Hippocampal memory consolidation during sleep: a comparison of mammals and birds

Abstract

The transition from wakefulness to sleep is marked by pronounced changes in brain activity. The brain rhythms that characterize the two main types of mammalian sleep, slow-wave sleep (SWS) and rapid eye movement (REM) sleep, are thought to be involved in the functions of sleep. In particular, recent theories suggest that the synchronous slow-oscillation of neocortical neuronal membrane potentials, the defining feature of SWS, is involved in processing information acquired during wakefulness. According to the Standard Model of memory consolidation, during wakefulness the hippocampus receives input from neocortical regions involved in the initial encoding of an experience and binds this information into a coherent memory trace that is then transferred to the neocortex during SWS where it is stored and integrated within preexisting memory traces. Evidence suggests that this process selectively involves direct connections from the hippocampus to the prefrontal cortex (PFC), a multimodal, high-order association region implicated in coordinating the storage and recall of remote memories in the neocortex. The slowoscillation is thought to orchestrate the transfer of information from the hippocampus by temporally coupling hippocampal sharp-wave/ripples (SWRs) and thalamocortical spindles. SWRs are synchronous bursts of hippocampal activity, during which waking neuronal firing patterns are reactivated in the hippocampus and neocortex in a coordinated manner. Thalamocortical spindles are brief 7-14 Hz oscillations that may facilitate the encoding of information reactivated during SWRs. By temporally coupling the readout of information from the hippocampus with conditions conducive to encoding in the neocortex, the slow-oscillation is thought to mediate the transfer of information from the hippocampus to the neocortex. Although several lines of evidence are consistent with this function for mammalian SWS, it is unclear whether SWS serves a similar function in birds, the only taxonomic group other than mammals to exhibit SWS and REM sleep. Based on our review of research on avian sleep, neuroanatomy, and memory, although involved in some forms of memory consolidation, avian sleep does not appear to be involved in transferring hippocampal memories to other brain regions. Despite exhibiting the slow-oscillation, SWRs and spindles have not been found in birds. Moreover, although birds independently evolved a brain region -the caudolateral nidopallium (NCL) -involved in performing high-order cognitive functions similar to those performed by the PFC, direct connections between the NCL and hippocampus have not been found in birds, and evidence for the transfer of information from the hippocampus to the NCL or other extra-hippocampal regions is lacking. Although based on the absence of evidence for various traits, collectively, these findings suggest that unlike mammalian SWS, avian SWS may not be involved in transferring memories from the hippocampus. Furthermore, it suggests that the slow-oscillation, the defining featur...

Sleeping on the wing

“…REM sleep is characterized by EEG activity remarkably similar to that occurring during wakefulness [79]. However, in contrast with wakefulness, the bird has its eyes closed and muscle tone reaches its lowest level, often causing the head to drop [84].…”

“…As in mammals, avian sleep consists of two distinct types of sleep, slow wave sleep (SWS, or non-REM sleep) and rapid eye movement (REM) sleep [79]. The two states are distinguished from one another and wakefulness in part by changes in brain activity measured with the EEG.…”

Complex Motor Sequence Skills Profit from Sleep

“…An overwhelming body of literature points to beneficial effects of sleep on memory consolidation [1,2,3]. This seems to be true for simple motor tasks (e.g.…”

Birds introduced in new areas show rest disorders

“…To maintain high, efficient cognitive performances, an organism has to rest and sleep [10]. The duration and the quality of the rest constrain the quality of cognitive operations [10,11], including predator avoidance [12].…”

“…To maintain high, efficient cognitive performances, an organism has to rest and sleep [10]. The duration and the quality of the rest constrain the quality of cognitive operations [10,11], including predator avoidance [12]. But the introduction process seems poorly compatible with a good rest and sleep: individuals arrive in a new environment, devoid of informed conspecifics that otherwise could help to find a safe resting place.…”