Songbirds provide an animal model for human speech perception and production, and for developmentally restricted forms of learning and memory, in which it is possible to causally link specific behaviors and their underlying neural circuitry. Birds, like humans, learn to produce a specific vocal pattern by hearing an acoustic model early in development, and by using auditory feedback of their own self-produced vocalizations in order to match their vocal motor commands to that acoustic model. This research will focus on a neural circuit that is essential for vocal learning to advance our understanding of its specific function. We will compare the synaptic physiology between two related pathways that form this circuit in order to characterize patterns of functional communication between neurons and how they change during learning. We will test whether N-methyl-D-aspartate (NMDA) receptors, which require both a chemical ligand and a voltage signal, carry the majority of synaptic current in these pathways as a means to filter highly related patterns of information in which vocal output matches the acoustic model. We will measure the activity of vocal-control neurons in response to hearing vocal sounds in both pathways of this circuit, and test whether NMDA receptors are normally involved in transmitting auditory input signals and/or vocal motor output signals in either or both of these pathways. We will examine the morphology of individual neurons in these two pathways to test whether regression of neuronal size and synaptic contacts correlates with a decreased role in function following the end of vocal learning. We will assess specific relationships between brain structure and function by separately lesioning these two pathways at different stages of vocal development and measuring resultant behavioral effects. The knowledge gained from these studies will advance our understanding of the neural mechanisms by which organisms learn sounds used for vocal communication during a sensitive period of development, and have implications for treatment of human speech disorders.