First published online May 30, 2008
Journal of Experimental Biology 211, 1978-1991 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014092
Two-voice complexity from a single side of the syrinx in northern mockingbird Mimus polyglottos vocalizations
Sue Anne Zollinger1,*,
Tobias Riede2,3 and
Roderick A. Suthers1,2,4
1 Department of Biology, Jordan Hall, Indiana University, Bloomington, IN 47405,
USA
2 School of Medicine, Jordan Hall, Indiana University, Bloomington, IN 47405,
USA
3 National Center for Voice and Speech, 1101 13th Street, Denver, CO 80204,
USA
4 Program in Neural Science, Jordan Hall, Indiana University, Bloomington, IN
47405, USA
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Fig. 1. Synthesized examples illustrating four types of biphonation and two-voice
phenomena observed in mockingbird songs. (A) The spectrogram (top) and
spectrum (bottom) illustrate the spectral properties typical of both `type A'
biphonation and two-voice phenomena, consisting of two independent
frequencies, f0 and g0. (B) `Type B'
biphonation consists of a fundamental frequency and sidebands. In this
synthesized example, f0 is a 1 kHz tone. The second
frequency is a 250 Hz modulation frequency, m0, which
appears spectrographically as sound energy 250 Hz above and below
f0. (C) A synthesized example of `dual biphonation'
similar to the type observed in mockingbird vocalizations. Each side of the
syrinx produces `type B' biphonation simultaneously. In this example, two
unrelated fundamental frequencies (f0 and
g0), originate from opposite sides of the syrinx. The two
`voices' are each modulated by an unrelated, lower modulating frequency (250
Hz, m0). (D) A synthesized example of `dual biphonation'
similar to C, but in this case m0 is also frequency
modulated, resulting in sidebands that are not parallel to
f0 or g0. BP, biphonation; 2VC,
two-voice phenomena; SB, sidebands; 2SB, dual biphonation (two
f0 values, each with sidebands).
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Fig. 2. Spectral properties typical of three types of nonlinear phenomena;
frequency jumps, subharmonics and deterministic chaos. (A) A 3500 Hz tone with
two 250 Hz frequency jumps. A power spectrum (a) taken at arrow a shows a
single peak of sound energy corresponding to f0. (B) A
3750 Hz tone, with a series of bifurcations, the first from a single
f0 to a 0.5f0 subharmonic regime, then
an abrupt transition to deterministic chaos, and then to a
0.33f0 subharmonic regime. Comparing spectra a and b
illustrates the increase in spectral complexity resulting from the addition of
subharmonic values and their harmonics (b in B). (C) A 10 ms section of
deterministic chaos (in this case, a low-dimensional noise, generated using a
Rossler attractor equation). The aperiodicity of the sound waveform (V) and
the sound energy fairly evenly distributed across the entire spectrum
(spectrogram C, and power spectrum, c) are indicators used to identify
potential chaos in mockingbird songs. Abbreviations as in Tables
1 and
2; FJ, frequency jump; SH,
subharmonics; CH, deterministic chaos; 1f0, single
frequency sounds; V, amplitude of sound waveform.
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Fig. 3. Examples of frequency jumps and subharmonics in mockingbird song (bird
m123). (A) Frequency jumps occurred with airflow through only the right (arrow
a) or left (arrow c) side of the syrinx. Arrow b indicates a shift from a
0.5f0 to 0.25f0 subharmonic regime.
Arrow c indicates a shift between a 0.33f0 and
0.5f0 subharmonic regime. (B) Expanded views of the sound
waveform at each arrow in spectrogram (A), showing the abrupt changes in
oscillation patterns at bifurcation points. (C) Power spectrum taken at arrow
b, showing spectral peaks at f0 and its harmonics
(2f0, 3f0, etc.), as well as at
fractional integer values corresponding with a 0.25f0
subharmonic, and its harmonics. fL and
fR, rate of airflow through left and right bronchus,
respectively. Airflow associated with positive pressure is expiratory, and
that associated with negative pressure is inspiratory. P, pressure in the
cranial thoracic air sac; V, oscillogram of vocalization (sound waveform).
Horizontal lines in (A) indicate ambient pressure or zero airflow.
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Fig. 4. Normalized rates of bronchial airflow 25 ms before to 5 ms after a
frequency jump (A) or the onset of subharmonics (B). Bifurcations occurred at
time 0. Slopes of linear regressions over three different 5 ms periods were
measured (25–20 ms before, 5–0 ms before, and 0–5 ms after
bifurcation points, indicated by vertical lines). (A) Rates of bronchial
airflow show greater variance in their slope 0–5 ms before and 0–5
ms after a frequency jump than at an earlier point in the syllable, where no
NLP were observed (failed Levene's test of equal variances below the 5% level,
P<0.05; variance at 25–20 ms prior to bifurcation,
 2=0.8; 5–0 ms prior, 2=7.7;
0–5 ms after, 2=7.2). Airflow was normalized to
percent of maximum flow rate during syllable. Grey lines, upward jump in
frequency; black lines, downward jump. (B) Rate of bronchial airflow
25–20 ms prior, 5–0 ms prior and 0–5 ms after onset of
subharmonics did not show significant differences in slope (passed Levene's
test for equal variances well above the 5% level, P=0.21).
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Fig. 5. Unilaterally produced subharmonics and deterministic chaos (mockingbird
m152). (A) Arrows indicate abrupt transitions from a harmonic vocalization to
a chaotic sound. The chaotic region is followed by a period of subharmonics
(arrow b) after which the vocalization returns to a periodic state. (B) A 25
ms segment of the sound waveform. The arrow indicates an abrupt transition
from a periodic to aperiodic oscillation. (C) Power spectrum taken at arrow b
(spectrogram, A). Spectral peaks show sound energy at f0
and associated harmonics (2f0, 3f0,
etc) as well as at 0.5f0 and related harmonics
(1.5f0, 2.5f0,
3.5f0, etc.) Abbreviations as in
Fig. 3.
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Fig. 6. Unilateral biphonation in two syllables produced by mockingbird m130. (A)
Two independent frequencies produced by a single side of the syrinx. Arrows a
and b indicate biphonic sounds concurrent with airflow through only the left
side of the syrinx. Similar, but less dramatic, biphonation events also occur
during the first syllable in A. At arrow a, sound energy is present at 990 Hz
(f0) and 1505 Hz (g0), and their
harmonics, 2f0 (1980 Hz) and 2g0 (3010
Hz). (B) An expanded view of the sound waveform at arrow a (spectrogram, A),
showing a change from the biphonation event to a single f0
tone. (C) Power spectrum taken at arrow b, in spectrogram (A), showing the two
fundamental frequencies (f0 and g0),
as well as sound energy at various linear combinations of the two
fundamentals. Abbreviations as in Fig.
3.
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Fig. 7. Unilateral biphonation and `dual biphonation' in mockingbird song. (A)
Spectrogram of two syllables produced in sequence by mockingbird m108. The
first syllable in the pair (a) is an example of unilateral biphonation. This
syllable is produced with the left side only, but two independent frequencies
are present, the fundamental frequency (f0) at  1800
Hz, and a lower, modulating frequency (m0) visible as
sidebands 115 Hz above and below f0. The bird adds
phonation from the right side of the syrinx in second syllable (b), and a
second fundamental (g0) appears, also with sidebands 115
Hz above and below, indicating that g0 is also modulated
by m0. This is a two-voice syllable in which each voice is
biphonic. (B) Expanded view of the sound waveform at arrows a and b in
spectrogram (A), showing the AM pattern on the waveforms at a rate of 115
Hz (period of one modulation cycle 11.5 ms) both during unilateral flow
(a) and bilateral flow (b). In addition to the 115 Hz modulation pattern, the
two-voiced sound exhibits a second pattern in the waveform, which is likely
the result of beating between f0 and
g0. A beat frequency is equal to the difference between
f0 and g0, in this example, the second
modulation rate in b is approximately equal to 550, which corresponds with the
difference frequency between g0 and f0
(2695–2135 Hz). (C) Power spectra taken at arrows a and b in (A). Sound
was filtered with a digital 800 Hz Hanning shape high pass filter. Peaks
labeled correspond with fundamental frequencies as well as the sidebands
resulting from the interaction of modulating frequency
(m0) with the carrier (f0) in a, and
with the two carrier frequencies (f0 and
g0) in b. Abbreviations as in
Fig. 3.
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Fig. 8. (A) Dual biphonation (two-voice phenomenon with biphonation in both
voices). The sound at the arrow consists of two fundamental frequencies
(f0 and g0), both simultaneously
modulated by a third frequency (m0). Although
f0 and g0 are modulated in opposite
patterns in the frequency domain (one downsweeping and the other upsweeping in
frequency), the rate of amplitude modulation frequency m0
is the same (approx. 250 Hz) for both, as evidenced by evenly spaced sidebands
the same distance from both f0 and g0.
(B) An expanded view of the sound waveform showing the pattern of amplitude
modulation. (C) Power spectrum taken at arrow a in spectrogram (A).
Abbreviations as in Fig. 3.
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Fig. 9. Unequal contribution of right and left sides to bilaterally produced chaos.
Chaotic sounds often contained harmonic windows, or brief moments of periodic
oscillation, such as that seen around arrow b. Mockingbird `loud hew' calls
(A) were always produced with flow through both sides of the syrinx for the
entire duration of the call. In most cases (56 of 67 calls) one side alone
appeared to contribute the majority of the aperiodicity in the call, while the
contribution of the other side is more pure-tonal. (B) The difference in
aperiodic behavior of the two sides is shown by examination of sound recorded
inside the right and left bronchi and cranial thoracic air sac (obtained by
high-pass filtering and amplifying thermistor and pressure transducer
outputs). (C) 20 ms segment of the sound waveform, at the time indicated by
arrow a in the spectrogram, illustrating the aperiodic nature of the
oscillation. (D) Power spectrum of the sound taken at arrow a in the
spectrogram (A). Additionally, the airflow in the right side
(FR) shows an aperiodic, rapid modulation, which are not
present in the flow on the left (fL). Other abbreviations
as in Fig. 3.
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© The Company of Biologists Ltd 2008
