According to researchers at the University of Arizona, during sleep the brain takes six to seven times less time to play the memory of an occurrence. Professor Bruce McNaughton and his colleague David Euston say that a memory of having lunch, for example, would involve a number of elements like the name of the restaurant, items served, surrounding noise level, payment of bill etc.
While an actual dining experience might have taken up an hour of actual time, replaying the memory of it would only take 8 to 10 minutes, say the researchers.
Memory stores patterns of activity in modular form in the brain's cortex. Different modules process different kinds of information like sounds, sights, tastes, smells etc.
The networks of activity of are then transferred by the cortex to a brain region called the hippocampus, which creates tags (temporary bar codes) for every memory and sends that signal back to the cortex.
Each module in the cortex uses these bar codes to retrieve its own part of the activity. McNaughton said that the speed of the consolidation process is not constrained by the real world physical laws that regulate activity in time and space, which is why processing the memory of an event takes such a less time during sleep.
The retrieval bar codes generated by the hippocampus are only temporary until the cortex can carry a given memory on its own. "It's a slow process," said McNaughton.
"The initial creation of the tag is made through existing connections. In order to do the rewiring necessary to have the intermodular connections carry the burden takes time. What you have to do is reinstate those memories multiple times. Every time you reinstate the memory, the modules make a little shift in the connection . . . something grows this way, grows that way, a connection gets made here, gets broken there. And eventually, after you do this multiple times, then an optimal set of connections gets constructed," he added.
The researchers say that the brain generally does all of this during sleep when it is not busy with processing real-time inputs. McNaughton has developed the technology to record from multiple probes, each of which can track the activity of a dozen or more brain cells. "We need groups of cells because in order to identify a pattern, you have to look at the collected activity of many neurons," McNaughton said.
The researchers now plan to find an experiment that will enable them to demonstrate that changes in the memory reactivation process would affect memory consolidation but not damage the brain in the process. "The more practical point, I think, is that this methodology, the ability to measure how fast the brain is processing at the level of changing the state of the brain from one 10- millisecond epoch to the next, how fast the internal state is sweeping through its memories or its allowable patterns is, I think, a model for thought speed," McNaughton said.
Knowing the determinants for the speed of thought, he said, might allow studies of the effects of drugs, developmental anomalies and the behavioural therapies that might improve them.