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Rats' brains shed light on memory mechanism

A new MIT faculty member has demonstrated that certain cells in rats' brains which fired during an activity also fired later when the rats were asleep, lending support to the theory that memories are consolidated through that particular area of the brain during sleep. Assistant Professor Matthew Wilson, who has a joint appointment in the Departments of Brain and Cognitive Science and in Biology, co-authored the paper describing research on the hippocampus in rats. The paper, which was co-authored by Bruce L. McNaughton of the University of Arizona, appeared in the July 29 issue of Science.

Dr. Wilson, who is coming to MIT from Arizona this month, is the first appointment in the new Center for Learning and Memory headed by Professor Susumu Tonegawa.

Drs. Wilson and McNaughton implanted fine electrodes in the hippocampi of three rats. The hippocampus is known to play an important role in transferring recent events and information into long-term memory. Humans with damaged hippocampi (seahorse-shaped structures deep in the brain) can't remember things that happened even minutes earlier, although memories acquired before the damage occurred are intact, Dr. Wilson explained.

Each electrode was equipped with a microdrive allowing the scientists to control placement in the hippocampus within a few microns. Using an array of these microdrives through a series of four wires attached to each electrode, they could monitor the activity of 50 to 100 cells around the electrodes. They gathered information before and during activity of the rats in a box, as well as afterwards when the rats were sleeping.

When the rats were moving around, different sets of cells in the hippocampus fired together, depending on where the rat was located in its physical environment. The cells' activity could be described as a kind of cognitive spatial map, with different patterns of activity in the hippocampus corresponding to different locations visited by the rat, Dr. Wilson explained.

Other cells didn't fire during the activity or fired individually rather than in groups. The same sets of cells that fired when the rat was awake also did so during sleep, with the effect gradually decreasing over time. Thus, information the rats had acquired about where they had been during their waking activity was re-expressed in the exact same parts of the hippocampus while they slept.

The firings during sleep increased during sharp-wave brain activity, the destination of which may be the neocortex (the exterior part of the brain associated with higher cognitive processes), suggesting that the memories were being consolidated and transferred to the cortex for long-term storage. Drs. Wilson and McNaughton theorize that changes in the hippocampal synapses that occur as a result of behavior may produce related changes in the neocortex during slow-wave sleep, completing the transfer.

Although the role of the hippocampus in consolidating memory of recent events has been recognized for some time, the work of Drs. Wilson and McNaughton provides a potential demonstration of the phenomenon in action. It also offers another bridge between behavioral and physiological psychology by showing cellular interactions and changes as a consequence of behavioral experience.

"Using these techniques, we can actually track this information, seeing where memories start and where they go," Dr. Wilson said. "It provides a window into the cellular substrate of higher-level cognitive functions."

The study was supported by the National Institute of Mental Health, the Office of Naval Research, the National Science Foundation and the McDonnell-Pew Cognitive Neuroscience program.

A version of this article appeared in the August 17, 1994 issue of MIT Tech Talk (Volume 39, Number 2).

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