Separate Human Memory Systems Found

Peering into the dark well of human memory, researchers have produced compelling new evidence for two independent systems of memory long theorized by neuroscientists that go to the heart of how the brain learns and remembers.

In a series of remarkable experiments with patients suffering from different kinds of brain damage, the researchers showed that some could not learn a new task but could easily recall being tutored in it, while others could learn the new task even though they were never able to recall the many sessions in which they tried it.

The experiments, conducted by researchers at UCLA and UC San Diego, highlight the difference between conscious memory and nonconscious learning abilities.

Published in today’s issue of the journal Science, the reports are the latest in a series of recent studies that shed new light into the intersection of the conscious mind, cognition, and the physical structure of the human brain.

“There are a lot of important clinical implications to this work,” said Richard Thompson, an authority on the biology of memory and learning and director of the USC neuroscience program.

“The more we know about where these memories are stored and about the neurochemistry of how they are stored and retrieved, the more likely we are to develop treatments to improve or repair brain damage,” he said.

In recent studies, other researchers have revealed the interplay between emotion and recollection, between learned habit and conscious thought, and between memories made for the moment and those that become a permanent part of the brain’s chemical structure. Researchers have even determined that the brain appears to handle false memories differently than those arising from actual events.

In all, scientists suspect that there may be four separate memory systems in the brain, rather than one, as long believed. Each system, they suggest, handles a different kind of information and experience.

Focusing on two of those proposed systems, the newest study compared the learning abilities of people with amnesia and those with Parkinson’s disease. The aim was to show that the anatomy of memory is so complex that the brain processes conscious memories of events in one region and the memories needed to retain learning in another.

Amnesia, stemming from damage to a part of the brain called the hippocampus, impairs the ability to acquire and consciously recall information about past facts and events but spares the brain’s capacity to learn new skills and habits.

Conversely, Parkinson’s disease, which affects the basal ganglia, has the opposite effect. The learning ability may be gone but conscious memory is left intact.

Consequently, the kinds of experiences one set of patients in the study could remember, the other set of patients could not, and vice versa.

By contrasting the performance of patients with different kinds of memory impairments, the researchers demonstrated that varieties of memory are so distinct that the human brain processes different types of visual information in separate areas, depending on the type of learning required. It confirms in human subjects an idea developed through extensive animal experiments.

“This is the strongest kind of evidence that one could get,” said UC San Diego memory expert Larry Squire, who performed the study with UCLA psychologist Barbara J. Knowlton and University of Toronto psychologist Jennifer Mangels.

Squire and his colleagues conducted the study with 12 patients with amnesia, 20 people with Parkinson’s disease and 15 control subjects who had no brain damage.

To test the effects of brain damage on memory, they devised an unusual game of weather forecasting. The subjects were asked to predict rain or sunshine based on cards flashed on a computer screen, which represented different probabilities of weather conditions. It was not a game anyone could easily memorize, but through trial and error, people could develop an intuitive “feel” for the right answer.

Those with amnesia learned to play the game fairly well, although they were not able to recall learning or playing it. Those with Parkinson’s could not learn the game even after more than 100 trials, but they had no trouble remembering their training sessions.

The different responses show that “these [areas of the brain] are very different machines in how they process information,” said Michael M. Merzenich, a brain expert at UC San Francisco.

(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)

The Human Memory Systems

Researchers at UCLA and UC San Diego have found new evidence of the existence of two independent systems of memory in the human brain.

In all, scientists suspect there may be four separate memory systems in the brain, rather than one as long believed, each handling a different kind of information and experience.

Memories of learned skills involve the structures of the basal ganglia.

Emotional memories appear to be handled through the amygdala.

Conscious memory of facts and events centers on the hippocampus.

Associative learning, like Pavlovian conditioning, is controlled through the cerebellum.