This project explores the neural mechanisms underlying the encoding of information in the brain and its stabilisation during sleep. It draws widely from a number of fields within the neurosciences, including brain imaging and in vivo electrophysiological approaches, in an attempt to bind these disparate areas of study together.
Background
Information that has been encoded in the brain during waking experience is thought to undergo a process of consolidation within the cortex during sleep, the repeated reactivation of memories during this process being thought to increase the strength of connectivity between the cortical modules in which they are encoded. This serves to both stabilise encoded memories as well as interleave them with older, previously consolidated experiences. This project focuses in particular on the interaction between the hippocampus, a structure thought to play a crucial role in the encoding of waking experience, and the cortex.
Cyclic changes in the depth of sleep are thought to contribute differently to this process of memory consolidation. Rapid eye movement (REM) sleep, for example, appears to contribute strongly to the consolidation of emotional memories or sequences of experience bound in time (including procedure-related memories). In contrast, slow wave sleep (SWS) may favour the consolidation of purely episodic or factual memories. Indeed, the different oscillatory brain states found during these periods of sleep (observed as cyclic voltage changes in regions of the brain) appear to process information in very contrasting ways. The theta oscillation (around 4-12 Hz) observed during REM sleep appears to act as timing signal against which sequence information can be encoded and preserved. The slow 1 Hz oscillatory activity observed during SWS, on the other hand, may serve to drive cyclic periods of high frequency activity (sharp wave associated ripple, SWR, events at 180-250 Hz) between the hippocampus and the cortex to drive the consolidation process. Given that these brain states vary with both waking behaviour as well as the depth of sleep, they appear to allow the coordination of initial encoding and then reactivation of specific memories/memory types during sleep.
Across the scope of oscillatory frequencies that appear to be involved, each can be considered to represent a different internal ‘time-metric’ against which information is encoded and replayed. Thus, the brain may [re]process ‘time’ in a way that strongly differs from how we experience it in real life.