Measuring habit formation through goal-directed response switching.

Reward-learning theory views habits as stimulus-response links formed through extended reward training.Accordingly, animal research has shown that actions that are initially goaldirected can become habitual after operant overtraining.However, a similar demonstration is absent in human research, which poses a serious problem for translational models of behavior.We propose that response-time (RT) switch cost after operant training can be used as a new, reliable marker for the operation of the habit system in humans.Using a new method, we show that RT switch cost demonstrates the properties that would be expected of a habitual behavior: (1) it increases with overtraining; (2) it increases when rewards are larger, and (3) it increases when time pressure is added to the task, thereby hindering the competing goal-directed system.These results offer a promising new pathway for studying the operation of the habit system in humans.