Sex steroid hormones and sensorimotor vocal development in songbirds

Vocal production learning is rare among animals with birdsong and human speech representing two of the clearest examples. There are two phases to this learning process: one involves the formation of a song memory or template, the second is a sensorimotor process requiring the imitation of the template by vocal output that in songbirds results in a stereotypic species-typical song. Recent advances in the understanding of neural processes required for sensorimotor learning, which includes error correction, have not considered the critical action of testosterone that is essential for song crystallization. We argue that nucleus LMAN of the avian song control system that expresses high densities of androgen receptors is one of the critical sites of testosterone action for crystallization. An additional role may be played by testosterone sensitive areas in the song system such as HVC and RA. Analogous processes for testosterone-dependent sensorimotor vocal learning may also occur in humans and other species.