Selectivity for conspecific song in the zebra finch auditory forebrain.

The selectivity of neurons in the zebra finch auditory forebrain for natural sounds was investigated systematically. The principal auditory forebrain area in songbirds consists of the tonotopically organized field L complex, which, by its location in the auditory processing stream, can be compared with the auditory cortex of mammals. We also recorded from a secondary auditory area, cHV. Field L and cHV are auditory processing stages that are presynaptic to the specialized song system nuclei where auditory neurons show an extremely selective response for the bird's own song, but weak response to almost any other sounds, including conspecific songs. In our study, we found that neurons in field L and cHV had stronger responses to conspecific song than to synthetic sounds that were designed to match the lower order acoustical properties of song, such as their overall power spectra and AM spectra. Such preferential responses to natural sounds cannot be explained by linear frequency tuning or simple nonlinear intensity tuning and requires linear or nonlinear spectro-temporal neuronal transfer functions tuned to the acoustical properties of song. The selectivity for conspecific songs in field L and cHV might reflect an intermediate auditory processing stage for vocalizations that then contributes to the generation of the very specific selectivity for the bird's own song seen in the postsynaptic song system.