variation (Green 1975). Much of this work has focused on two particular
subtypes of coo vocalizations, the smooth-early-high (SEH) and smoothlate-high (SLH), so-called because of the relative position of the peak of
one frequency component that sweeps up and then down over the course
of the call. Although Japanese macaque mothers can discriminate the coos
of their young from others (Pereira 1986), the role of individual recognition cues in the coo vocalizations has not been well-studied. Nevertheless,
the SEH and SLH call types function in different behavioral contexts
(Green 1975), and Japanese macaques possess a species-specific bias for discriminations involving these coos when the relevant variation is in the relative timing of the FM peak (Zoloth et al. 1979).
Although initial data suggested that the SEH and SLH calls are perceived
categorically (May et al. 1989), more recent data from the field indicate that
many adult female coo vocalizations have FM peaks within the ambiguous
zone between “early” and “late” prototypes (Owren and Casale 1994). The
categorical boundaries determined in laboratory tests do not coincide with
natural variation in the distribution of calls in the field. Nonetheless, all data
to date show clear evidence that the coo calls are perceived as perceptually
distinct classes (if not categorically), and several studies now substantiate
the notion that the relative position of the FM peak within the coo is the
most salient cue to discrimination among different coos (May et al. 1988;
Le Prell and Moody 2000). Amplitude cues also appear to function in call
discrimination (Le Prell and Moody 1997). Thus, the stimulus dimensions
involved in “real-world” classification and/or categorization of coo calls
may involve a more complex stimulus space than that suggested by the original categorical perception studies.
Interestingly, lesions to the left, but not the right, superior temporal
gyrus impair discrimination between the SEH and SLH coos by Japanese
macaques, suggesting a human language-like hemispheric dominance
(Heffner and Heffner 1984). Similarly, behavioral tests among free-ranging
rhesus macaques, Macaca mulatta, indicate a left hemisphere dominance for
the processing of conspecific compared to heterospecific vocalizations that
is present in adults but not infants (Hauser and Andersson 1994). Apart
from their comparative value in understanding the evolutionary origin of
human cognition, the lateralization of specific cognitive functions can in
theory allow for identification of at least some of the brain regions mediating these behaviors. Comparisons of activation in each hemisphere during
tasks that call on lateralized functions, using either fMRI or immediate early
gene-expression techniques, should yield regions of differential activity in
the appropriate hemisphere worthy of closer study.
3.4.2. Primate Auditory System: Cortical Anatomy and Physiology
The mammalian auditory cortex extends across the superior temporal plane
and onto the adjacent superior temporal gyrus. The primary auditory cortex
in primates can be divided into separate core and belt areas on the basis of
7. Neuroethology of Vocal Communication
365
subtypes of coo vocalizations, the smooth-early-high (SEH) and smoothlate-high (SLH), so-called because of the relative position of the peak of
one frequency component that sweeps up and then down over the course
of the call. Although Japanese macaque mothers can discriminate the coos
of their young from others (Pereira 1986), the role of individual recognition cues in the coo vocalizations has not been well-studied. Nevertheless,
the SEH and SLH call types function in different behavioral contexts
(Green 1975), and Japanese macaques possess a species-specific bias for discriminations involving these coos when the relevant variation is in the relative timing of the FM peak (Zoloth et al. 1979).
Although initial data suggested that the SEH and SLH calls are perceived
categorically (May et al. 1989), more recent data from the field indicate that
many adult female coo vocalizations have FM peaks within the ambiguous
zone between “early” and “late” prototypes (Owren and Casale 1994). The
categorical boundaries determined in laboratory tests do not coincide with
natural variation in the distribution of calls in the field. Nonetheless, all data
to date show clear evidence that the coo calls are perceived as perceptually
distinct classes (if not categorically), and several studies now substantiate
the notion that the relative position of the FM peak within the coo is the
most salient cue to discrimination among different coos (May et al. 1988;
Le Prell and Moody 2000). Amplitude cues also appear to function in call
discrimination (Le Prell and Moody 1997). Thus, the stimulus dimensions
involved in “real-world” classification and/or categorization of coo calls
may involve a more complex stimulus space than that suggested by the original categorical perception studies.
Interestingly, lesions to the left, but not the right, superior temporal
gyrus impair discrimination between the SEH and SLH coos by Japanese
macaques, suggesting a human language-like hemispheric dominance
(Heffner and Heffner 1984). Similarly, behavioral tests among free-ranging
rhesus macaques, Macaca mulatta, indicate a left hemisphere dominance for
the processing of conspecific compared to heterospecific vocalizations that
is present in adults but not infants (Hauser and Andersson 1994). Apart
from their comparative value in understanding the evolutionary origin of
human cognition, the lateralization of specific cognitive functions can in
theory allow for identification of at least some of the brain regions mediating these behaviors. Comparisons of activation in each hemisphere during
tasks that call on lateralized functions, using either fMRI or immediate early
gene-expression techniques, should yield regions of differential activity in
the appropriate hemisphere worthy of closer study.
3.4.2. Primate Auditory System: Cortical Anatomy and Physiology
The mammalian auditory cortex extends across the superior temporal plane
and onto the adjacent superior temporal gyrus. The primary auditory cortex
in primates can be divided into separate core and belt areas on the basis of
7. Neuroethology of Vocal Communication
365