New discovery helps amnesiacs keep their memories

All people can lose their short-term memories within one minute of the initial thought. However, most people form long-term memories of the event during that time, thus preserving the memories longer. Amnesiacs do not form these memories — at least, not consciously accessible memories. While such things as head injuries and strokes are known to cause amnesia, very little is known about why such severe memory loss occurs. A University of Missouri-Columbia researcher and his collaborators have discovered that some amnesiacs can maintain their memories for a longer period of time if they spend time sitting in a dark, quiet room.

"This discovery might provide us with a better understanding of what causes amnesia," said Nelson Cowan, professor of Psychological Sciences, who conducted the study along with Nicoletta Beschin of the Department of Rehabilitation Unit in Somma Lombardo, Italy, and Sergio Della Sala of the University of Aberdeen, United Kingdom. "It also will help us better determine different types of amnesia as well as better therapeutic applications."

Cowan conducted two experiments with people who have amnesia caused by either a head injury or stroke. In the first experiment, participants reviewed six lists of 15 unrelated words. After the participants read the words they were tested in six different conditions, one of which was to lie quietly in a darkened room with no outside interference, trying not to fall asleep, for 10 minutes.

After 10 minutes, the participants were asked to recall any of the words. Cowan found that although severe memory loss occurred following activity-filled conditions, 60 percent of the patients experienced much less memory loss after sitting in the darkened room.

In the second experiment, Cowan and his team extended the retention time to one hour. Each participant listened to stories and were then tested using four of the six conditions used in the first experiment. At the end of the hour they were asked to tell the researchers as much about the story as they could. Cowan found that when the participants were left alone in the darkened room, they were able to remember more than seven times the amount of information as those in the activity-filled conditions. This was true even when the participants appeared to sleep during the hour. The fact that they remembered considerable information after an hour rules out the possibility that these patients rehearsed the information continually during the hour.

"One intriguing aspect of the study was that there was similar preservation of the participant’s memory in a quiet, dark room both when they were awake and when they were asleep," Cowan said. "The data we present sheds light on why short-term memories fade away in all of us. Do they fade inevitably over time, or do they fade only if displaced by newer events in the environment?"

Because four of six patients retained memories of parts of a story for an hour in a quiet, dark room, this suggests that memories are preserved in short-term memory until they are displaced. The finding that the two amnesiacs with apparent temporal lobe damage did NOT benefit from the quiet, dark room suggests that temporal lobe structures in the brain hold on to information at least until it is displaced by other information in short-term memory.

The differences in the results suggest that the method may provide a way to distinguish between different types of amnesiacs. Only some of them hold on to information if interference from subsequent events is minimized.

Cowan’s study [ abstract ] was published in the April 2004 issue of the neurological journal Brain.

Additional links:

Nelson Cowan
Department of Psychological Sciences

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