Researchers working with monkeys have observed the formation of new memories in the hippocampus, a finding that sheds new light on the physiological mechanisms through which the brain acquires information.

The hippocampus is the region of the brain most often associated with memory, learning, and emotion. Although scientists have linked the hippocampus with the acquisition of long-term memories for nearly half a century, few studies have examined neural activity during the acquisition of associative memories, which establish new connections between unrelated items.

To examine patterns of neural activity connected with associative memories, a joint team of New York University and Harvard researchers led by Wendy Suzuki trained two macaque monkeys to perform location–scene association tasks on a daily basis for more than 18 months (Science, 6 June). Each day the monkeys were shown four identical targets superimposed over a complex visual scene. Learning by trial and error which combination of scene and target location earned them a reward, the monkeys demonstrated a distinct behavioral learning curve. Using electrodes to monitor the monkeys' brains, the researchers identified 25 cells in the hippocampus—dubbed “changing cells”—that showed increased activity that closely corresponded to the monkeys' learning curve. The parallel between these cells and the learning curve suggested these specific neurons were responsible for developing associative memories. Additional experimentation indicated that changing cells recruited a network of hippocampal neurons gradually over a period of time as the new memories were formed.

“We have known since the 1950s that the hippocampus is important for memory because damage to the hippocampus causes profound memory impairment,” Suzuki tells Lab Animal. “However, very little was understood about exactly how the normal hippocampus signals new memory formation. We show that the 'changing cells' in the hippocampus signal learning by dramatic changes in their firing rate.” In the future, she hopes to examine how the major cortical inputs to the hippocampus participate in the formation of new memories. Suzuki tells Lab Animal, “We are also interested in moving beyond the formation of new memories to ask, how does the hippocampus participate in the consolidation of new memories over the course of several days or months.”