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Abstract:
Tracing the evolution of speech includes understanding the
vocalizations of the earliest hominids. Assuming that their
repertoire resembled those of extant monkeys and apes, it is of
great interest to note that the source-filter model developed for
human speech production also provides a useful framework for
sounds of other primates. As in humans, some vocalizations reveal
the imprint of supralaryngeal resonances. Such effects are
particularly evident when calls include either a rich harmonic
spectrum resulting from stable, periodic vocal-fold vibration, or
show noisier, but relatively stable broad-band characteristics.
In contrast, many vocalizations do not show evidence of
supralaryngeal filtering, even though they are periodic, noisy,
or a mix. In some cases, filtering effects may be disappearing
because vocal-fold vibration rate is simply too high--fundamental
frequency in nonhuman primate routinely varies by an order of
magnitude. However, supralaryngeal effects are also lost in
high-amplitude sounds that show prominent nonlinear effects--for
instance including sidebands and otherinteraction products due to
amplitude modulation, vocal-fold desynchronization, or likely
deterministic chaos. These observations suggest that calls should
understood by viewing the vocal-folds as a nonlinear system
composed of coupled, driven oscillators. Under some production
conditions, vibration is stable, stays within a narrow range of
modes, and provides a good medium for "displaying" the
supralaryngeal filter. Resulting calls resemble normative speech
sounds like vowels, and their formants are highly perceptible to
both monkeys and apes. These vocalizations are stable and
well-suited for use in stable categories. Like human vowels, they
are rich in cues to vocalizer characteristics, and likely used
when individuality or phonetic-like signaling is central. Under
other production conditions, however, vocal-fold vibration is
strongly nonlinear, highly unstable, and shifts abruptly among
different modes. Resulting calls are similar to crying and
shrieking of human infants and children-sounds that few adult
humans can produce. While perceptual evidence is limited,
acoustic features in these vocalizations seem extremely salient
and aversive. These calls are well-suited to circumstances in
which sounds function to draw the attention of others or to
provoke auditory discomfort or annoyance. These effects may be
most useful to younger, smaller, or more socially impotent
individuals who use them to shape the behavior of larger, more
powerful caretakers and other group members.
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