Changes in activity of the striatum during formation of a motor habit

To examine experience-dependent plasticity of striatal neurons during habit learning in awake, freely moving animals, single neurons in the dorsolateral striatum (54 neurons related specifically to vertical head movement and 14 unresponsive neurons, i.e. not related to any body movement) were recorded and tracked off-line to assess changes in firing rate over sessions as performance of instrumental head movement became automatic and habitual. Rats were trained to emit operant vertical head movements that triggered water delivery for 14 sessions (2 h per session, one session per day). Rats significantly increased the number and efficiency of head movements over sessions until reaching asymptotic behaviour. Habit formation was indicated by significantly higher levels of instrumental responding exhibited by rats during a late, relative to an early, session in which the reward was devalued. As head movements became habitual across sessions, most head movement-related neurons (89%) exhibited decreased firing rate, while a small population (11%) exhibited increased or maintained firing rate. The rate of decrease in firing by the majority correlated with the rate of improvement in movement efficiency over sessions. All unresponsive neurons, though not apparently related to movement, exhibited decreased firing rate over sessions. Our findings suggest that, during habit learning, the striatum may shift from facilitating acquisition of efficient movement with a large population of neurons to modulating or maintaining habitual movement with stronger firing of fewer movement-related neurons.