When you remember a friend or your first day of work, you're fully aware of what you're remembering. But memory has another guise: nonconscious skills like riding a bicycle or knowing how to tie your shoes. Results reported in this month's issue of Nature Neuroscience suggest that the hippocampus, a twist of tissue deep in the brain long believed to help form only the first kind of memories, also serves certain memories that don't rise to the level of awareness. The finding bolsters an emerging theory that the role of the hippocampus in memory is to relate different elements of experience.
Like most memory researchers, psychologists Marvin Chun of Vanderbilt University in Nashville, Tennessee, and Elizabeth Phelps of New York University base their conclusions on studying patients with severe anterograde amnesia. Because of hippocampal damage, say from stroke or a brain infection such as encephalitis, such people can remember their distant pasts but cannot create new memories. However, they can learn new skills and habits, which had persuaded researchers that the hippocampus is needed for forming conscious memories but not for learning unconscious skills.
Chun and Phelps chose a mental skill for their study. The task was to pick a sideways letter T out of a field of 11 Ls and push a button to indicate which way the T was tilting. Over the course of 240 trials, the 15 control subjects and 4 amnesic patients got faster at finding the target T. Half of the patterns were generated randomly and were unique. But unbeknownst to the subjects, the other half were repeated over and over again throughout the course of the experiment. None of the subjects noticed the repetition, and no one could pick out the repeated patterns at the end of the experiment. Even so, as the experiment proceeded, the control subjects responded faster to these repeated patterns than they did to newly created ones. The amnesic patients, however, responded to the old forms only as fast as they responded to new. The work shows that even though people with damage to the hippocampus can learn simple patterns and skills, this brain structure is crucial to unconscious recognition of more complex patterns.
"What they've contributed," says neuroscientist Larry Squire of the University of California, San Diego, "is probably the first demonstration that the hippocampal system can be dissociated from conscious awareness." The hippocampus registers and temporarily holds new information, says Squire, binding together all the things--place, odors, sounds, people--that constitute a remembered episode. This study suggests, says Squire, "that relational work may be more fundamental to the work of the hippocampus than awareness."