Fig 1. The Alternating Serial Reaction Time (ASRT) task with second-order dependence structure.

(a) Participants had to press the key corresponding to the current sequence element (i.e. cue location) on the screen as accurately and quickly as possible, using the index and middle fingers of both hands. In the display, the possible locations were outlined in black and the cue always looked the same, fill color and saturation are only used here for explanatory purposes. (b) The structure of the example sequence segment in (a). Color saturation and outline indicate the element that was presented on a trial. The vertical arrow indicates the current trial. The task was generated from an eight element second-order sequence where every second element was deterministic and the elements in between were random. The deterministic components in this example are: red-blue-yellow-green. The element on any random trial (including the current one) is unpredictable. However, this current trial happens to mimic the deterministic second-order dependence where green is followed by red after a gap of one trial, making it a high probability trigram trial (H). The other random elements were associated with lower probability trigrams (L). (c) Under the true generative model, when in a random state, high-probability trigrams (rH) and low-probability trigrams (rL) are equally unexpected. (d) A learner who can pick up second-order dependencies, but who is agnostic to the higher-order alternating state structure, would expect rH more than rL. (e) In the last training session (session 8; after more than 14,000 trials), participants responded faster to deterministic than random trials, suggesting that they learned to predict the upcoming element. They also responded quickly even on random trials if those happened to complete a high probability trigram (rH). The y axis shows the standardised reaction time (RT) averaged over the different trial types on the last session of learning. The error bars indicate the 95% CI.