Figure 2. Learning curves did not differ between the groups.
Mean probabilities (± s.e.m) of choosing the cue associated with the higher outcome, on average, (a) among pairs of two sure cues (15 trials per ‘block’) or (b) when the risky 0/10¢ cue was paired with a sure cue of 0¢ or 10¢ value (20 trials per ‘block’) confirmed that both groups quickly learned to choose the best cue in trials in which one cue was explicitly better than the other. These results verify that both groups understood the task instructions and could perform the task similarly well (in terms of choosing and executing their responses fast enough, etc.). Participants evidenced learning of values for deterministically-rewarded cues even in the first choice trials despite the fact that they were never informed verbally or otherwise of the monetary outcomes associated with each of the cues, and thus could only learn these from experience. However, for cues leading to deterministic outcomes, a little experience can go a long way (Shteingart et al., 2013), and participants received 16 training trials prior to the test phase. Our data suggest that learning in this phase did not differ between the groups: in the first 5 choice trials in the test phase that involved a pair of sure cues, the probability of a correct response was 0.78 ± 0.18 in the DYT group and 0.81 ± 0.07 in the CTL group (Mann-Whitney U test, df = 24, P = 0.59). We verified that that this level of performance could result from trial-and-error learning by simulating the behaviors of individuals using the best-fit learning rates (see Materials and methods). The simulation confirmed that both groups should show similar rates of success on the first 5 choice trials (DYT 0.81 ± 0.17 probability for correct choice, CTL 0.87 ± 0.13, Mann-Whitney U test z = 0.88, df = 24, P = 0.38) despite differences in learning rates from positive and negative prediction errors (see Results). Indeed the model, which started from initial values of 0 and learned only via reinforcement learning, performed on average better than participants.