The brain is divided into three main parts: the hindbrain, the midbrain, and the forebrain.
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 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 formation runs through the hindbrain and the midbrain and is involved in sleep and wakefulness, pain perception, breathing, and muscle reflexes.
The biggest and most complex part of the brain is the forebrain, which includes the thalamus, the hypothalamus, the limbic system, and the cerebrum.
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.
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.
The limbic system includes the hippocampus, 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.
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:
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.
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.
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.