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N E U R O S C I E N C E
Left Hemisphere Cerebral
Specialization for Babies While
Babbling
Siobhan Holowka1 and Laura Ann Petitto1,2*
Baby babbling is the universal developmental
milestone before the onset of language production
in humans, yet little is known about
whether the neural determinants of this behavior
are fundamentally linguistic (1, 2) or
reflect only oral-motor developments(3, 4).
In adults, the presence of right asymmetry in
mouth aperture during linguistic tasks as
contrasted with left or equal mouth opening
during nonlinguistic tasks has been widely
used as a key measure of left hemisphere
cerebral specialization for language (5).
Given the noninvasive nature of
mouth asymmetry studies, this
technique is ideally suited to inferring
whether functionalcerebral
asymmetries of babies’ earliest
productions exist. If babbling
is fundamentally linguistic
in nature, then left hemispheric
specialization should be reflected
in right mouth asymmetry
while babbling. If babbling is
fundamentally motoric in nature,
then equal hemispheric
participation should be reflected
in equal mouth opening while
babbling. The results will provide
insightinto the neural basis
of babbling and hence into the origins of
human language.
To control for any language-specific effects
of mouth asymmetry, we videotaped 10 babies
acquiring either English (n
5) or French (n

5). The babies were studied between the ages of
5 and12 months, according to the age at which
each baby first entered the syllabic babbling
stage. Once this developmentalmilestone was
achieved, we examined three types of oral activity
produced by the babies: babbles, nonbabbles,
and smiles (table S1). Babbles were
defined as vocalizations that contained a reduced
subset of possible sounds (phonetic
units) found in spoken language, had reduplicated
(repeated) syllabic organization (consonant-
vowel alternations), and were produced
without apparent meaning orreference; all vocalizations
lacking any of these three criteria
were coded as nonbabbles. Spontaneous smiles
were coded as an additional control of babies’
specificity of mouth opening for distinctive
types of oral activity (5).
At 50 ms (three video frames) from initial lip
opening, two “blind” independent coders scored
150 randomly selected segments of babbles,
nonbabbles, and smilesaccording to whether
greater right, left, or equal mouth opening was
observed. A standard Laterality Index (LI) (5)
was computed for each baby for their production
of babbles, nonbabbles, and smiles: LI
(R 
L/R  L  E), and mean LI scores were calculated
for each group of babies (English and
French). Thus, a mean positive LI score indicated
more instances of right mouth opening, and amean negative LI score indicated more instances
of left mouth opening for the given production.
The mean LI scores indicated that all babies
had right mouth asymmetry while babbling,
equal mouth opening while nonbabbles were
produced, and left mouth asymmetry while
smiling (Fig. 1). Statistical analysis was performed
with a two-way mixed analysis of variance:
group (English and French) production
(babble, nonbabble, and smile). No significant
effect of group was detected (F
0.09, NS),
indicating that no significant differences were
found between the English and French babies
(table S1). A significant main effect was discovered
for production (F
236.91, P 
0.001), and all pair-wise comparisons were significant
(P  0.001), indicating that the babies’
mouth openingdiffered depending on whether
a babble, nonbabble, or smile was produced
(Videos S1 to S3).
The origins of language in humans have
remained elusive as a result of controversy
over the neural basis of babbling. Like adults,
the right mouth asymmetry observed in babies
suggests left hemisphere asymmetry for
babbling, reflecting the human left hemisphere
control of natural language. If...