Temporal sequences of spikes during practice code for time in a complex motor sequence

Practice of a complex motor gesture involves exploration of motor space to attain a better match to target output, but little is known about the neural code for such exploration. Here, we examine spiking in an area of the songbird brain known to contribute to modification of song output. We find that neurons in the outflow nucleus of a specialized basal ganglia- thalamocortical circuit, the lateral magnocellular nucleus of the anterior nidopallium (LMAN), code for time in the motor gesture (song) both during singing directed to a female bird (performance) and when the bird sings alone (practice). Using mutual information to quantify the correlation between temporal sequences of spikes and time in song, we find that different symbols code for time in the two singing states. While isolated spikes code for particular parts of song during performance, extended strings of spiking and silence, particularly burst events, code for time in song during practice. This temporal coding during practice can be as precise as isolated spiking during performance to a female, supporting the hypothesis that neurons in LMAN actively sample motor space, guiding song modification at local instances in time.