The Brain and Spinal Cord

Forebrain Structures

The two hemispheres of the cerebral cortex are part of the forebrain (Figure), which is the largest part of the brain. The forebrain contains the cerebral cortex and a number of other structures that lie beneath the cortex (called subcortical structures): thalamus, hypothalamus, pituitary gland, and the limbic system (collection of structures). The cerebral cortex, which is the outer surface of the brain, is associated with higher level processes such as consciousness, thought, emotion, reasoning, language, and memory. Each cerebral hemisphere can be subdivided into four lobes, each associated with different functions.

An illustration shows the position and size of the forebrain (the largest portion), midbrain (a small central portion), and hindbrain (a portion in the lower back part of the brain).
The brain and its parts can be divided into three main categories: the forebrain, midbrain, and hindbrain.

Lobes of the Brain

The four lobes of the brain are the frontal, parietal, temporal, and occipital lobes (Figure). The frontal lobe is located in the forward part of the brain, extending back to a fissure known as the central sulcus. The frontal lobe is involved in reasoning, motor control, emotion, and language. It contains the motor cortex, which is involved in planning and coordinating movement; the prefrontal cortex, which is responsible for higher-level cognitive functioning; and Broca’s area, which is essential for language production.

An illustration shows the four lobes of the brain.
The lobes of the brain are shown.

People who suffer damage to Broca’s area have great difficulty producing language of any form (Figure). For example, Padma was an electrical engineer who was socially active and a caring, involved mother. About twenty years ago, she was in a car accident and suffered damage to her Broca’s area. She completely lost the ability to speak and form any kind of meaningful language. There is nothing wrong with her mouth or her vocal cords, but she is unable to produce words. She can follow directions but can’t respond verbally, and she can read but no longer write. She can do routine tasks like running to the market to buy milk, but she could not communicate verbally if a situation called for it.

Probably the most famous case of frontal lobe damage is that of a man by the name of Phineas Gage. On September 13, 1848, Gage (age 25) was working as a railroad foreman in Vermont. He and his crew were using an iron rod to tamp explosives down into a blasting hole to remove rock along the railway’s path. Unfortunately, the iron rod created a spark and caused the rod to explode out of the blasting hole, into Gage’s face, and through his skull (Figure). Although lying in a pool of his own blood with brain matter emerging from his head, Gage was conscious and able to get up, walk, and speak. But in the months following his accident, people noticed that his personality had changed. Many of his friends described him as no longer being himself. Before the accident, it was said that Gage was a well-mannered, soft-spoken man, but he began to behave in odd and inappropriate ways after the accident. Such changes in personality would be consistent with loss of impulse control—a frontal lobe function.

Beyond the damage to the frontal lobe itself, subsequent investigations into the rod's path also identified probable damage to pathways between the frontal lobe and other brain structures, including the limbic system. With connections between the planning functions of the frontal lobe and the emotional processes of the limbic system severed, Gage had difficulty controlling his emotional impulses.

However, there is some evidence suggesting that the dramatic changes in Gage’s personality were exaggerated and embellished. Gage's case occurred in the midst of a 19th century debate over localization—regarding whether certain areas of the brain are associated with particular functions. On the basis of extremely limited information about Gage, the extent of his injury, and his life before and after the accident, scientists tended to find support for their own views, on whichever side of the debate they fell (Macmillan, 1999).

Image (a) is a photograph of Phineas Gage holding a metal rod. Image (b) is an illustration of a skull with a metal rod passing through it from the cheek area to the top of the skull.
(a) Phineas Gage holds the iron rod that penetrated his skull in an 1848 railroad construction accident. (b) Gage’s prefrontal cortex was severely damaged in the left hemisphere. The rod entered Gage’s face on the left side, passed behind his eye, and exited through the top of his skull, before landing about 80 feet away. (credit a: modification of work by Jack and Beverly Wilgus)

The brain’s parietal lobe is located immediately behind the frontal lobe, and is involved in processing information from the body’s senses. It contains the somatosensory cortex, which is essential for processing sensory information from across the body, such as touch, temperature, and pain. The somatosensory cortex is organized topographically, which means that spatial relationships that exist in the body are maintained on the surface of the somatosensory cortex (Figure). For example, the portion of the cortex that processes sensory information from the hand is adjacent to the portion that processes information from the wrist.

A diagram shows the organization in the somatosensory cortex, with functions for these parts in this proximal sequential order: toes, ankles, knees, hips, trunk, shoulders, elbows, wrists, hands, fingers, thumbs, neck, eyebrows and eyelids, eyeballs, face, lips, jaw, tongue, salivation, chewing, and swallowing.
Spatial relationships in the body are mirrored in the organization of the somatosensory cortex.

The temporal lobe is located on the side of the head (temporal means “near the temples”), and is associated with hearing, memory, emotion, and some aspects of language. The auditory cortex, the main area responsible for processing auditory information, is located within the temporal lobe. Wernicke’s area, important for speech comprehension, is also located here. Whereas individuals with damage to Broca’s area have difficulty producing language, those with damage to Wernicke’s area can produce sensible language, but they are unable to understand it (Figure).

An illustration shows the locations of Broca’s and Wernicke’s areas.
Damage to either Broca’s area or Wernicke’s area can result in language deficits. The types of deficits are very different, however, depending on which area is affected.

The occipital lobe is located at the very back of the brain, and contains the primary visual cortex, which is responsible for interpreting incoming visual information. The occipital cortex is organized retinotopically, which means there is a close relationship between the position of an object in a person’s visual field and the position of that object’s representation on the cortex. You will learn much more about how visual information is processed in the occipital lobe when you study sensation and perception.

Other Areas of the Forebrain

Other areas of the forebrain, located beneath the cerebral cortex, include the thalamus and the limbic system. The thalamus is a sensory relay for the brain. All of our senses, with the exception of smell, are routed through the thalamus before being directed to other areas of the brain for processing (Figure).

An illustration shows the location of the thalamus in the brain.
The thalamus serves as the relay center of the brain where most senses are routed for processing.

The limbic system is involved in processing both emotion and memory. Interestingly, the sense of smell projects directly to the limbic system; therefore, not surprisingly, smell can evoke emotional responses in ways that other sensory modalities cannot. The limbic system is made up of a number of different structures, but three of the most important are the hippocampus, the amygdala, and the hypothalamus (Figure). The hippocampus is an essential structure for learning and memory. The amygdala is involved in our experience of emotion and in tying emotional meaning to our memories. The hypothalamus regulates a number of homeostatic processes, including the regulation of body temperature, appetite, and blood pressure. The hypothalamus also serves as an interface between the nervous system and the endocrine system and in the regulation of sexual motivation and behavior.

An illustration shows the locations of parts of the brain involved in the limbic system: the hypothalamus, amygdala, and hippocampus.
The limbic system is involved in mediating emotional response and memory.

The Case of Henry Molaison (H.M.)

In 1953, Henry Gustav Molaison (H. M.) was a 27-year-old man who experienced severe seizures. In an attempt to control his seizures, H. M. underwent brain surgery to remove his hippocampus and amygdala. Following the surgery, H.M’s seizures became much less severe, but he also suffered some unexpected—and devastating—consequences of the surgery: he lost his ability to form many types of new memories. For example, he was unable to learn new facts, such as who was president of the United States. He was able to learn new skills, but afterward he had no recollection of learning them. For example, while he might learn to use a computer, he would have no conscious memory of ever having used one. He could not remember new faces, and he was unable to remember events, even immediately after they occurred. Researchers were fascinated by his experience, and he is considered one of the most studied cases in medical and psychological history (Hardt, Einarsson, & Nader, 2010; Squire, 2009). Indeed, his case has provided tremendous insight into the role that the hippocampus plays in the consolidation of new learning into explicit memory.

Clive Wearing, an accomplished musician, lost the ability to form new memories when his hippocampus was damaged through illness. Check out the first few minutes of this documentary video for an introduction to this man and his condition.