Brain Damage Limits Working Memory

New research carried out at the University of California, San Diego School of Medicine, and led by Larry R. Squire, PhD, professor of psychiatry, psychology and neurosciences at UC San Diego and a scientist at the VA San Diego Healthcare System, proved that working memory of objects' location is not affected if important brain structures like the hippocampus are damaged.

The scientists looked at four memory-impaired patients that suffered damage to their medial temporal lobes – MTL, a part of the cerebral cortex that contains the hippocampus and is linked to long-term memory.

The patients were asked to look at an arrangement of objects on a table and try to reproduce it on another table.

If the arrangement had no more than three objects, then the patients had no trouble reproducing their relative position, their ability to remember being the same as that of control subjects without brain damage.

Also, if the objects had been placed in relation with one another, the patients reproduced the arrangement much faster.

However, if on the table there were four or more objects, they could no longer remember their location, because they no longer had long-term memory functions in the MTL.

Their limit of working memory was reached and “their performance abruptly collapsed,” Squire said.

“The findings provide strong evidence for a fundamental distinction in the brain between working memory and long-term memory, even in the realm of spatial information and spatial-object associations,” he added.

This goes against what was previously believed, that spacial information needs the hippocampus and other memory-associated regions.

The work “indicates that patients with memory impairment due to MTL damage, including early stage Alzheimer's disease, have a narrower difficulty than what one might have thought.
“They have an intact ability to hold information in mind, and an ability to work with it on a temporary basis.”

The research was published in the October 13, 2010 issue of The Journal of Neuroscience.