2012年3月29日木曜日

Separating value from choice: delay discounting activity in the lateral intraparietal area.


Kenway Louie and Paul W Glimcher
Journal of Neuroscience, 30(16), 5498–5507.

LIP neuronsは意思決定過程の前半は「選択肢の価値」、後半は「その選択確率」をコードしている。価値と選択確率を分離できる実験課題("価値の高い方を選ぶ"ので両者の分離は通常困難)がポイント。 http://www.jneurosci.org/content/30/16/5498

The mathematical formulations used to study the neurophysiological signals governing choice behavior fall under one of two major theoretical frameworks: "choice probability" or "subjective value." These two formulations represent behavioral quantities closely tied to the decision process, but it is unknown whether one of these variables, or both, dominates the neural mechanisms that mediate choice. Value and choice probability are difficult to distinguish in practice, because higher-valued options are chosen more frequently in free-choice tasks. This distinction is particularly relevant for sensorimotor areas such as parietal cortex, where both value information and motor signals related to choice have been observed. We recorded the activity of neurons in the lateral intraparietal area while monkeys performed an intertemporal choice task for rewards differing in delay to reinforcement. Here we show that the activity of parietal neurons is precisely correlated with the individual-specific discounted value of delayed rewards, with peak subjective value modulation occurring early in task trials. In contrast, late in the decision process these same neurons transition to encode the selected action. When directly compared, the strong delay-related modulation early during decision making is driven by subjective value rather than the monkey's probability of choice. These findings show that in addition to information about gains, parietal cortex also incorporates information about delay into a precise physiological correlate of economic value functions, independent of the probability of choice.

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