Howard Hughes Medical Institute researchers have identified a circuit in the brain that appears crucial in converting short-term memories into long-term memories. The circuit links the major learning-related area of the brain to another region that governs the brain's higher functions.
The studies open the way for eavesdropping on one of the central processes in learning and memory, says HHMI investigator Erin M. Schuman. She and graduate student Miguel Remondes of the California Institute of Technology published their findings in the October 7, 2004, issue of the journal Nature.
According to Schuman, the finding sheds light on a central question in learning and memory research that concerns the roles of two brain structures, the hippocampus, which is involved in memory formation, and the neocortex, which is associated with higher brain functions.
“There are two key findings required to understand the present work,” said Schuman. “First, lesions of the hippocampus prior to training can prevent the formation of some kinds of short-term memory. Second, if one delays the hippocampal lesion to days after training, one can observe that as the delay increases, the memory deficit decreases. These data suggest that the importance of the neocortex as a memory storage site increases with the lifetime of the memory. In addition, there is a clear need for the hippocampus and cortex to talk to one another.”
One candidate for the communication conduit is the temporoammonic (TA) projection, “a pathway that we have been chipping away at understanding for years,” said Schuman. “We and others had studied the physiology of this very direct connection between the two areas, but no one had directly studied this pathway's importance in learning.”
For the experiments, Remondes perfected a technique to make precise electrical lesions of the TA projection in the brains of rats. In the first set of experiments, he created the lesions in animals and then tested their ability to learn to navigate a tank full of opaque water to find a submerged platform. When the researchers tested the rats the day after the electrical lesions were made, they still recalled the platform's location. But they lost that memory four weeks after training.
“There were two possible explanations for this result,” said Schuman. “Either we had selectively impaired the process of converting short-term memories into long-term memories. Or, short-term and long-term memories are on parallel pathways, and the lesion had selectively affected the long-term memory pathway.”