Sensitivity to chunk order in a premotor song nucleus of canaries

Sequences of distinct, ordered elements can be organized hierarchically across multiple levels. Representing such sequences in the brain may require extracting information about transition probabilities and chunks. How these processes are implemented at the neuronal level and the extent to which they are intermingled remain unclear. To address these questions, we took advantage of the multilevel structure of domesticated canary song, which comprises syllables, phrases, and chunks, as well as the selective auditory responses of neurons in the premotor HVC nucleus to playback of the bird’s own song. Neuronal activity was recorded in response to a selected song bout and to variants of that song bout generated by altering the sequential organization at either the phrase or the chunk level. Reordering chunks could strongly affect responses to one or more phrases. Moreover, the lower the transition probabilities due to chunk rearrangement, the weaker the responses. In contrast, reordering phrases within a given chunk had little impact on responsiveness, despite a dramatic decrease in transition probabilities. The present study also suggests an encoding of chunk sequencing through the integration of information provided at chunk boundaries and over a preceding period of several seconds.