Long-term Memory - Sleep

Sleep

Some theories consider sleep to be an important factor in establishing well-organized long-term memories. (See also sleep and learning.) It is said that sleep plays a key function in the consolidation of new memories.

According to Tarnow's theory, long-term memories are stored in dream format (reminiscent of the Penfield & Rasmussen’s findings that electrical excitations of cortex give rise to experiences similar to dreams). During waking life an executive function interprets long-term memory consistent with reality checking (Tarnow 2003). Also, that the information stored in memory, no matter how it was learned, can affect performance on a particular task without the subject being aware that this memory is being used. Newly acquired declarative memory traces are believed to be reactivated during NonREM sleep to promote their hippocampo-neocortical transfer for long-term storage.Specifically new declarative memories are better remembered if it follows Stage II non-rapid eye movement sleep. The reactivation of memories during sleep can lead to lasting synaptic changes within certain neural networks. It is the high spindle activity, low oscillation activity, and delta wave activity during NREM sleep that helps to contribute to declarative memory consolidation, In learning before sleep spindles are redistributed to neuronally active upstates within slow oscillations.. Sleep spindles are thought to induce synaptic changes and thereby contribute to memory consolidation during sleep. Here, we examined the role of sleep in the object-place recognition task, a task closely comparable to tasks typically applied for testing human declarative memory: It is a one-trial task, hippocampus-dependent, not stressful and can be repeated within the same animal. Sleep deprivation reduces vigilance or arousal levels, affecting the efficiency of certain cognitive functions such as learning and memory.

The theory that sleep benefits memory retention is not a new idea. It has been around since Ebbinghaus's experiment on forgetting in 1885. More recently studies have been done by Payne and colleagues and Holtz and colleagues. In Payne's experiment participants were randomly selected and split into two groups. Both groups were given semantically related or unrelated word pairs, but one group was given the information at 9am and the other group received theirs at 9pm. Participants were then tested on the word pairs at one of three intervals 30 minutes, 12 hours, or 24 hours later. It was found that participants who had a period of sleep between the learning and testing sessions did better on the memory tests. This information is similar to other results found by previous experiments by Jenkins and Dallenbach (1924). It has also been found that many domains of declarative memory are affected by sleep such as emotional memory, semantic memory, and direct encoding.

Holtz found that not only does sleep affect consolidation of declarative memories, but also procedural memories. In this experiment fifty adolescent participants were taught either word pairs (which represents declarative memory) and a finger taping task(procedural memory at one of two different times of day. The first group was taught at 3pm several hours before sleep and the other group taught directly before sleep at 9pm. Their memories were then tested at 24 hours after learning and 7 days after learning. What they found was that the procedural finger taping task was best encoded and remembered directly before sleep, but the declarative word pairs task was better remembered and encoded if learned at 3 in the afternoon.