The effect of acute tryptophan depletion on probabilistic choice

I. M. Anderson, R. A. Richell, C. M. Bradshaw

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    Choice behaviour can be viewed as a response to reinforcement determined by an interaction between the quantities, delays and probabilities of two outcomes. The variation in the perceived value of a reinforcer with alteration of these factors (discounting) can be modelled mathematically by hyperbolic discounting functions. Making risky choices is a feature of impulsivity and has been associated with reduced serotonin (5-hydroxytryptamine, 5-HT) function. In this study, we investigated the possible role of 5-HT in modulating probability discounting using the technique of acute tryptophan (TRP) depletion in subjects undertaking an imaginary gambling task. The gambling task consisted of choosing between two 'roulette-like' dials: 'A' which provided a smaller but nearly certain 'win' and 'B' which gave a 'win' 2.5 times the amount with a probability that was systematically varied. A series of reward sizes on dial 'A' was presented ranging from 10 pence to £10 000. The probability of winning on dial 'B' at which the subjects valued the two dials equally (indifference point) was determined as a measure of willingness to take a risk. Subjects were more likely to take a risk for smaller rewards but the indifference points in the 15 subjects who received TRP depletion did not differ from 13 who had the control drink. On a surprise retesting 1 week later there was a trend (p <0.07) for subjects to be more willing to take risks the second time, particularly in the case of small rewards. This study does not support a role for 5-HT in modulating probabilistic choice in agreement with recent evidence from experiments with animals; however, the imaginal nature of the task and modest numbers may have influenced the result.
    Original languageEnglish
    Pages (from-to)3-7
    Number of pages4
    JournalJournal of Psychopharmacology
    Issue number1
    Publication statusPublished - Mar 2003


    • Discounting
    • Gambling
    • Probability
    • Serotonin
    • Tryptophan depletion


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