Brain scans show two-sided memory flow.

Brain-imaging studies suggest that a cellular information highway speeds through disparate parts of the brain to coordinate memories for personally experienced events, also known as episodic memories. Researchers now offer the first data suggesting that this memory thoroughfare turns left at the front of the brain to store recollections and veers right to retrieve them.

Positron emission tomography (PET) images show that blood flow jumps sharply

on the left side of the prefrontal cortex - which lies just behind the forehead - as novel information enters episodic memory, argue Endel Tulving, a psychologist at the University of Toronto, and his colleagues. In contrast, Tulving's group finds that blood surges in the right prefrontal cortex as episodic memories resurface.

"Prefrontal asymmetry in episodic memory has not been previously reported, or even suspected," Tulving says.

Tulving and his associates cannot yet identify precise prefrontal regions involved in different facets of episodic memory. The new data and the few related PET studies of human memory - most of which concentrate on learning, or "encoding," of new information - support a general split between left-side storage and right-side retrieval, the scientists hold.

The team describes its PET experiments in the March 15 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

In one study, 12 right-handed men age 21 to 35 studied two lists of common nouns presented on a computer screen as they lay under a PET scanner. For one list, participants indicated whether words contained the letter "a"; for the other, they noted whether words referred to animate or inanimate objects.

The men showed stronger memory 20 minutes later for words analyzed by meaning, compared to words perused on a letter-by-letter basis. This effect probably reflects the greater amount of brain work required by the former task, according to Thlving and his coworkers.

Moreover, PET scans displayed substantial blood-flow rises in the left prefrontal cortex during performance of the noun-meaning task.

Left prefrontal activation encourages storage of new verbal material interpreted in light of prior general knowledge, the investigators contend.

In a second study, 12 right-handed men age 19 to 30 listened to a taperecorded list of novel word definitions, such as "a form of recreation for the jumpy - trampoline." The next day, while lying under the PET scanner, they heard previously studied and new word definitions. Novel information triggers more mental work than familiar material, the researchers note.

Mathematical subtraction of PET images obtained during presentations of new definitions from those acquired during study of familiar definitions yielded areas of greatly increased blood flow in the right prefrontal cortex.

This part of the brain apparently helps to reclaim information from episodic memory, the researchers maintain.

Blood-flow data revealed several other brain regions that help to store and retrieve episodic memories, they add.

Prefrontal asymmetry on the tasks used by Tulving's group may reflect general brain processes, such as response generation, rather than memory, contends Larry R. Squire, a memory investigator at the Veterans Affairs Medical Center in San Diego.

"We need to see if prefrontal asymmetry generalizes to other encoding and retrieval tasks for episodic memory," adds Steven E. Petersen, a neuroscientist at Washington University School of Medicine in St. Louis.