Speech and the Brain
Dr. C. George Boeree
The brain is divided into two halves, a left hemisphere and a right
hemisphere. This is called lateralization,
and applies to any animal further up the evolutionary tree than, say, a
worm. In animals that are particularly vocal, such as canaries,
dolphins, and chimpanzees, it seems that one hemisphere or another is
dedicated to controlling those behaviors and the responses to
In human beings, it is the left hemisphere that usually contains the
specialized language areas. While this holds true for 97% of
right-handed people, about 19% of left-handed people have their
language areas in the right hemisphere and as many as 68% of them have
some language abilities in both the left and the right hemispheres.
Lateralization was first discovered in the 1800's by physicians (such
as Broca and Wernicke, who we will discuss in a bit) who did autopsies
on patients who had had several language difficulties before their
deaths. These physicians found damage to particular areas of the
brain now named after them, and these areas were consistently on the
These discoveries were later confirmed in the 1950's by researchers,
such as Wilder Penfield and Herbert Jasper. Brain surgery is
normally done using only local anesthesia. The patient is sedated
but not unconscious. This is done in part in order to be able to
keep tabs on the patient's experiences. Penfield and Jasper took
advantage of these patients (with their permission, of course) and
stimulated certain areas electrically. When asked questions, the
patients were unable to reply during te stimulation of the left
hemisphere, but had no problem responding during stimulation of
the right hemisphere.
In the 1960's, another, rather bizarre, set of experiments involved
using sodium amytal, a very fast-acting anesthetic. Researchers
were able to put one or the other hemisphere of volunteers to sleep by
injecting the anesthetic into either the right or left carotid artery
(which supply blood to the same-side hemispheres). Lo and behold,
the patients who had the left hemisphere put to sleep did not respond
Most recently, researchers have taken advantage of the huge advances
made in brain imaging. In particular, the PET scan ("positron emission
tomography") provides a computer with the information needed to
construct a three dimensional map of a persons brain including the
relative activity of different areas. PET scans involve injecting
someone with a radioactive glucose solution. Since active areas
of the brain use more energy, and therefore more glucose, they release
more radiation, which the computer translates into "warmer" colors such
as yellow and red. Areas that are less active are shown with
"cooler" colors such as green and blue. As by now you should
expect, certain areas of the left hemisphere were more active while
people were engaged in linguistic activities.
Most early studies were done on mature male volunteers. Since
then, scientists have gone on to study female volunteers and even
children. Interestingly, women seem to be slightly less
"lateralized" than men, being more likely to use portions of the right
hemisphere as well as the left.
Studies of children have provided some fascinating information:
If a child has damage to the left hemisphere, the child may develop
language in the right hemisphere instead. The younger the child,
the better the recovery. So, although the "natural" tendency is
for language to develop on the left, our brains are capable of adapting
to difficult circumstances, if the damage occurs early enough.
The first language area within the left hemisphere to be discovered is
called Broca's Area, after Paul Broca. Broca was a French
neurologist who had a patient with severe language
problems: Although he could understand the speech of others with
little difficulty, the only word he could produce was "tan."
Because of this, Broca gave the patient the pseudonym "Tan."
After the patient died, Broca performed an autopsy, and discovered that
an area of the frontal lobe, just ahead of the motor cortex controlling
the mouth, had been seriously damaged. He correctly hypothesized
that this area was responsible for speech production.
Physicians called the inability to speak aphasia, and the inability to
produce speech was therefore called Broca's
aphasia, or expressive aphasia.
Someone with this kind of aphasia has little problem understanding
speech. But when trying to speak themselves are capable only of
slow, laborious, often slurred sequences of words. They don't
produce complete sentences, seldom use regular grammatical endings such
as -ed for the past tense, and tend to leave out small grammatical
It turns out that Broca's area is not just a matter of getting language
out in a motor sense, though. It seems to be more generally
involved in the ability to deal with grammar itself, at least the more
complex aspects of grammar. For example, when they hear sentences
that are put into a passive form, they often misunderstand: If
you say "the boy was slapped by the girl," they may understand you as
communicating that the boy slapped the girl instead.
The second language area to be discovered is called Wernicke's Area, after Carl
Wernicke, a German neurologist. Wernicke had a patient who could
speak quite well, but was unable to understand the speech of
others. After the patient's death, Wernicke performed an autopsy
and found damage to an area at the upper portion of the temporal lobe,
just behind the auditory cortex. He correctly hypothesized that
this area was responsible for speech comprehension.
This kind of aphasia is known as Wernicke's
Aphasia, or receptive aphasia.
When you ask a person with this problem a question, they will respond
with a sentence that is more or less grammatical, but which contains
words that have little to do with the question or, for that matter,
with each other. Strange, meaningless, but grammatical sentences
come forth, a phenomenon called "word salad."
Like Broca's area is not just about speech production, Wernicke's is
not just about speech comprehension. People with Wernicke's
Aphasia also have difficulty naming things, often responding with words
that sound similar, or the names of related things, as if they are
having a very hard time with their mental "dictionaries."
Despite the fact that Broca's and Wernicke's Areas are in different
lobes, they are actually quite near each other and intimately connected
by a tract of nerves called the arcuate
fasciculus. There are also people who have damage to the
arcuate fasciculus, which results in an aphasia known as conduction aphasia. These
people have it a bit better than other aphasias: They can
understand speech, and they can (although with difficulty) produce
coherent speech, they cannot repeat words or sentences that they hear.
Reading and writing are a part of language as well, of course.
But since these skills have only been around a few thousand years, they
are not as clearly marked in terms of brain functioning as the basic
comprehension and production areas. But there is an area of the
brain called the angular gyrus
that lies about halfway between Wernicke's area and the visual cortex
of the occipital lobe. It was discovered, after a young patient
with reading problems died and his brain was examined during
autopsy. The angular gyrus showed significant abnormalities.
The angular gyrus has been implicated in problems such as alexia (the inability to read), dyslexia (difficulties with
reading), and agraphia (the
inability to write). In research involving the use of PET scans
on people with these problems, the angular gyrus is not as active as it
is in other people while engaged in reading or writing. However,
problems such as dyslexia also can involve other areas of the brain, or
not involve brain disorders at all.
© Copyright 2004, C. George Boeree