State and neuronal class-dependent reconfiguration in the avian song system

Sensory systems may adapt to behavioral requirements through state-dependent changes. In the forebrain song-system nucleus HVc of zebra finches, state-dependent auditory responses have been described in multiunit recordings. Here we report on behavioral state-dependent changes in the activity of distinct HVc neuronal classes. HVc projection neurons were identified by electrically stimulating HVc's target nuclei, the robust nucleus of the archistriatum and Area X, in anesthetized zebra finches. Projection neurons and two classes of putative interneurons could be distinguished on the basis of extracellular spike waveforms, with the first two factors of a principal components analysis accounting for 81% of the variance in spike morphometric values. Spike width was the best single variable for distinguishing among the neuronal classes. Putative interneurons had much higher firing rates spontaneously and in response to song than did projection neurons, which had extremely low spontaneous rates and phasic responses to song. Recordings from HVc in behaving animals were dominated by the two classes of putative interneurons. Both classes showed strong, selective, and temporally similar auditory responses during sleep, but only one class of interneurons reliably maintained auditory responses on waking. These responses were weaker and less selective than those seen during sleep. The observation that HVc auditory responsiveness in awake zebra finches is restricted to some classes of neurons may help explain prior multiunit results that suggested nearly complete suppression of HVc auditory responses in awake birds. We propose that the heterogeneous effects of behavioral state on distinct subpopulations of HVc neurons allow HVc to participate in multiple roles during song production, conspecific song recognition, and possibly memory consolidation during sleep.