Neural correlates of sensitive periods in avian song learning.

A wide range of neural events takes place during avian song learning, including changes in receptor systems and synaptic organization and the wholesale production and growth of new cells and their connections. Any (or all) of these events may be involved in song learning, and therefore their timing may restrict when auditory memories are laid down in the song system and when these memories are used to pattern vocal behavior. In particular, mounting evidence indicates that NMDA receptor activation participates in the acquisition of auditory memories during song learning, and developmental changes in the number and function of NMDA receptors within the song system may regulate the capacity for song learning. These developmental changes in NMDA receptors occur in conjunction with a period of synapse elimination, and NMDA receptor activation during song learning may regulate this regressive event. In addition, both developmental and seasonal periods of vocal learning correlate well with neuronal addition and turnover within the vocal motor pathway. This temporal overlap provides the opportunity for auditory experience to shape the initial organization of naive motor circuitry. Thus, a variety of cellular substrates present themselves as candidates for providing the neural plasticity necessary for song learning and may together regulate how and when experience is stored in the nervous system.