Structure
and Functions of the Brain
The brain is divided into
three main parts: the hindbrain, the midbrain, and the forebrain.
The
Hindbrain
The hindbrain is
composed of the medulla, the pons, and the cerebellum. The medulla lies
next to the spinal cord and controls functions outside conscious control, such
as breathing and blood flow. In other words, the medulla controls essential
functions. The pons affects activities such as sleeping,
waking, and dreaming. The cerebellum controls balance and
coordination of movement. Damage to the cerebellum impairs fine motor skills,
so a person with an injury in this area would have trouble playing the guitar
or typing a term paper.
The
Midbrain
The midbrain is
the part of the brain that lies between the hindbrain and the forebrain. The
midbrain helps us to locate events in space. It also contains a system of
neurons that releases the neurotransmitter dopamine. The reticular
formationruns through the hindbrain and the midbrain and is involved in
sleep and wakefulness, pain perception, breathing, and muscle reflexes.
The
Forebrain
The biggest and most
complex part of the brain is the forebrain, which includes the
thalamus, the hypothalamus, the limbic system, and the cerebrum.
Thalamus
The thalamus is
a sensory way station. All sensory information except smell-related data must
go through the thalamus on the way to the cerebrum.
Hypothalamus
The hypothalamus lies
under the thalamus and helps to control the pituitary gland and the autonomic
nervous system. The hypothalamus plays an important role in regulating body
temperature and biological drives such as hunger, thirst, sex, and aggression.
Limbic
System
The limbic system includes
thehippocampus, the amygdala, and the septum. Parts of the
limbic system also lie in the thalamus and the hypothalamus. The limbic system
processes emotional experience. The amygdala plays a role in aggression and
fear, while the hippocampus plays a role in memory.
Cerebrum
The cerebrum,
the biggest part of the brain, controls complex processes such as abstract
thought and learning. The wrinkled, highly folded outer layer of the cerebrum
is called the cerebral cortex. The corpus callosum is a band of
fibers that runs along the cerebrum from the front of the skull to the back. It
divides the cerebrum into two halves, or hemispheres. Each hemisphere is
divided into four lobes or segments: the occipital lobe, the parietal lobe, the
temporal lobe, and the frontal lobe:
·
The occipital lobe contains the primary
visual cortex, which handles visual information.
·
The parietal lobe contains the primary
somatosensory cortex, which handles information related to the sense of touch.
The parietal lobe also plays a part in sensing body position and integrating
visual information.
·
The temporal lobe contains the primary
auditory cortex, which is involved in processing auditory information. The left
temporal lobe also contains Wernicke’s area, a part of the brain
involved in language comprehension.
·
The frontal lobe contains the primary motor
cortex, which controls muscle movement. The left frontal lobe contains Broca’s
area, which influences speech production. The frontal lobe also processes
memory, planning, goal-setting, creativity, rational decision making, and
social judgment.
Brain
Hemispheres
Lateralization refers
to the fact that the right and left hemispheres of the brain regulate different
functions. The left hemisphere specializes in verbal processing tasks such as
writing, reading, and talking. The right hemisphere specializes in nonverbal
processing tasks such as playing music, drawing, and recognizing childhood
friends.
Roger Sperry,
Michael Gazzaniga, and their colleagues conducted some of the early research in
lateralization. They examined people who had gone through split-brain
surgery, an operation done to cut the corpus callosum and separate the two
brain hemispheres. Doctors sometimes use split-brain surgery as a treatment for
epileptic seizures.
Control
of the Body
Because of the organization
of the nervous system, the left hemisphere of the brain controls the
functioning of the right side of the body. Likewise, the right hemisphere
controls the functioning of the left side of the body.
Vision and hearing operate
a bit differently. What the left eye and right eye see goes to the entire
brain. However, images in the left visual field stimulate receptors on the
right side of each eye, and in-formation goes from those points to the right
hemisphere. Information perceived by the right visual field ends up in the left
hemisphere.
In the case of auditory
information, both hemispheres receive input about what each ear hears. However,
information first goes to the opposite hemisphere. If the left ear hears a
sound, the right hemisphere registers the sound first.
The fact that the brain’s
hemispheres communicate with opposite sides of the body does not affect most
people’s day-to-day functioning because the two hemispheres constantly share
information via the corpus callosum. However, severing the corpus callosum and
separating the hemispheres causes impaired perception.
Split-Brain
Studies
If a researcher presented a
picture of a Frisbee to a split-brain patient’s right visual field, information
about the Frisbee would go to his left hemisphere. Because language functions
reside in the left hemisphere, he’d be able to say that he saw a Frisbee and
describe it. However, if the researcher presented the Frisbee to the patient’s
left visual field, information about it would go to his right hemisphere.
Because his right hemisphere can’t communicate with his left hemisphere when
the corpus callosum is cut, the patient would not be able to name or describe
the Frisbee.
The same phenomenon occurs
if the Frisbee is hidden from sight and placed in the patient’s left hand,
which communicates with the right hemisphere. When the Frisbee is in the
patient’s left visual field or in his left hand, the patient may not be able to
say what it is, although he would be able to point to a picture of what he saw.
Picture recognition requires no verbal language and is also a visual-spatial
task, which the right hemisphere controls.
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