Behavioral and neural signatures of readiness to initiate a learned motor sequence

All movements are thought to be "prepared" in the brain before initiation, and preparation can be impaired in motor diseases. However, little is known about what sort of preparation precedes self-initiated, naturally learned sequences of movements. Here we took advantage of a canonical example of a precisely timed learned motor sequence, adult zebra finch song, to examine motor preparation. We found that the sequences of short vocalizations, or introductory notes (INs), preceding song gradually increased in speed and converged on an acoustic endpoint highly similar across renditions, just before song initiation. The more the initial IN differed acoustically from the final IN, the greater the number of INs produced presong. Moreover, the song premotor nucleus HVC exhibited IN-related neural activity that progressed to a distinctive endpoint immediately before song. Together, our behavioral and neural data suggest that INs reflect a variable period of preparation during which the brain attains a common "ready" state each time sequence generation is about to begin.