Memory research supports mind displacement theory

Work with amnesiacs shows memory loss may be due to “interference.”
Tuesday, July 20, 2004 | 12:00 a.m. CDT; updated 12:50 p.m. CDT, Wednesday, July 16, 2008

The key to unlocking the mystery of human memory may be to ask someone who can’t remember.

Nelson Cowan, an MU psychology professor, and researchers from Italy and the United Kingdom have used amnesia patients to study how information is lost in a person’s short-term memory. Their study tackled two competing theories about memory. The decay theory holds that information is lost after a passage of time. The displacement theory claims that information is lost when other information is presented to replace it.

Cowan performed the study in Italy, using six amnesia patients and six control subjects with normal memories. The subjects were read a list of 15 words, then were either asked to perform cognitive tasks or put into a dark, quiet room for a time before being asked to recall the words. In a second test, subjects were read a short story.

In both experiments, Cowan said the researchers found that four of the six subjects were able to remember a greater proportion of the words or the story if they were put in a dark, quiet room where there was no “interference.”

“The point is that there was nothing to displace the information, so the research favors the displacement theory,” Cowan said.

The study was done over several years, and the results were published in the April issue of the neurological science journal Brain.

Patients with amnesia — or amnesiacs — who cannot remember events from the past or after they happen, were the ideal subjects for researchers studying working memory, Cowan said. Amnesiacs cannot form long-term memory — information that is permanently stored in the brain. They can, however, store information in short-term memory, which Cowan said is believed to be around 30 seconds to a minute after information is presented.

“We studied amnesic individuals because they are not forming any long-term memory, so if they recall new information, we make the assumption that new information was in short-term memory,” he said.

Amnesia is an acquired condition, said Douglass Herrmann, a psychology professor specializing in memory at Indiana State University. It can be caused by physical reasons, such as head injury or stroke, or it can come from psychiatric problems, often classified as dissociative disorders, where someone is reacting to a traumatic event and is separating themselves from that event.

Herrmann said amnesia is “really sort of a broad topic,” in that if a person has a brain injury he is likely to have some degree of amnesia. People with retrograde amnesia cannot remember things from their past; those with anterograde amnesia cannot remember events after they happen.

“The amnesia that is sort of classic is what you see on soap operas, where someone forgets being married to someone,” Hermann said. “That kind of character appears on soap operas, but far more often than in real life. Those are very rare.”

Cowan said the goal of his research, which was conducted with Nicoletta Beschin of the Department of Rehabilitation Unit in Somma Lombardo, Italy, and Sergio Della Sala of the University of Aberdeen in the United Kingdom, was to discover “the limit on how much a person can hold in mind” and what factors limit the amount of information a person can remember.

Cowan said there is little knowledge about how a person recalls information from memory.

“If you tell someone your phone number and they repeat it back 20 seconds later, you don’t know if that information was in short-term memory during those 20 seconds or whether they have memorized it and repeated it from long-term memory,” he said.

Cowan said the research could provide clues to treating amnesia. “The patients were very excited that they could remember something for an hour,” he said. “We are looking into whether we can use this technique to help patients.”

Even those with normal memory should not take it for granted, Cowan said. The control subjects in the word-list test correctly recalled no more than 20 percent of what they had been read when “interference” was presented. When put into a dark, quiet place, they recalled 33 percent of the information.

“A lot of people underestimate how easily they forget, and therefore they do not do enough to prevent it,” he said. “Write it down, rehearse it, connect it up with other information.”

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