Author: Kamran Diba

Today, I want to introduce a relatively short, but interesting and suggestive paper.

Fast-Forward Playback of Recent Memory Sequences in Prefrontal Cortex During Sleep
Euston DR, Tatsuno M, McNaughton BL.

This paper shows that mPFC neuronal activity was replayed during sleep following task period, and this replay was task dependent. In addition, it is temporally compressed. The authors confirmed these results using two different methods; cross-correlation and template matching. Both analysis shows the pattern is compressed by a factor about 6. It is convincing that similar values are acquired with two distinct ways.
But they don’t mention to correlation between the replay and sleep stages (slow-wave sleep / REM sleep). I wonder whether frequency of mPFC replay were depend on sleep stage. Regarding to this, I’m also interested in whether it is correlated with EEG patterns, especially sharp-waves.
The authors suggest that processing speed of brain is faster than physical limitation. This is unique and interesting view points. But what does the replay do? The replay seems in hundred millisecond order, so it may be too slow to induce synaptic plasticity.

Today, I pick up the following paper (because of my personal reflection).

Preplay of future place cell sequences by hippocampal cellular assemblies.
Dragoi G, Tonegawa S.

Temporal sequences of place cell firing are observed during awake quiescence and slow-wave sleep following exploring behaviors. This is called “replay”. This paper shows that such temporal sequences are also observed before exploring (figure 1). They call this “preplay”. Preplay is also observed in animals which haven’t explored linear tracks (figure 4). This phenomenon may reflect network dynamics.

This paper may indicate that many (but less than number of all combination of firing order) activity patterns exist as a candidate sequence for information coding. If a candidate pattern matches the sequence during exploration, it will be strengthen during replay. In this view, preplay is a preparation for future learning, and learning is selecting suitable pattern from prepared patterns. Preparing many candidates may require much energy, but it may enable animals to learn quickly, so such mechanism may have advantage in evolutions.

This is my first post to this blog. I’ll post brief reviews about papers which interest me. The first paper is

Awake replay of remote experiences in the hippocampus.
Karlsson MP, Frank LM. 2009, Nat Neurosci.

Replay events of place cells firing sequences are separated in two categories; one occurs during experience, and the other occurs following experience. An awake replay was thought to be depend on sensory inputs, thus it didn’t contain a later type of a replay event. However, this paper shows that both types of replay occur during waking state (figure 2 and 4). In addition, the paper shows firing rate during SWRs are lower in awake state than in quiescent state (figure 3) but replay pattern is more robust in awake state than in quiescent state (figure 3 and 4). I think the paper indicate awake replay has a role for memory consolidation, and the role might be different from that of replay during sleep.

First, I thought the difference of robustness of replay sequence might correlate with that dream is obscurer and more fragmentized than recalling during waking. But now I don’t think it is such simple because human dreams during REM sleep during which sharp-wave ripple isn’t observed. There are many discussions about correlation between memory consolidation and dream. They may be complicated, but interesting.