language even though they can move their lips and tongues.
They understand spoken language and know what they want
to say but have trouble forming words and putting them into
The potential for the development of language-speciﬁ c
mechanisms in the left hemisphere is present at birth, but
the assignment of language functions to speciﬁ c brain areas
is fairly ﬂ exible in the early years of life. Thus, for example,
damage to the language areas of the left hemisphere dur-
ing infancy or early childhood causes temporary, minor lan-
guage impairment until the right hemisphere can take over.
However, similar damage acquired during adulthood typically
causes permanent, devastating language deﬁ cits. By puberty,
the brain’s ability to transfer language functions to the right
hemisphere is less successful, and often language skills are lost
Differences between the two hemispheres are usually
masked by the integration that occurs via the corpus callosum
and other pathways that connect the two sides of the brain.
However, the separate functions of the left and right hemi-
spheres have been uncovered by studying patients in which
the two hemispheres have been separated surgically for treat-
ment of severe epilepsy. These so-called
participated in studies in which they were asked to hold and
Areas found clinically to be involved in the comprehension (Wernicke’s area) and motor (Broca’s area) aspects of language. Blue lines indicate
divisions of the cortex into frontal, parietal, temporal, and occipital lobes.
PET scans reveal areas of increased blood ﬂ ow in speciﬁ c parts of
the temporal, occipital, parietal, and frontal lobes during various
Courtesy of Dr. Marcus E. Raichle.
Images of the active areas of the brain in a male (left) and a female
(right) during a language task. Note that both sides of the woman’s
brain are used in processing language, but the man’s brain is more
Shaywitz et al., 1995 NMR Research/Yale Medical School.