Brain Plasticity and Behavior

Bryan Kolb

• Publisher: Psychology Press
• Year: 2013
• ISBN:
  9781134784219
• Edition: 1
• Topic: Psychology
• Subtopic: History & Theory in Psychology
• Index: Psychology

There are few books devoted to the topic of brain plasticity and behavior. Most previous works that cover topics related to brain plasticity do not include extensive discussions of behavior. The first to try to address the relationship between recovery from brain damage and changes in the brain that might support the recovery, this volume includes studies of humans as well as laboratory species, particularly rats. The subject matter identifies a consistent correlation between specific changes in the brain and behavioral recovery, as well as various factors such as sex and experience that influence this correlation in consistent ways. Evolving from a series of lectures given as the McEachran Lectures at the University of Alberta, this volume originally began as a summary of the lectures, but has expanded to include more background literature, allowing the reader to see the author's biases, assumptions, and hunches in a broader perspective. In writing this volume, the author had two goals in mind: * to initiate senior undergraduates or graduate psychology, biology, neuroscience or other interested students to the issues and questions regarding the nature of brain plasticity, and * to provide a monograph in the form of an extended summary of the work the author and his colleagues have done on brain plasticity and recovery of function.

I BRAIN PLASTICITY

In principle, it would seem easy to study brain function. Brain cells are relatively large and can be seen in a light microscope. Brain cells have electrical properties that tell us when they are and are not active. Brain cells are chemical so we can do various assays. There are several fields devoted to the study of behavior (comparative psychology, ethology, kinesthesiology). Thus, it would seem that by putting the information from these areas together one would be able to get a pretty good idea of how the brain works. One of the formidable problems, however, is that the organization of the brain can be fundamentally altered by experience. Experience includes not only external events, but also internal events such as the actions of hormones, the effects of injury, the relentless effects of development and aging, and even thoughts. An understanding of how the brain changes is therefore an important topic in neuroscience. Chapters 1 and 2 provide basic information on the nature of brain plasticity. Chapter 1 considers basic properties of the nervous system that contribute to plasticity and provides examples of brain plasticity. Chapter 2 gives more details on how plasticity is measured and the conditions under which it occurs.

1 Some Basic Concepts, Examples, and Biases

BRAIN PLASTICITY

Donna was born on June 14, 1933.¹ Her memory of her early life is sketchy, but those who saw her early on report that she did not seem to know anything for some time. She could neither talk, nor walk, nor even use a toilet. Indeed, she did not even seem to know who her father was, although her mother seemed more familiar to her. Like all children, Donna grew quickly, and in no time she was using and understanding simple language and could recognize lots of people by sight almost instantly. Donna began taking dancing lessons when she was 4 years old and was a “natural.” By the time she finished high school she was ready for a career as a dancer with a major dance company. Her career as a dancer was interrupted in 1958 when she married and had two children. Donna never lost interest in dancing and kept fit in her years at home with the kids. In 1968 her children were in school so she began dancing again with a local company. To her amazement, she still could do most of the movements, although she was pretty rusty on the classic dances that she had once so meticulously memorized. Nonetheless, she quickly relearned. In retrospect she should not have been so surprised, as she had always been known as a person with a fabulous memory.

In 1990 Donna was struck by a drunk driver as she was out on an evening bicycle ride. Although she was wearing a helmet she suffered a closed head injury (among other injuries!) and was in a coma for several weeks. As she awakened from the coma she was confused and had difficulty in talking and in understanding others, she had very poor memory, she had spatial disorientation and often got lost, she had various motor disturbances, and she had difficulty recognizing anyone but her family and closest friends. Brain scans revealed diffuse cerebral injury with some focal injury on the ventral surface of the temporal and frontal lobes where the brain presumably was banged against the skull in her fall.

Over the ensuing 10 months she regained most of her motoric abilities and language skills, and her spatial abilities improved significantly. Nonetheless, she found herself to be short-tempered and easily frustrated with her slow recovery. She suffered periods of depression. Two years later she was once again dancing, but she now found it very difficult to learn new steps. Her emotions were still labile, which was a strain on her family, but her episodes of frustration and temper outbursts were becoming much less common. A year later they were gone and her life was not obviously different from that of other 55-year-old women. She did have some cognitive changes that persisted, however. She could not seem to be able to remember the names or faces of people that she met and was unable to concentrate if there were distractions such as a television or radio playing in the background. She did not seem able to dance as she had before her injury, and she retired from her life’s first love.

Donna provides a pretty typical example of one of the most intriguing and important properties of the human brain: It has a capacity for continuously changing its structure, and ultimately its function, throughout a lifetime. This capacity to change, which is known as brain plasticity, allows the brain to respond to environmental changes or changes within the organism itself. Consider the plasticity in Donna’s brain. When she was a newborn she was confronted with a world that nature could not possibly have prepared her for. She had to learn language, to distinguish different faces, to walk, to ride a bicycle, to read, to dance, and so on. Because her brain is solely responsible for her behavior, this means that her brain somehow had to change to reflect her experiences. When Donna reached puberty her body changed and so did her thoughts. Her dreams often had sexual content and, because her dreams are a product of her brain, there must have been some change in her brain activity to change her dreams so dramatically. This change was likely induced by the estrogen surge of adolescence. When Donna returned to dancing after a 10-year break, she had retained much of her skill, even though she had not practiced at all. In this case the brain somehow did not change and she could quickly relearn what she had lost. After her accident, Donna had to “relearn” how to talk and walk and so on. In actual fact she did not go through the same process that she had as a baby, but something in her brain had to change in order to allow her to regain her lost abilities. Whatever changed in her brain must have had some limits, however, because she never did fully recover her memory or her ability to learn new dances.

Thus, in the life of Donna we can see several different types of brain plasticity. First, during her early childhood the brain changed dramatically in its structure, organization, and behavior. These changes were not accomplished quickly: Her brain was fundamentally different from its adult form until at least at 12 or 14 years. Indeed, the plastic changes in the developing brain are so profound that a child is effectively a different creature at different stages of its own development! The brain’s plasticity reflects more than mere maturational change, however, as it includes the ability to change with experience. Indeed, the capacity to alter brain structure and function in response to experience provides the nervous system with the ability to learn and to remember information. Some experiential changes are self-evident, such as the acquisition of specific bits of knowledge, whereas other changes are more subtle, such as perceptual learning or the development of different problem-solving strategies. Nonetheless, regardless of the nature of experiential change, the brain has altered its form and function. Finally, after a brain injury, processes are recruited to change the brain again. In this case the brain must reorganize, at least in part, in order to allow the production of behaviors that have been lost.

Although the property of brain plasticity is most obvious during development, the brain remains malleable throughout the life span. It is evident that we can learn and remember information long after maturation. Furthermore, although it is not as obvious, the adult brain retains its capacity to be influenced by “general” experience. For example, being exposed to fine wine or Pavorotti changes one’s later appreciation of wine or music, even if encountered in late adulthood. The adult brain is plastic in other ways, too. For example, one of the characteristics of normal aging is that neurons die and are not replaced. This process begins in adolescence, yet most of us will not suffer any significant cognitive loss for decades because the brain compensates for the slow neuron loss by changing its structure. Similarly, although complete restitution of function is not possible, the brain has the capacity to change in response to injury in order to at least partly compensate for the damage.

The brain is plastic in another way, too. Imagine the problem of learning a completely new skill, such as juggling while perched on a unicycle. Initially one is totally inept, but with practice at least some people can master the task. Thus, a new behavior, or set of behaviors, has been acquired. From what we have just discussed, it should be obvious that the brain has changed. But what has changed it? One candidate is the behavior itself. That is, if we repeatedly engage in a particular behavior, the behavior itself can alter the brain, which in turn facilitates the behavior. The idea that activity might change the heart or muscles is seldom questioned. The possibility that behavior could change the structure and function of the brain is seldom considered! Nevertheless, it is an important aspect of brain plasticity. Indeed, there is little doubt that even thought can change the brain. Consider the now extensive research on the variables influencing eyewitness testimony. Different people’s accounts of the same events are notoriously inconsistent, in part because they are altered significantly by questions or thoughts “planted” by others. That is, the “memory,” and therefore by inference the brain, is altered by cognitive activity. In a general sense this is the process of perceptual learning where we learn about the world by observing and thinking about sensory experience.

The property of brain plasticity confronts us with a host of fundamental questions. First, as we assume that the brain produces behavior, then how is that a changing brain can produce the same behavior at different times? Shouldn’t behavior change if the brain is changing? Indeed, how do we remember anything if the brain is changing every time we learn something? Second, it has been assumed since the time of Broca (i.e., the mid-1800s) that at least some functions are localized in the cortex. If the brain is plastic, what does this imply for the nature of cortical organization? Third, what are the constraints on plasticity? There must be factors such as hormones or other chemicals that can directly control processes fundamental to plasticity. Fourth, it seems likely that the brain has some type of limits in the extent to which it is plastic. What are the limits and what determines them? Fifth, there is the general question of establishing what the properties of the nervous system are that enable it to be plastic. And, more specifically, are all regions of the brain equally plastic? Sixth, there is a clinical, and even potential educational, question: Can we gain control of the plasticity and turn it on or off at opportune times? Finally, there is the interesting question of what factors influence plasticity and whether individual differences in different abilities may at least partly reflect differences in the brain’s capacity for plasticity.

In sum, the study of brain plasticity provides a window on some of the fundamental questions in psychology and neuroscience. In particular, it allows a way of looking at the neurological bases of fundamental psychological processes such as learning and thought, and the manner in which these develop. It also leads to consideration of important clinical issues surrounding behavioral change, whether it be related to recovery from neurological injury or disease, or psychopathology related to other causes.

ASSUMPTIONS AND BIASES

As we consider the properties of the brain that make it plastic, we need to consider several biases and assumptions that underlie thinking about plasticity.

1. Behavioral states, including mind states, correspond to brain states. Although this proposition is not novel, and probably appears to be self-evident to most neuroscientists, it has been a central philosophical issue since the time of Descartes. In fact, modern-day philosophers still debate this issue seriously, in large part because mind (or “cognitive processes” in modern jargon) is the central problem of psychology. In this volume I assume that it is the brain that thinks and controls behavior, and try to show that an understanding of plasticity will be enlightening with respect to how it does these tricks!

2. The structural properties of the brain are important in understanding its function. It follows from my first assertion that changes in the physical structure of the brain will be reflected in changes in its functioning. Although many behavioral scientists (e.g., Pylyshyn, 1980; Skinner, 1938) have seen the structure of the brain as virtually incidental to the study of its function, this is not my view. Rather, I assume not only that changes in structure underlie behavioral change but also that it is possible to identify and potentially influence those changes. This does not imply that a single structural change is responsible for all behavioral change, nor that a particular behavioral change is due solely to one morphological change, nor that an understanding of morphology means an understanding of the functional properties that emerge from the morphology. It does mean, however, that the cerebral organization places significant constraints on the computations of the brain and may provide important clues for understanding the nature of those computations.

3. Plasticity is a property of the synapse. The Spanish anatomist Ramon y Cajal postulated in the early part of this century (1928) that the process of learning might produce prolonged morphological changes in the efficiency of the connections between neurons (Fig. 1.1). However, it was not until 1948 that a Polish neuroscientist, Jerzy Konorski, formally proposed a mechanism. He suggested that appropriate combinations of sensory stimuli could produce two types of changes in neurons and their connections: (a) an invariant but transitory change in the excitability of neurons, and (b) an enduring plastic change in neurons. In other words, Konorski suggested that when neurons are active they change. This change might be transitory, much as when one looks up a phone number and then forgets it, or it might be enduring, such as the case in which a telephone number is memorized. The idea that neurons are somehow changed with use is important, for it means that one could look at the neuron and try to identify the changes. The question is, however, where do you look? A Canadian psychologist, Donald Hebb, proposed in 1949 that the logical place to look is at the synapse (Fig. 1.2). He suggested that when synapses are active, they change if the conditions are right. For Hebb, the most important condition was that two neurons had to be coincidentally active and if so, then the connection between them was strengthened.² Hebb’s addition was important, for it (a) specified the conditions under which plasticity would occur and (b) pointed to a role of both the pre- and postsynaptic side of a connection in plasticity. This latter conclusion means that plasticity can be measured either pre- or postsynaptically. It also follows from Hebb’s proposals that during development, learning, recovery from injury, and aging, there are changes at the synapse that allow the b...