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A neural mechanism underlying failure of optimal choice with multiple alternatives
Bolton K H Chau1, Nils Kolling1, Laurence T Hunt2, Mark E Walton1 & Matthew F S Rushworth1,2
Despite widespread interest in neural mechanisms of decision-making, most investigations focus on decisions between just two options. Here we adapt a biophysically plausible model of decision-making to predict how a key decision variable, the value difference signalencoding how much better one choice is than anotherchanges with the value of a third, but unavailable, alternative. The model predicts a surprising failure of optimal decision-making: greater difficulty choosing between two options in the presence of a third very poor, as opposed to very good, alternative. Both investigation of human decision-making and functional magnetic resonance imagingbased measurements of value difference signals in ventromedial prefrontal cortex (vmPFC) bore out this prediction. The vmPFC signal decreased in the presence of low-value third alternatives, and vmPFC effect sizes predicted individual variation in suboptimal decision-making in the presence of multiple alternatives. The effect contrasts with that of divisive normalization in parietal cortex.
npg 201 4 Nature America, Inc. All rights reserved.
Value representations are ubiquitous in the brain1, but there is special interest in vmPFC and the intraparietal sulcus (IPS) because several lines of evidence suggest they use value information to guide a decision process26. Not only do these areas carry information about reward expectation3,715, but vmPFC, and sometimes IPS, also carries value difference signals reflecting the difference in value between chosen and unchosen options during decision-making in a way that suggests that value comparison between options is taking place3,5,9,1619.
Despite broad interest in the neural mechanisms of decision-making, little is known about the comparison process when there are more than two alternatives to choose among. One important study of visually guided decisions (evidence accumulation processes in value-guided and visually guided decisions share many features3,17,20,21)
explicitly compared multi-alternative and binary visual decisions, but the degree of evidence for only one of the two or four possible choices was manipulated on each trial22. By contrast, in the present study, we investigated decisions where the value related to each of several potential options was parametrically varied on each decision so that the interactions between neural representations of several...