of song, varies across species depending on the degree of sexual dimorphism
in the brain of the animals.
2.3.3.3. The Anterior Forebrain Pathway
A second pathway in the songbird forebrain involves an avian corticobasal
ganglia-thalamocortical pathway (Bottjer and Johnson 1997; Farries and
Perkel 2000; Perkel and Farries 2000). This three-nucleus pathway makes
an obligatory contribution to song development (Bottjer et al. 1984;
Sohrabji et al. 1990; Scharff and Nottebohm 1991). In zebra finches, the AFP
has also been shown to contribute to song maintenance (Williams and
Mehta 1999; Brainard and Doupe 2000). Although neurons in the AFP in
zebra finches show premotor activity during singing (Jarvis and Nottebohm
1997; Hessler and Doupe 1999), lesions of AFP nuclei—in contrast to
lesions of HVc or RA—have little disruptive effect on motor output
(Nordeen and Nordeen 1993). In Bengalese finches, partial lesions of an
AFP nucleus result in the transient disruption of song (Kobayashi et al.
2001). The premotor activity in the AFP has been interpreted as an efference copy signal that combines with auditory feedback to create an error
signal that helps stabilize vocal output (Williams and Mehta 1999; Brainard
and Doupe 2000, 2001; Solis et al. 2000). The differences between zebra and
Bengalese finches may result from the apparent greater reliance on auditory feedback, and hence a greater effect of the AFP, in song maintenance
in adult Bengalese finches (Okanoya and Yamaguchi 1997).
Different populations of HVc neurons project to RA in the VMP and
area X in the AFP. Thus, the apparent role of HVc in some aspects of song
perception does not distinguish between potential differential roles of the
VMP and AFP in song perception. If the VMP is involved in song perception, this would provide strong evidence for a “motor” theory of birdsong
perception (Williams and Nottebohm 1985) and its related theory for
speech perception by reference to production (Liberman et al. 1967;
Liberman and Mattingly 1985). Alternatively, the perceptual role of HVc
could be mediated through its AFP projections. In this case, the observed
linkage between production and perception would arise through a shared
developmental history, not through direct moment-to-moment coupling.
The data appear to support the ontogenetic theory. In what is perhaps
the most convincing evidence that the song system is involved in non-BOS
acoustic stimulus recognition, lesions to lMAN in female canaries disrupt
the retention of auditory discriminations (Burt et al. 2000). These lesions
effect discrimination among pairs of conspecific songs, heterospecific songs,
and synthetic sounds, but not among pairs of visual stimuli, and therefore
suggest a general auditory processing role for the AFP rather than one
specifically related to conspecific songs or a bird’s own song. In contrast,
the effects of lesions to the AFP in adult male zebra finches appear to be
restricted to conspecific discriminations involving a bird’s own song (Scharff
7. Neuroethology of Vocal Communication
349
in the brain of the animals.
2.3.3.3. The Anterior Forebrain Pathway
A second pathway in the songbird forebrain involves an avian corticobasal
ganglia-thalamocortical pathway (Bottjer and Johnson 1997; Farries and
Perkel 2000; Perkel and Farries 2000). This three-nucleus pathway makes
an obligatory contribution to song development (Bottjer et al. 1984;
Sohrabji et al. 1990; Scharff and Nottebohm 1991). In zebra finches, the AFP
has also been shown to contribute to song maintenance (Williams and
Mehta 1999; Brainard and Doupe 2000). Although neurons in the AFP in
zebra finches show premotor activity during singing (Jarvis and Nottebohm
1997; Hessler and Doupe 1999), lesions of AFP nuclei—in contrast to
lesions of HVc or RA—have little disruptive effect on motor output
(Nordeen and Nordeen 1993). In Bengalese finches, partial lesions of an
AFP nucleus result in the transient disruption of song (Kobayashi et al.
2001). The premotor activity in the AFP has been interpreted as an efference copy signal that combines with auditory feedback to create an error
signal that helps stabilize vocal output (Williams and Mehta 1999; Brainard
and Doupe 2000, 2001; Solis et al. 2000). The differences between zebra and
Bengalese finches may result from the apparent greater reliance on auditory feedback, and hence a greater effect of the AFP, in song maintenance
in adult Bengalese finches (Okanoya and Yamaguchi 1997).
Different populations of HVc neurons project to RA in the VMP and
area X in the AFP. Thus, the apparent role of HVc in some aspects of song
perception does not distinguish between potential differential roles of the
VMP and AFP in song perception. If the VMP is involved in song perception, this would provide strong evidence for a “motor” theory of birdsong
perception (Williams and Nottebohm 1985) and its related theory for
speech perception by reference to production (Liberman et al. 1967;
Liberman and Mattingly 1985). Alternatively, the perceptual role of HVc
could be mediated through its AFP projections. In this case, the observed
linkage between production and perception would arise through a shared
developmental history, not through direct moment-to-moment coupling.
The data appear to support the ontogenetic theory. In what is perhaps
the most convincing evidence that the song system is involved in non-BOS
acoustic stimulus recognition, lesions to lMAN in female canaries disrupt
the retention of auditory discriminations (Burt et al. 2000). These lesions
effect discrimination among pairs of conspecific songs, heterospecific songs,
and synthetic sounds, but not among pairs of visual stimuli, and therefore
suggest a general auditory processing role for the AFP rather than one
specifically related to conspecific songs or a bird’s own song. In contrast,
the effects of lesions to the AFP in adult male zebra finches appear to be
restricted to conspecific discriminations involving a bird’s own song (Scharff
7. Neuroethology of Vocal Communication
349