and in some cases of damage to the left hemisphere,evidence indicates
that the right hemisphere can take on a number of significant linguistic
functions.Similarly,studies show that musicians with perfect pitch evi-
dence left hemisphere dominance during music perception.
Recent electroencephalographic data suggest that the right hemi-
sphere may play a dominant role in negative-withdrawal emotion
whereas the left hemisphere appears dominant for positive-approach
emotion (Davidson 1992,1995;see Lee et al.1990 for the reverse pattern
of emotional valence and hemisphere bias using epileptic patients
receiving intracarotid administration of sodium amobarbital).Thus,for
example,when people are given explicit instructions to move their face
into a Duchenne smile—what Ekman and colleagues (1988,1990) con-
sider to be the only true or honest smile—they have far greater left hemi-
sphere activation than right (Ekman,Davidson,and Friesen 1990).In
contrast,Gazzaniga and Smiley (1991) provided important information
on split-brain patients who have much greater asymmetries in smiling on
the left side of the face than on the right.
If we are to understand how and why hemispheric specialization
evolved,it is important to look more closely at the neural specializations
of our closest living relatives,monkeys and apes.This movement has
begun thanks in part to MacNeilage,Studdert-Kennedy,and Lindblom
(1987) who critically examined the evidence for hand preferences in non-
human primates.This work (see also updated review by MacNeilage
1991) demonstrated that individuals in several nonhuman primate
species preferentially use one hand more than the other in both uni-
manual and bimanual tasks (Ward and Hopkins 1993).For most
monkeys,the left hand appears dominant,whereas some studies of apes
reveal right hand dominance.Data on handedness,coupled with work
on asymmetries in nonhuman primate neuroanatomy (Falk 1987;
Heilbroner and Holloway 1988;Perrett et al.1988;Falk et al.1990;
Cheverud et al.1991) and cognitive function (Hopkins,Washburn,and
Rumbaugh 1990;Hamilton and Vermeire 1991;Hopkins,Morris,and
Savage-Rumbaugh 1991;Vauclair,Fagot,and Hopkins 1993),are impor-
tant in that they provide insights into the phylogenetic precursors of
human hemispheric specialization (for recent synthetic discussions of
this point,see Bradshaw and Rogers 1993;Hauser 1996).Call Perception
One of the earliest attempts to assess hemispheric biases in acoustic per-
ception in primates took advantage of a detailed field study and psy-
chophysical techniques.The empirical foundation for this research was
Green’s (1975) in-depth analysis of wild Japanese macaque vocalizations,
in particular,their coo.This call type is acoustically variable,with much89 Primate Vocalizations in Emotion and Thought