A two-sided mirror
When he was working at Rockefeller University in New York, Marler had two extremely brilliant students: Fernando Nottebohm and Mark Konishi. For a long time, scientists had been trying to understand the neuroanatomical bases on which the control of complex behaviours was based. The song system offered a particular advantage, insofar as bird vocalisations are emitted by a specific unique organ: the syrinx. Nottebohm’s idea was that it must be possible to map the areas of the brain connected to the muscles activating the syrinx, by studying degeneration resulting from the severing of the axonal endings.
In the beginning, he pinpointed an intermediary motor nucleus situated in the brain stem, the nucleus of nerve 12 (nXIIts), before identifying several other interconnected nuclei in the brain: those that control song. “The latter define a caudal motor pathway and a more rostral pathway”, explains Jacques Balthazart. “The first one is composed of the HVC (formerly known as the “High Vocal Center”), which unilaterally projects onto another nucleus, the RA (Robustus archistriatalis). In turn, the latter projects onto the nucleus of nerve 12, which innervates the syrinx. Besides the motor pathway, there is also a far more complex frontal pathway, where the HVC projects over an area called Area X. The neuronal connections take the following route: from Area X to a nucleus called DLM, from here to another nucleus called IMAN, and from IMAN to the RA.”
In other words, there are two ways the information from the HVC can end up in the RA, directly or indirectly. The work of Nottebohm led to a complete description of the neuroanatomical substrates of the control of song. Hence, it attracted the attention of numerous neurobiologists because it offered them a perfectly defined frame of reference to carry out their studies on the neuroendocrine and neurochemical mechanisms governing complex behaviour.
What do we know about the role of the various nuclei that constitute
the song system? Firstly, various experiments have revealed that the
lesion of a nucleus in the motor pathway makes the bird completely
silent. Secondly, it was discovered that the destruction of the rostral
pathway in the adult had no immediate effect on song. However, in the
case of a lesion in Area X or the IMAN nucleus, the stability of the
vocalisations produced is somewhat altered after a few months: the
rhythm slows down or accelerates and the quality of the harmonics
diminishes. On the other hand, if any link in the rostral chain of a
young bird is damaged, it will never learn to sing.