Time-dependent competition between habitual and goal-directed response preparation

Converging evidence indicates that separate goal-directed and habitual systems compete to control behavior1. However, it has proven difficult to reliably induce habitual behavior in human participants2-4. We reasoned that habits may be present in the form of habitually prepared responses, but are overridden by goal-directed processes, preventing their overt expression. Here we show that latent habits can be unmasked by limiting the time participants have to respond to a stimulus. Participants trained for 4 days on a visuomotor association task. By continuously varying the time allowed to prepare responses, we found that the probability of expressing a learned habit followed a stereotyped time course, peaking 300-600ms after stimulus presentation. This time course was captured by a computational model of response preparation in which habitual responses are automatically prepared at short latency, but are replaced by goal-directed responses at longer latency. A more extensive period of practice (20 days) led to increased habit expression by reducing the average time of movement initiation. These findings refine our understanding of habits, and show that practice can influence habitual behavior in distinct ways: by promoting habit formation, and by modulating the likelihood of habit expression.