LEARNING GOAL
Compare the three types of brain plasticity.
2.14 Your Environment Changes Your Brain
When Michelle Mack was a youngster, her parents realized that she was different from other children but they could not explain these differences. When Michelle was 27 years old, they learned that she was missing the left hemisphere of her brain (2.33a). Doctors suspected that Michelle’s condition was the result of a stroke she experienced in the womb.
More information
The brain scan photo with almost nothing showing on the left hemisphere. The right hemisphere seems normal-sized.
FIGURE 2.33 Michelle Mack and a Case of Extreme Plasticity
(a) While in her mother’s womb, Michelle suffered a stroke that destroyed her left hemisphere (shown here as the black area on the right). (b) Over time, Michelle’s right hemisphere took over the duties of the left hemisphere—language production and moving the right side of the body—to a surprising extent. Michelle’s case shows the plasticity of the brain.
Without a left hemisphere, Michelle should have shown severe deficits in skills processed in that half of the brain. For example, the left hemisphere controls language, and it controls motor actions for the right side of the body. Losing a hemisphere as an adult would result in devastating loss of function. But Michelle’s speech is only minimally affected. And she can move the right side of her body, although with some difficulty (2.33b). Michelle is able to lead a surprisingly independent life. She graduated from high school, has a job, pays her bills, and does chores. Where did her abilities come from? Her right hemisphere developed language and the ability to move the right side of her body, as well as functions that ordinarily occur across both hemispheres.
Michelle’s case shows that nurture can influence nature. Over time, Michelle interacted with the world. Her experiences let her brain reorganize itself. Her right hemisphere took over processing for the missing left hemisphere. In fact, despite the great precision and the specificity of its connections, the brain is extremely adaptable. Over the course of development and our constant stream of experience, and after injury, the brain continually changes. This property, known as plasticity, reflects the interactive nature of biological and environmental influences. Because of exposure to certain environments and experiences, the brain can change in three ways: by growing new neurons, by changing existing neural connections, and by reorganizing.
Growing New Neurons Brain plasticity decreases with age. Even into very old age, however, the brain can grow new connections among neurons and even grow new neurons. This rewiring and growth within the brain represents the biological basis of learning. That is, as you learn, your brain changes. Until about 30 years ago, scientists believed that adult brains produced no new brain cells. There is now evidence that new neurons are produced in some brain regions (Eriksson et al., 1998; Frisén, 2016). The production of new neurons is called neurogenesis. A fair amount of neurogenesis may occur in the hippocampus (Christian et al., 2014). As you learned in study unit 2.6, the hippocampus is involved in the storage of new memories. These memories are eventually transferred to the cortex as information in the hippocampus is continuously overwritten. Perhaps, without disrupting memory, neurons in the hippocampus can be lost and replaced. Further research may allow us to develop techniques to use neurogenesis to reverse the brain’s natural loss of neurons as we grow older. Curbing neuron loss could slow the mental decline that comes with aging. There is some controversy, however, as to how much neurogenesis occurs in adults (Boldrini et al., 2018; Sorrells et al., 2018).
FIGURE 2.34 Learning About the Environment
As children learn about the world around them, neural connections in the brain establish these experiences as memories. Neurons that fire together wire together to create permanent connections that are the physical basis of these memories.
Changing Existing Neural Connections Throughout life, you learn new things about the world. As a child, your everyday interactions with the environment led you to learn things that you still remember today (2.34). Thanks to brain plasticity, all your memories are reflected in your brain’s physical changes. Psychologists now believe that these changes are mainly in the strength of existing neural connections. One possibility is that when one neuron activates another, the connection between them strengthens. The strengthened connection then makes these neurons more likely to fire together in the future. This theory can be summarized by psychologist Donald Hebb’s catchphrase: “Neurons that fire together wire together.”
By contrast, the connection between two neurons tends to get weaker if the neurons do not continue to communicate regularly. Early in life, overabundant connections form among the brain’s neurons. Subsequently, life experiences help “prune” some of these connections to strengthen the rest, much as pruning weak or nonproductive branches will strengthen a fruit tree. Much of this neural pruning happens by the time you reach adolescence, and it results in a more efficient brain.
Brain Reorganization Sometimes the brain undergoes some reorganization, which is another example of brain plasticity. That is, entirely new connections develop between nearby brain regions. This new growth is a major factor in recovery from brain injury. As we saw in the case of Phineas Gage, whose left hemisphere was severely damaged by a tamping rod, people with brain injuries are often able to function successfully after recovery. Following an injury in the cortex, the surrounding brain areas assume the function of the damaged area (Mikellidou et al., 2019). Think of the healthy brain areas as local businesses scrambling to pick up the customers of a newly closed competitor. The remapping seems to begin immediately, and it continues for years. Such plasticity involves all levels of the CNS, from the cortex down to the spinal cord.
Brain reorganization is much more common in children than in adults (Asaridou et al., 2020). As an extreme example, consider young children who have epilepsy so severe that it paralyzes one or more of their limbs. To control the epilepsy, surgeons may remove an entire cerebral hemisphere. As in the case of Michelle Mack, the remaining hemisphere eventually takes on many of the lost hemisphere’s functions. The children regain almost complete use of their limbs. If this procedure were performed on adults, however, the lack of a cerebral hemisphere would result in severe deficits. With less chance of brain reorganization, the adults will most likely lose all the functions of the missing hemisphere.
As you have seen in this chapter, this is an exciting time in psychology. Researchers are greatly increasing our understanding of how the brain works with the rest of the body to produce behavior and mental activity. The knowledge you have gained about biology’s role in psychology will serve as a foundation for your learning in later chapters.
LEARNING GOAL CHECK: REVIEW
Glossary
- plasticity
- A property of the brain that causes it to change as a result of experience or injury.