Song selectivity in the song system and in the auditory forebrain.

The sensorimotor neurons found in the song-system nuclei are responsive to the sounds of the bird's own song. This selectivity emerges during vocal learning and appears to follow the development of the bird's song vocalization in two ways: at each stage, the neurons are most selective for the bird's current vocalizations and this selectivity increases as the bird learns to produce a stable adult song. Also, because of their location in the sensori-vocal pathway and because their physiological properties are correlated with the motor program, it is postulated that these neurons play a crucial role in interpreting the auditory feedback during song to preserve a desirable vocal output. The neurons found in presynaptic auditory areas lack this selectivity for the bird's own song. Auditory neurons in the secondary auditory areas caudal nidopallium and caudal mesopallium show specific responses to familiar songs or behaviorally relevant songs. These auditory areas might therefore be involved in perceptual tasks. Neurons in the primary forebrain auditory area are selective for the spectrotemporal modulations that are common in song, yielding an efficient neural representation of those sounds. Neurons that are particularly selective for the tutor song at the end of the sensory period have not yet been described in any areas. Although these three levels of selectivity found in the primary auditory forebrain areas, the secondary auditory forebrain areas, and the song system suggest a form of hierarchical sensory processing, the functional connectivity between these areas and the mechanisms generating the specific selectivity for songs that are behaviorally relevant or crucial in song learning and production have yet to be revealed.