand the fine neurophysiological mechanisms at stake during NREM sleep.