2011年2月21日月曜日

Separate value comparison and learning mechanisms in macaque medial and lateral orbitofrontal cortex.

サルの損傷研究。報酬に基づく意思決定において、mOFC(内側眼窩前頭野)はdecisionに、lOFC(背側眼窩前頭野)はlearningに関与。 http://www.ncbi.nlm.nih.gov/pubmed/21059901 http://www.ncbi.nlm.nih.gov/pubmed/20346766

「lOFCがないと、credit assignmentに失敗する」とかすごくおもしろい。単純な「条件付け」にもまだまだフロンティアがありそう。

Proc Natl Acad Sci U S A. 2010 Nov 23;107(47):20547-52. Epub 2010 Nov 8.
Separate value comparison and learning mechanisms in macaque medial and lateral orbitofrontal cortex.
Noonan MP, Walton ME, Behrens TE, Sallet J, Buckley MJ, Rushworth MF.

Uncertainty about the function of orbitofrontal cortex (OFC) in guiding decision-making may be a result of its medial (mOFC) and lateral (lOFC) divisions having distinct functions. Here we test the hypothesis that the mOFC is more concerned with reward-guided decision making, in contrast with the lOFC's role in reward-guided learning. Macaques performed three-armed bandit tasks and the effects of selective mOFC lesions were contrasted against lOFC lesions. First, we present analyses that make it possible to measure reward-credit assignment--a crucial component of reward-value learning--independently of the decisions animals make. The mOFC lesions do not lead to impairments in reward-credit assignment that are seen after lOFC lesions. Second, we examined how the reward values of choice options were compared. We present three analyses, one of which examines reward-guided decision making independently of reward-value learning. Lesions of the mOFC, but not the lOFC, disrupted reward-guided decision making. Impairments after mOFC lesions were a function of the multiple option contexts in which decisions were made. Contrary to axiomatic assumptions of decision theory, the mOFC-lesioned animals' value comparisons were no longer independent of irrelevant alternatives.

Neuron. 2010 Mar 25;65(6):927-39.
Separable learning systems in the macaque brain and the role of orbitofrontal cortex in contingent learning.
Walton ME, Behrens TE, Buckley MJ, Rudebeck PH, Rushworth MF.

Orbitofrontal cortex (OFC) is widely held to be critical for flexibility in decision-making when established choice values change. OFC's role in such decision making was investigated in macaques performing dynamically changing three-armed bandit tasks. After selective OFC lesions, animals were impaired at discovering the identity of the highest value stimulus following reversals. However, this was not caused either by diminished behavioral flexibility or by insensitivity to reinforcement changes, but instead by paradoxical increases in switching between all stimuli. This pattern of choice behavior could be explained by a causal role for OFC in appropriate contingent learning, the process by which causal responsibility for a particular reward is assigned to a particular choice. After OFC lesions, animals' choice behavior no longer reflected the history of precise conjoint relationships between particular choices and particular rewards. Nonetheless, OFC-lesioned animals could still approximate choice-outcome associations using a recency-weighted history of choices and rewards.

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