Songbirds are the best known group of vocal learners. By using recent techniques of molecular biology in combination with electrophysiological, anatomical, and behavioral studies we have gained new insights into the organization of brain structures used in vocal learning.
Image taken from Jarvis and Nottebohm, “Motor Driven Gene Expression”, PNAS USA 94 (1997).
Of the three orders of birds in which vocal learning has been found to occur, the neurobiology of vocal communication has been most extensively studied in songbirds (oscine passeriformes). As set of discrete forebrain nuclei involved in song learning and production were first mapped in oscines. These nuclei, collectively known as the ‘song system’, project to motor neurons that innervate the trachea and syrinx and control their function in coordination with respiratory movements.
Because forebrain vocal nuclei and vocal learning have not been found in suboscines, the closest relative of oscine songbirds, nor in interrelated groups such as columbiformes (doves and pigeons), or distantly related ones, such as galliformes (chicken and fowl), it has been proposed that vocal learning and associated neural structures evolved independently in songbirds, parrots, and hummingbirds.
The use of cDNA cloning and in situ hybridization techniques in combination with the study of alert behaving animals has recently allowed a high-resolution mapping of brain areas involved in perceptual and motor aspects of vocal communication in songbirds. This approach generated new insights into the functional organization of the song system that were not readily apparent from electrophysiological or anatomical studies alone.
Specifically, expression analysis of transcriptional regulators has shown a clear separation of brain areas that have gene activation following song perception or production. The auditory ZENK response is species-specific, is tuned to specific features of song, occurs with novel song stimuli, and requires early juvenile experience with adult tutors. The vocal gene response is dependent on behavioral context and different genes.
“Decrements in auditory responses to a repeated conspecific song are long-lasting and require two periods of protein synthesis in the songbird forebrain.” – a paper that appeared in Proceedings of the National Academy of Science of the USA in April 1995.
“Motor-driven gene expression.” – a paper that appeared in Proceedings of the National Academy of Science of the USA in April 1997. It shows that even though the same auditory stimulus activates sensory and motor pathways, perception and production of song are accompanied by anatomically distinct patterns of gene expression.
“Selective expression of insulin-like growth factor II in the songbird brain.” – a paper that appeared in Journal of Neuroscience in 1997.
“A relationship between behavior, neurotrophin expression, and new neuron survival.” – a paper that appeared in Proceedings of the National Academy of Science of the USA in May 2000. It shows a relationship between the occurrence of a learned behavior, an increase in neurotrophin expression by neurons that produce this behavior, and an increase in the survival of new neurons of this type.