The apparatus, trial structure and block structure of the two-step task are described in the results section and Figure 1. Block transitions were triggered based on subject’s behavior, occurring 20 trials after an exponential moving average (tau = 8 trials) of subject’s choices crossed a 75% correct threshold. The 20 trial delay between the threshold crossing and block transition allowed subjects performance at the end of blocks to be assessed without selection bias due to the block transition rule. In neutral blocks where there was no correct choice, block transitions occurred with 0.1 probability on each trial after the 40th, giving a mean neutral block length of 50 trials. Transitions from non-neutral blocks occurred with equal probability (25%) to either to another non-neutral block via reversal in the reward or transition probabilities, or to one of the two neutral blocks. Transition from neutral blocks occurred via a change in the reward probabilities only to one of the non-neutral blocks with the same transition probabilities.
