2014年1月17日金曜日

Phasic Dopamine Release in the Rat Nucleus Accumbens Symmetrically Encodes a Reward Prediction Error Term

Andrew S. Hart, Robb B. Rutledge, Paul W. Glimcher, and Paul E. M. Phillips
J. Neurosci. 2014;34 698-704
http://www.jneurosci.org/cgi/content/abstract/34/3/698?etoc

ドーパミンの放出そのものが報酬予測誤差をコードするのか?
負と正の予測誤差の両方を保持するのか?正の予測誤差だけなのか?
よく分かっていなかった。
→ ラットの側坐核における「ドーパミンの放出」は(負と正、両方の)報酬予測誤差と一致する。

Making predictions about the rewards associated with environmental stimuli and updating those predictions through feedback is an essential aspect of adaptive behavior. Theorists have argued that dopamine encodes a reward prediction error (RPE) signal that is used in such a reinforcement learning process. Recent work with fMRI has demonstrated that the BOLD signal in dopaminergic target areas meets both necessary and sufficient conditions of an axiomatic model of the RPE hypothesis. However, there has been no direct evidence that dopamine release itself also meets necessary and sufficient criteria for encoding an RPE signal. Further, the fact that dopamine neurons have low tonic firing rates that yield a limited dynamic range for encoding negative RPEs has led to significant debate about whether positive and negative prediction errors are encoded on a similar scale. To address both of these issues, we used fast-scan cyclic voltammetry to measure reward-evoked dopamine release at carbon fiber electrodes chronically implanted in the nucleus accumbens core of rats trained on a probabilistic decision-making task. We demonstrate that dopamine concentrations transmit a bidirectional RPE signal with symmetrical encoding of positive and negative RPEs. Our findings strengthen the case that changes in dopamine concentration alone are sufficient to encode the full range of RPEs necessary for reinforcement learning.

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