International neuroscientists discuss recent progress

Internationally recognized scientists gathered at Brandeis October 3-5 to discuss recent progress in understanding the neural mechanisms that promote learning. The workshop, attended by 25 scientists, was sponsored by the Science of Learning Division of the National Science Foundation through a grant to Professor John Lisman, the Zalman Abraham Kekst Chair in Neuroscience. Lisman and Dr. Emrah Duzel, a neurologist from University College London, were the co-organizers of the workshop. Among the leading scientists attending were Mortimer Mishkin, chief of the Cognitive Section on Neuroscience at the National Institute of Mental Health and the Nobel Prize recipient Susumu Tonegawa.

The question of how the brain changes during learning has long fascinated scientists. In 1949 the Canadian psychologist Donald Hebb proposed that learning new associations involves changes in the strength of synapses. Subsequent work in many laboratories established that synapses do change as we learn and that the process rather closely follows the specific rule that Hebb postulated.  Recent work, however, has revealed a limitation of Hebb’s rule; the forming of associations depends on the novelty of incoming information and on the motivation to learn, factors for which Hebb’s rule cannot account. The purpose of the workshop was to see how Hebb’s rule could be revised to take into consideration the new findings.

Participants reviewed mounting evidence that the neurotransmitter, dopamine, is released into a brain structure called the hippocampus and that it is important for memory formation. If released, dopamine acts on synapses that have been strengthened during learning and makes the strengthening permanent. Without dopamine, the synapses rapidly weaken, a process that would lead to forgetting. The evidence for this new view was presented by Richard Morris, Julietta Frey, Anthony Grace, and John Lisman, scientists whose discoveries have contributed to this new theory.

One way of summarizing the new findings is that the Hebb rule governs the initial learning and sets a “tag” at synapses that strengthens the synapse. Then, if dopamine arrives, because the information is novel if the subject is motivated, it triggers the synthesis of new protein. These proteins interact with the tag and make these specific synapses become stably strengthened, thereby forming the long-term memory. The overall process is termed “neoHebbian.”

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