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Older memories 'stored differently'

3rd February 2009

A new study shows that, the older a memory is, the less activity seen in the central parts of the brain when it is recalled.

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The act of generating memories must therefore be tied to some role in the brain's interior, though the same structures are not involved in forming new ones.

The finding seems to explain the reason why people with Alzheimer's disease can recall events from long ago, though they forget what is happening around them.

The research focused especially on a structure in the central part of the brain known as the hippocampus.

It is a well known scientific fact that any kind of hippocampal injury takes away the ability to form new memories.

Areas of the brain close to the hippocampus are often the first to show the effects of Alzheimer's disease.

Larry Squire of the University of California said that this is why Alzheimer's typically begins with memory problems.

However, scientists still do not fully understand the brain's protection system for older memories.

Squire researched the brains of 15 healthy people in their 50s and 60s.

They were monitored with a scanner while they responded to questions about news events from the past 30 years.

Squire said that researchers found the hippocampus was most active when subjects were recalling memories about new events that occurred just a year or two earlier.

He said that the hippocampus became less active as subjects recalled memories that were five years or 10 years old.

Activation of the cerebral cortex on the brain's surface increased as people tried to remember older events.

This shows the hippocampus is just as necessary for forming new memories as it is for retrieving recent ones, though it plays little or no role in older memories.

Russell Poldrack said that the evidence gathered in the study was the clearest demonstration yet of what happens as memories get older.

He said that it is for this reason that Alzheimer's patients are more likely to forget where they put their keys yesterday than to forget something that happened in their distant past.

In a related study by Fred Gage, it was found that memories stored contemporaneously receive a 'time stamp' that sets them apart from other memories, after they pass through the hippocampus.

 

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