Biomedical Engineering Reference
In-Depth Information
8 From the Syrinx to the Brain
In the last few years there has been enormous progress in the understand-
ing of the brain structure of birds. At least, this is the case for song-
birds, which constitute some 4000 of the approximately 10 000 species of
birds known to exist. The reason for such interest is that, for these species,
the learning of song has many parallels with the acquisition of speech by
humans [Doupe and Kuhl 1999]. Notice that we write “speech” and not
“language”, since there is no evidence of birds using their songs in a com-
binatorial way in order to convey flexible syntactic content like that char-
acterizing human speech. However, songbirds require (as do humans) a
tutor whose song is memorized in the early stages of life [Thorpe 1961,
Marler 1970, Nottebohm 1970]. Later, they try to reproduce the songs previ-
ously memorized, correcting their vocalizations thanks to an auditory feed-
back [Konishi 1965]. For this reason, songbirds have been used as a test bench
on which fundamental aspects of a complex learned behavior can be studied.
This has led to the accumulation of an important amount of information on
the physical substrate of these operations. This will allow us to advance fur-
ther in our discussion, establishing the place in the brain where the precise
gestures (responsible for the execution of the commands driving the vocal
organ in order to produce a song) are controlled.
The fundamental operations involved in song learning (memorization of
the song of the tutor, auditory feedback from different attempts leading to
corrections, etc.) are interesting and sophisticated. For this reason, detailed
experiments have been performed in order to determine whether there were
specific neural circuits that would serve as the physical substrate. The first
step consisted in describing morphologically the brains of songbirds. From
these studies, we learned that there are sets of neurons (called nuclei) clearly
grouped into different regions in the brain, interconnected in a very complex
way through long axons. The way in which knowledge was generated about
the role played by these nuclei was basically through the study of behavioral
changes after localized lesions. As much as they help us, however, classifica-
tions can be dangerous. We shall say in this chapter that two major pathways
organize these nuclei, running the risk of caricaturing a most complex biolog-
ical device. In any case, it is well established in the field that a set of nuclei
is necessary to generate song. Systematic studies on lesioned birds indicate
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