1. Biological Computation Research Department, Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974, USA
2. Howard Hughes Medical Institute, Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
3. These authors contributed equally to this work

Correspondence to: Michale S. Fee¹ Correspondence and requests for materials should be addressed to M.S.F. (e-mail: Email: fee@lucent.com).

Abstract

Sequences of motor activity are encoded in many vertebrate brains by complex spatio-temporal patterns of neural activity; however, the neural circuit mechanisms underlying the generation of these pre-motor patterns are poorly understood. In songbirds, one prominent site of pre-motor activity is the forebrain robust nucleus of the archistriatum (RA), which generates stereotyped sequences of spike bursts during song¹ and recapitulates these sequences during sleep². We show that the stereotyped sequences in RA are driven from nucleus HVC (high vocal centre), the principal pre-motor input to RA^{3, 4}. Recordings of identified HVC neurons in sleeping and singing birds show that individual HVC neurons projecting onto RA neurons produce bursts sparsely, at a single, precise time during the RA sequence. These HVC neurons burst sequentially with respect to one another. We suggest that at each time in the RA sequence, the ensemble of active RA neurons is driven by a subpopulation of RA-projecting HVC neurons that is active only at that time. As a population, these HVC neurons may form an explicit representation of time in the sequence. Such a sparse representation, a temporal analogue of the 'grandmother cell'⁵ concept for object recognition, eliminates the problem of temporal interference during sequence generation and learning attributed to more distributed representations^{6, 7}.