Neuroplasticity, Neurogenesis, and Brain Lateralization
The control of some bodily functions, such as movement, vision, and hearing, is performed in specific areas of the cortex, and if an area is damaged, the individual will likely lose the ability to perform the corresponding function. For instance, if an infant suffers damage to facial recognition areas in the temporal lobe, it is likely that he or she will never be able to recognize faces. [ 1 ] However, the brain is not divided in an entirely rigid way. The brain’s neurons have a remarkable capacity to reorganize and extend themselves to carry out particular functions in response to the needs of the organism and to repair damage. As a result, the brain constantly creates new neural communication routes and rewires existing ones. Neuroplasticity is the brain’s ability to change its structure and function in response to experience or damage .
Neuroplasticity enables us to learn and remember new things and adjust to new experiences.
Our brains are the most “plastic” when we are young children, as it is during this time that we learn the most about our environment. And neuroplasticity continues to be observed even in adults. [ 2 ] The principles of neuroplasticity help us understand how our brains develop to reflect our experiences. For instance, accomplished musicians have a larger auditory cortex compared with the general population [ 3 ] and also require less neural activity to play their instruments than do novices. [ 4 ] These observations reflect the changes in the brain that follow our experiences.
Plasticity is also observed when damage occurs to the brain or to parts of the body that are represented in the motor and sensory cortexes. When a tumor in the left hemisphere of the brain impairs language, the right hemisphere begins to compensate to help the person recover the ability to speak. [ 5 ] And if a person loses a finger, the area of the sensory cortex that previously received information from the missing finger begins to receive input from adjacent fingers, causing the remaining digits to become more sensitive to touch. [ 6 ]
Although neurons cannot repair or regenerate themselves as skin and blood vessels can, new evidence suggests that the brain can engage in neurogenesis , the forming of new neurons . [ 7 ] These new neurons originate deep in the brain and may then migrate to other brain areas where they form new connections with other neurons. [ 8 ] This leaves open the possibility that someday scientists might be able to “rebuild” damaged brains by creating drugs that help grow neurons.
Unique Functions of the Left and Right Hemispheres Using Split-Brain Patients We learned that the left hemisphere of the brain primarily senses and controls the motor movements on the right side of the body, and vice versa. This fact provides an interesting way to study brain lateralization—the idea that the left and the right hemispheres of the brain are specialized to perform different functions. Gazzaniga, Bogen, and Sperry [ 9 ] studied a patient, known as W. J., who had undergone an operation to relieve severe seizures. In this surgery, the region that normally connects the two halves of the brain and supports communication between the hemispheres, known as the corpus callosum, is severed. As a result, the patient essentially becomes a person with two separate brains. Because the left and right hemispheres are separated, each hemisphere develops a mind of its own, with its own sensations, concepts, and motivations. [ 10 ]
In their research, Gazzaniga and his colleagues tested the ability of W. J. to recognize and respond to objects and written passages that were presented to only the left or to only the right brain hemispheres. The researchers had W. J. look straight ahead and then flashed, for a fraction of a second, a picture of a geometric shape to the left of where he was looking. By doing so, they assured that—because the two hemispheres had been separated—the image of the shape was experienced only in the right brain hemisphere (remember that sensory input from the left side of the body is sent to the right side of the brain). Gazzaniga and his colleagues found that W. J. was able to identify what he had been shown when he was asked to pick the object from a series of shapes, using his left hand, but that he could not do so when the object was shown in the right visual field. Conversely, W. J. could easily read written material presented in the right visual field (and thus experienced in the left hemisphere) but not when it was presented in the left visual field.
The information presented on the left side of our field of vision is transmitted to the right brain hemisphere, and vice versa. In split-brain patients, the severed corpus callosum does not permit information to be transferred between hemispheres, which allows researchers to learn about the functions of each hemisphere.
This research, and many other studies following it, demonstrated that the two brain hemispheres specialize in different abilities. In most people, the ability to speak, write, and understand language is located in the left hemisphere. This is why W. J. could read passages that were presented on the right side and thus transmitted to the left hemisphere, but could not read passages that were only experienced in the right brain hemisphere. The left hemisphere is also better at math and at judging time and rhythm. It is also superior in coordinating the order of complex movements—for example, lip movements needed for speech. The right hemisphere has only limited verbal abilities, and yet it excels in perceptual skills. The right hemisphere is able to recognize objects, including faces, patterns, and melodies, and it can put a puzzle together or draw a picture. This is why W. J. could pick out the image when he saw it on the left, but not the right, visual field.
Although Gazzaniga’s research demonstrated that the brain is in fact lateralized, such that the two hemispheres specialize in different activities, this does not mean that when people behave in a certain way or perform a certain activity they are using only one hemisphere of their brains at a time. That would be drastically oversimplifying the concept of brain differences. We normally use both hemispheres at the same time, and the difference between the abilities of the two hemispheres is not absolute. [ 11 ]