Rhythms of the Night: New Insights into the Neurophysiology of Avian Sleep

Abstract

The current memory consolidation models suggest that the various brain rhythms (i.e. neocortical slow-waves, hippocampal sharp-wave ripples (SWRs) and thalamocortical spindles) occurring throughout the mammalian brain during sleep form an oscillatory network thought to be involved in the processing of information acquired during wakefulness. While sleep’s role in memory consolidation is widely accepted, the exact role of the different sleep states and accompanying neurophysiological activity is still actively debated. Most theories of sleep-related memory consolidation are solely based on studies in a few mammalian species. In contrast, virtually nothing is known about how the avian brain operates as a system during sleep and how this compares to what is known in mammals. In addition, though avian sleep has been implicated in various forms of memory consolidation, e.g. imprinting, song learning and in auditory memories, the existence and role of rhythms like thalamocortical spindles and hippocampal SWRs has not been studied in depth so far. In this thesis I investigated the neurophysiology of avian rapid eye-movement (REM) and non-REM (NREM) sleep, using intra-cortical recordings made with high-density electrode probes in anesthetized and naturally sleeping birds. I aimed to characterize the thalamocortical and hippocampal network activity underlying NREM and REM sleep in birds in order to deepen our understanding of the mechanisms and functions of avian sleep-related brain activity. I show that, as in mammals, traveling slow-waves are found in most of the avian pallium, both under isoflurane anesthesia and natural NREM sleep. Though, unlike mammals, slow-waves appear not to be present in the avian hippocampus. Moreover, the apparent absence of two of the rhythms (i.e. thalamocortical spindles and hippocampal SWRs), implicated in hippocampal memory transfer in mammals, questions whether hippocampal memory consolidation takes place during avian sleep. This result would fit with the fact that, even though the avian hippocampus is involved in storing certain types of information (e.g. spatial memories), there is so far no evidence for hippocampal memories being transferred to other brain regions in birds. I furthermore compared the neurophysiology of NREM sleep and isoflurane induced anesthesia using a within-bird design, in order to compare the underlying intra-cortical dynamics during these to recording conditions. I found that, though there are some spectral differences between these states, the similarities predominated, namely that travelling slow-waves are present in both NREM sleep and in the anesthetized brain regions examined. Taken together, the results from research in birds, which exhibit sleep states that are in most respects similar to those found in mammals (despite being distantly related), suggest that the way some types of memories are consolidated during sleep might be fundamentally different in taxa other than mammals.