Why a good night's rest makes all the difference to memory...
Although many coffee-guzzling late night-studying students will claim otherwise, one of the classic pieces of academic advice is ‘sleep will improve your memory’. Whilst some claims, such as remembering information by listening to tapes while asleep, may be tenuous, there’s clear evidence for the role of sleep in memory consolidation.
Memory consolidation refers to the process by which new memories are transformed and reinforced to become more stable and permanent. It occurs soon after new information is laid down, but continues to affect memories for weeks or even months afterwards. Typically, the process involves the systematic reactivations of memories - in the order they were created - which strengthens the synaptic connections amongst the appropriate neurones. For example, a memory that involves motor control will see reactivation in the motor region during consolidation. However, it is both time-consuming and resource-draining, so sleep is thought to be an ideal time to reinforce memories, as more energy is available while the body rests.
The benefits of sleep on memory retention are well documented in cognitive experiments. In a classic example, Wagner et al. (2004) reported that participants gained insight into a challenging maths problem after sleeping, whilst they also improved at a task that involved texture discrimination in another study (Stickgold, 2005). Another piece of evidence suggesting sleep increases consolidation comes from Gais et al. (2006), who found that a period of sleep immediately after learning a list of English and German words led to better retention compared to staying awake and then sleeping. This indicates not only the direct benefits of sleep, but also suggest exactly when important consolidation takes place - soon after the encoding of the memory.
The key to consolidation is the formation of schemas, a collection of information about a specific event or object. As previously mentioned, consolidation involves the reactivation of specific memories in a pattern that strengthens the synaptic connections between the corresponding neurones. Repeated reactivation is often called long term potentiation (LTP). Similar or familiar items of information tend to be more strongly reinforced, and eventually are connected enough to form a schema for the memory. Less salient or familiar information is often neglected during reactivation, and may even be lost from the memory.
Reactivation during sleep consolidation also works to combine facets of memory that come from different perception modules. For example, visual information, audio information and pre-existing knowledge may be reactivated in tandem to form a single memory of the entire event or object.
Recent research has begun to reveal the possible biochemical mechanisms underpinning memory consolidation during sleep. Two key stages of sleep involved in the process are "slow wave sleep" (SWS) and "rapid eye movement" (REM) sleep, which occur during deep slumber later in the night. SWS has typically been shown to play a role in the retention of declarative (conscious recollections) memories, whilst REM is more closely associated with procedural (skills based) memories. Both stages feature distinct patterns of brain wave activity, which can be measured using techniques such as electroencephalography (EEG), where electrodes on the scalp measure electrical currents created by neurons firing.
The most prominent electrical activity during SWS are slow-wave oscillations, which originate in the neocortex of the brain. These waves work to synchronise neuronal activity and induce top-down control in rhythmic cycles. As well as slow-wave oscillations, "spindle waves" from the thalamus and "sharp wave" ripples from the hippocampus coordinate their activity to provide temporal rhythm during consolidation. Slow-wave oscillation also seems to induce widespread hyperpolarisation, which suppresses neural activity and favours the storage of new information via long term potentiation. During SWS, this specific electrical activity is thought to elicit the reactivation of recent declarative (hippocampus based) memories and control their transfer to the neocortex, where they remain as permanent memories. In support of this hypothesis, the suppression of hippocampal sharp wave ripples has been found to impair spatial memory retention (Girardeau et al, 2009).
REM sleep, by comparison, involves a different mechanism. It features ponto-geniculo-occipital (PGO) waves from the brainstem and visual cortex, and theta oscillations from the hippocampus (though the latter have been observed in rats more than humans). This activity is accompanied by high levels of the neurotransmitter acetylcholine, which appears to promote further synaptic consolidation without interference from external stimuli. Synapses also seem to show greater plasticity during REM sleep, which promotes long term potentiation.
So, judging by the significant role sleep plays in memory consolidation and reinforcement, it seems that a good eight hours will aid most people with remembering information at a later date. Whilst the exact mechanism isn’t fully clear yet, it is an exciting area of research that could provide real benefits for those who struggle to retain new memories… and shows that pulling an all nighter may not be the best strategy for learning!
Scientific American July 2014 issue page 50 will help to explain the new neurones that grow daily and the effect they have. Allan marsh allan marsh, Sun, 6th Jul 2014