Analyzing Probabilistic Reward Task Data

In this paper, we re-analyse 392 sessions of behavioural data derived from a probabilistic reward task ([10 (link)]; adapted from [55 (link)]), which is displayed schematically in Figure 1A. Central to the task is that an asymmetrical reinforcement scheme was used to induce a response bias: Correct responses to one stimulus, designated “rich”, were more likely to be rewarded than correct responses to the other stimulus, designated “lean” (Figure 1B). No feedback was given on other trials, including incorrect trials, and no explicit information about the asymmetry was provided. Participants were explicitly encouraged to win as much money as possible, and so could benefit from reporting the rich, rather than the lean, stimulus when in doubt. One measure of the tendency to do this is the response bias [10 (link)]:
$\begin{array}{ll}\frac{1}{2}log\left(\frac{n\left(a_1\right|s_r\left)n\right(a_1\left|s_l\right)}{n\left(a_2\right|s_r\left)n\right(a_2\left|s_l\right)}\right)& \end{array}$
where s_r and s_l indicate presentation of the rich and lean stimulus, respectively, a₁ and a₂ are the two possible key presses, and n(a|s) is the number of times a particular choice was made in response to that stimulus. Each count n was augmented by $\frac{1}{2}$ to avoid numerical instabilities. Outlier trials with very short (<150 ms) or very long (>1500 ms) reaction times are excluded (see [10 (link)] for a full description of the 2-step procedure used to exclude trials with outlier responses). Figure 1F shows the fraction of correct responses for each of the 392 individual experimental sessions. In addition to the computer task, participants completed self-report questionnaires (see Table 2). The datasets and manipulations are shown in Table 1. Briefly, the studies examined the effect of i) depression (categorical diagnosis according to DSM-IV; continuous quantification based on self-report measures of depressive features and anhedonia; and past history of MDD); ii) bipolar disorder, currently euthymic (categorical diagnosis according to DSM-IV); iii) stress; and iv) low-dose D ₂ agonist pramipexole. The low dose (0.5 mg) of pramipexole was assumed to reduce phasic DA bursts to unexpected rewards due to presynaptic (autoreceptor) activation [53 ,56 (link)]. We note that the dataset ‘Stress’ differs from the others because a more difficult version of the task was used [54 (link)].