Memory and the Hippocampus

By Ellen Kuwana
Neuroscience for Kids Staff Writer
February 4, 2000

"Memories, light the corners of my mind"

So goes the popular song from the 1970s, but actually it is believed that memories are formed deep within the brain rather than in the corners. One brain structure known to be involved in the complex processes of forming, sorting, and storing memories is the hippocampus. Several studies have shed new light on the contribution of the hippocampus to memory.

Many experiments have shown that the hippocampus is "critical to learning and remembering relationships that characterize spatial layouts, items in the particular context in which they have been experienced, and other associative, sequential or logical relationships among experiences" (reference 1). Not only is the hippocampus filing away memories, it is connecting them with other related memories and giving the memories meaning. In other words, the hippocampus might be connecting the memory of your first day at school with information about the physical surroundings, the smells, and the sounds of that event.

One way to look at what the hippocampus does in regard to memory is to examine a person with a damaged hippocampus. Many people with damage to the hippocampus have anterograde amnesia: they can remember the distant past but cannot form new memories. They can, however, learn new skills. In other studies, the ability to respond to complex patterns was compared in people with and without hippocampal damage. The test was to find the letter 'L' (in various orientations: upside down, sideways) among a background of 'T's (see the figure on the left).

Some of the patterns generated were random and new to the viewer. Some patterns were repeated, so that they were "familiar" to the person's brain, even if the person was not aware of having seen the pattern before. In fact, no one in either group was aware that some of the patterns were repeating. How did the groups do?

The control group's reaction time to all patterns improved over time. For the repeated patterns (even when they were not conscious that the pattern was a repeat), the reaction time was even faster than for new patterns. Therefore, the subjects in the control group "could learn to remember repeated patterns they weren't consciously aware of" (reference 3). They got faster at finding the 'L' in the repeated patterns than in the new patterns.

The people with amnesia had reaction times slower than the control group overall, but showed improvement over time to the new patterns. Unlike the control group, though, they did not get even better at the repeated patterns. This suggests that the hippocampus plays a role in "encoding implicit contextual information from the environment" (reference 2). In other words, the hippocampus is needed for learning contextual information.

Many structures are involved in the complex process of encoding memories. How each structure contributes to this process is not fully known. It is known that there are different types of memories: conscious memories such as your first day at school, and non-conscious memories or skills such as knowing how to ride a bike. It is also known that the hippocampus helps form conscious memories. This new research demonstrates that the hippocampus is "crucial to unconscious recognition of more complex patterns" (reference 3).


Hear It
"Hippocampus"

References:

  1. "Conscious awareness, memory, and the hippocampus," by Howard Eichenbaum. Nature Neuroscience, Vol. 2, No. 9, September 1999, pp. 775-776.
  2. Chun, Marvin and Phelps, Elizabeth, "Memory deficits for implicit contextual information in amnesic subjects with hippocampal damage," Nature Neuroscience, Vol. 2, No. 9, September 1999, pp. 844-847.
  3. "New Role Found for The Hippocampus," by Laura Helmuth. Science, Vol. 285, August 27, 1999, pp. 1339-1341.

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