Complex Oscillations - The Physics of Birdsong - page 94

It is natural to ask whether all the muscles involved in birdsong production
reflect the lateralized activity of the vocal organ during singing. This question
was settled by Goller and Suthers [Goller and Suthers 1999], who monitored
bilateral airflow and subsyringeal air sac pressure in brown thrashers ( Toxos-
toma rufum ) during singing, together with the electromyographic activity of
the abdominal muscles. They found that expiratory muscles showed activity
on both sides, regardless of whether the song was produced bilaterally or on
one side of the syrinx. The motor commands to the respiratory muscles ap-
pear to be bilaterally distributed, in contrast to what happens with the motor
control of the syrinx. In our model, this is guaranteed by the introduction of
a unique parameter p s as the sublabial pressure.
Anatomical maneuvers at the level of the syrinx or the vocal tract, other
than active lateral gating, can make the coupling parameter α change its
value and vary the coupling strength as the bird sings. Coupling can then
be turned on and off “on the fly”. On the other hand, coupling may always
be present but have little effect unless the sources are vibrating at nearby
frequencies (in terms of our model, when the parameters k i governing the
frequencies on each side are very close to one another). This seems to be the
case for the sonogram in Fig. 6.9.
Acoustic interaction, however, is not the only way to achieve interaction.
The coupling between the sources may also be structural, for example involv-
ing the cartilaginous pessulus to which the labia on both sides of the syrinx
are attached [Nowicki and Capranica 1986]. Little is known about this, but
the dynamics displayed by two nonlinear, mechanically coupled oscillators
cannot but be exciting.
These examples of complex sounds are extremely interesting, since the
source of their complexity should not be traced up towards the brain, but
to the nonlinearities of the peripheral system. Chaotic calls [Fletcher 2000],
period doubling [Fee et al. 1998], etc. are only a few examples of a
large class of nonlinear phenomena present in vocalizations across the
animal kingdom [Wilden et al. 1998]. In recent work, Tchernikovski et al.
[Tchernikovski et al. 2001] even showed that zebra finches take advantage
of period-doubling bifurcations in their learning processes. These authors
showed that the acoustic features of syllables would show, during develop-
ment, an increasing acoustic mismatch until an abrupt correction would take
place (a period-doubling bifurcation). This process might reflect the physi-
cal and neural constraints on the production and imitation of song. Along
the same lines, Podos [Podos 1996] showed that syntax development could
also be affected by the physical limits of how birds sing, by the introduc-
tion of pauses, stops, etc., opening a new perspective on the issue of how
vocal diversity arose. Altogether, these results illustrate that the final vocal
output is the result of a rich interplay between neural instructions and a
nontrivial physical organ that provides limitations as well as opportunities
[Goller 1998, Fee et al. 1998, Chiel and Beer 1997].