Brain region linked to conscious memories.

New findings from brain scans indicate that the hippocampus, a seahorse-shaped structure, located beneath the cerebral cortex, and nearby tissue spring into action when a person remembers a previously studied word. Other areas, at the front of the brain, orchestrate attempts to retrieve the memory of the word prior to its conscious return, according to a report in the Jan. 9 Proceedings of the National Academy of Sciences.

"Our results suggest that hippocampal activation is more closely associated with the actual recollection of a past event than with the effort involved in attempting to remember the event," contend Daniel L. Schacter, a psychologist at Harvard University, and his colleagues.

Implicit memory, the unintentional retrieval of previously studied information without any reported awareness of the material, relies on brain systems that direct the perception of words and objects. These systems lie outside the hippocampus and the related tissue that wraps around it-known collectively as the hippocampal formation-the scientists maintain.

They administered memory tests to 16 physically and mentally healthy adults. Half of them completed conscious, or explicit, recall tests; the rest performed implicit memory tasks.

Explicit testing included both "high recall" and "low recall" tasks. In the former task, participants thought about the meanings associated with each word in a list and then later were asked to remember those words. In the latter task, volunteers tried to remember words they had monitored for the letter t. Only the high recall test resulted in largely accurate word memories.

On the implicit trials, volunteers saw a series of words and paid attention only to the number of times the letter t appeared in them. They then completed strings of three letters with the first word that came to mind, such as saying "garnish" for "gar___." Some letter strings represented the first parts of words from the previously presented list.

As in many studies of implicit memory, participants usually produced an already viewed word when presented with a letter string from that word (SN: 11/17/90, p. 312).

Positron emission tomography (PET) scans of blood flow in the brain were taken for each person during the memory tests.

Activity in the hippocampal formation showed a substantial increase only when the researchers subtracted PET data for volunteers on low recall trials from the data for high recall trials. This procedure isolated brain activity uniquely linked to the conscious recollection of words, the researchers argue.

In contrast, areas at the front of the brain showed activity boosts only during volunteers' low recall efforts to retrieve words they could not remember.

Regions at the back of the brain involved in vision displayed increased blood flow when participants retrieved implicit memories.

Several earlier PET studies have reported heightened activity in the hippocampal formation for implicit as well as explicit memories. But in those instances, individuals may have had unintentional conscious recollections of some words presented in implicit trials, Schacter's team suggests.

The act of consciously retrieving a word from memory may stimulate the hippocampal formation much more than simply recognizing a previously studied word, they add.

However, other studies have found relatively weak responses of the hippocampal formation on memory tests similar to Schacter's high recall, explicit condition, according to Steven E. Petersen, a neuroscientist at Washington University School of Medicine in St. Louis.

"I suspect that the hippocampal formation plays a key role in the reorganization of one's knowledge about the world, but the extent to which consciousness enters into that process remains unclear," Petersen holds. "We're slowly approaching a good explanation of what the hippocampal formation does."