Revealing abrupt transitions from goal-directed to habitual behavior.

The speed of goal-directed to habit transitions has been debated since Clark Hull asked in 1943: is habit formation slow or sudden? To address this, male mice were given home-cage access to citric-acid water that reduced-without eliminating-reward-seeking for plain water in an auditory go/no-go task. Animals learned to discriminate quickly but exhibited ongoing state-like fluctuations in engagement. Strikingly, these fluctuations abruptly ceased (transition) long after discrimination stabilized, with HMM-GLM modeling pinpointing a ~3-trial transition. We confirmed this as a goal-directed to habit transition using sensory-specific outcome devaluation, DLS lesions, motor stereotypy, and pupillary responses. Dual-site fiber photometry showed equally abrupt DLS dynamics at the transition: outcome-related activity dropped and stimulus-response activity sharpened, suggesting a switch-like mechanism that recruits a readily available habit circuit rather than gradual changes across a threshold. Thus, habits can emerge suddenly, mediated by an abrupt dorsostriatal shift from outcome- to stimulus-driven processing.