Scientists have discovered that individual neurons in the monkey hippocampus can signal information about the outcome of experimental trials during an associative learning task. The research, published by Cell Press in the March 26th issue of the journal Neuron, furthers our understanding of outcome-selective cells in the primate brain and provides important insight into how information about trial outcomes may influence learning.
It is well established that the hippocampus plays an important role in the acquisition of new memories for facts, events and relationships. Several previous studies have shown that neurons in the hippocampus signal acquisition of new associations by altering the rate at which they "fire" electrical signals called action potentials. Further, there is some evidence that some hippocampal neurons may serve as outcome-selective cells and signal information about reward delivery or lack of reward.
"These outcome-selective cells have generally been interpreted as signaling the consequence of the animal's response, though their contribution to learning has not been examined. To study how hippocampal neurons convey information about reward and trial outcome during new associative learning, we recorded hippocampal neurons as monkeys learned novel object-place associations," explains senior study author Dr. Wendy A. Suzuki from the Center for Neural Science at New York University.
Dr. Suzuki and colleagues identified two discrete populations of neurons that signaled trial outcome by differentiating between correct and error trials during the period after behavioral response. About half of these cells exhibited increased activity following correct trials (called "correct up cells") while the other half exhibited increased activity after error trials (called "error up cells"). Importantly, correct up cells, but not error up cells or control cells, conveyed information about learning by increasing their stimulus- response properties with behavioral learning.
The results suggest that hippocampal correct up and error up cells serve distinct kinds of monitoring functions linked with trial outcome. "These findings not only expand the categories of associative-learning related signals in the hippocampus, but they suggest a way that information about successful trial outcome conveyed by correct up cells may influence new associative learning," concludes Dr. Suzuki. "Further studies are needed to explore the specific mechanisms underlying this relationship."
The researchers include Sylvia Wirth, Centre de Neuroscience Cognitive, CNRS, CEDEX, France; Emin Avsar, New York University, New York, NY; Cindy C. Chiu, New York University, New York, NY; Varun Sharma, New York University, New York, NY; Anne C. Smith, University of California, Davis, Davis, CA; Emery Brown, MGH, Harvard Medical School, Boston, MA, MIT-Harvard Division of Health Science and Technology, Massachusetts Institute of Technology, Cambridge, MA; and Wendy A. Suzuki, New York University, New York, NY.