2015年3月11日水曜日

Dopamine-associated cached values are not sufficient as the basis for action selection

Nick G. Hollon, Monica M. Arnold, Jerylin O. Gan, Mark E. Walton, and Paul E. M. Phillips
PNAS December 23, 2014 vol. 111 no. 51 18357-18362

側坐核(nucleus accumbens)のドーパミンは、意思決定において「学習された価値(cached values)」の処理に関与しているが、「価値とコストを統合した効用」の処理には関与していない。

Phasic dopamine transmission is posited to act as a critical teaching signal that updates the stored (or “cached”) values assigned to reward-predictive stimuli and actions. It is widely hypothesized that these cached values determine the selection among multiple courses of action, a premise that has provided a foundation for contemporary theories of decision making. In the current work we used fast-scan cyclic voltammetry to probe dopamine-associated cached values from cue-evoked dopamine release in the nucleus accumbens of rats performing cost–benefit decision-making paradigms to evaluate critically the relationship between dopamine-associated cached values and preferences. By manipulating the amount of effort required to obtain rewards of different sizes, we were able to bias rats toward preferring an option yielding a high-value reward in some sessions and toward instead preferring an option yielding a low-value reward in others. Therefore, this approach permitted the investigation of dopamine-associated cached values in a context in which reward magnitude and subjective preference were dissociated. We observed greater cue-evoked mesolimbic dopamine release to options yielding the high-value reward even when rats preferred the option yielding the low-value reward. This result identifies a clear mismatch between the ordinal utility of the available options and the rank ordering of their cached values, thereby providing robust evidence that dopamine-associated cached values cannot be the sole determinant of choices in simple economic decision making.

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