A small, inner-brain region called the hippocampus boasts a well-earned reputation as a memory hub. However, researchers disagree about whether the hippocampus specializes in remembering only experiences or instead coordinates recall of both experiences and factual information.
Support for the structure's double-barreled role comes from a group of six adults who suffered rare brain damage limited largely to the hippocampus. The analysis appears in a pair of reports in the April 10 Neuron.
"It looks like the human hippocampus is normally needed for semantic [factual] memories as well as for episodic [event] memories," says Larry R. Squire of the University of California, San Diego in La Jolla, who directed the investigations.
Passage of time loosens the injured hippocampus' cloaking of both forms of memory, Squire adds. All six brain-damaged patients remembered facts and events from more than a decade before their injuries occurred. They largely lacked recollections for material encountered in the 10 years before hippocampus damage and in its aftermath.
The patients, ages 36 to 64, had developed brain damage and memory loss after age 30 as a result of medical conditions such as viral encephalitis.
The first study explored factual memory. Five of the patients and 12 adults with uninjured brains answered multiple-choice questions about notable news events that occurred between 1950 and 2002.
Then, all six patients and 14 adults with healthy brains heard a list of famous and fictitious names. Famous names referred to people who became well known before 1970. Half remain alive today, and half had died between 1990 and 2001.
Participants decided whether each name referred to a famous person and, if so, whether that person was still alive.
Patients remembered little about news events that happened after they suffered brain damage or in the 10 years before, Squire's team found. However, patients and healthy adults alike recalled much of the news from the distant past and identified most of the famous people they had known about for decades. Only the patients, though, couldn't remember which famous people had died since 1990.
In a second study, the six patients with hippocampus damage and two others with broader injuries to the brain region that includes the hippocampus reported detailed autobiographical memories from childhood and adolescence. Their reports contained as much detail, including factual information, as those of 25 healthy adults. Previous studies had documented amnesia in these patients for personal events that occurred in the years shortly before and after their injuries.
In a commentary published with the new studies, Wendy A. Suzuki of New York University says the findings contrast with an earlier report that three children with hippocampus damage retained enough new factual knowledge to perform adequately in school.
The brain may undergo dramatic reorganization to shore up factual memories after hippocampus damage in childhood, Suzuki proposes. It's also possible that even without marked brain changes, memories of day-to-day experiences in the classroom enable such children to pass their tests, Squire says.
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Larry R. Squire
Veterans Affairs Medical Center
(V-116) 3350 La Jolla Village Drive
San Diego, CA 92161
Wendy A. Suzuki
Center for Neural Science
New York University
New York, NY 10003