Sections include: the CEO, brain tracks, the influential referee
Our brains do more than tell us when to act or speak. They also tell us when not to do those things, and that function is just as important. Inhibition and control are fundamental components of our higher thinking processes and provide the underpinnings for several more complex and characteristically human faculties, including language and decision making, problem solving, and planning.
Loosely defined, inhibition refers to the suppression of a reflexive, habitual, or otherwise highly compelling behavior that is disadvantageous or inappropriate for the context (for instance, the urge to tell someone, “Your hairdo is really ugly”). Control refers to the ability to direct mental function and behavior in accord with an intention or set of intentions. Clearly, control is closely related to inhibition—suppressing a statement that might offend someone else is an expression of the intention to be polite. However, control can also involve other functions, such as coordinating a sequence of behaviors needed to achieve a goal (for example, planning and carrying out a series of chess moves) or properly weighing alternative actions (eating now or waiting until your date arrives).
Our understanding of the brain mechanisms responsible for inhibition and control is still at a very early stage. For a long time, researchers were able to study such “high level” functions only by examining people with brain damage (lesion studies) or by recording the electrical activity of neurons in other animal species, such as nonhuman primates, our closest animal relatives. Such research suggested that an area at the front-most part of the brain—the prefrontal cortex—plays a critical role in inhibitions and control. For example, neurologists have long recognized that many patients with damage to this area exhibit what are referred to as frontal release signs: these are reflexes that infants or very young children display but that normally disappear as we mature. One example is the grasp reflex, which is useful for an infant in holding on to objects or its mother.
You can easily try out these reflexes yourself. The next time you are with an infant, try gently stroking his or her palm with your forefinger. The baby will reflexively grab your finger, a reaction sensibly called the grasp reflex. Try it on your adult friends: they will not exhibit the same response. However, a patient with damage to the prefrontal cortex may very well grip your finger. This suggests that the developing brain does not take away the reflex but rather suppresses it, since it is not needed in adult life, and that damage to the frontal cortex can “release” the old instinct. Thus, a healthy frontal cortex must play an active role in suppressing this unnecessary behavior, and probably many others.
The return of the grasp reflex is a common consequence of frontal brain damage, but not an especially debilitating one. However, consider the landmark case of Phineas Gage, described in chapter 1. He was foreman of a railway construction gang in Vermont in 1848 when an accidental explosion blew an iron bar through the front part of his head. The bar was over 3 feet (1 m) long and 11⁄4 inches (3 cm) in diameter. Remarkably, Gage recovered physically from his injury. But his behavior changed profoundly. Whereas before the accident he had been a highly capable and efficient foreman, after the accident his behavior was described as “fitful, irreverent, and grossly profane, showing little deference for his fellows. He was also impatient and obstinate, yet capricious and vacillating, unable to settle on any of the plans he devised for future action.” Reconstruction of Gage’s injury from the wounds in his skull indicates that the bar was shot through his prefrontal cortex.
What this suggests, and what many studies have since confirmed, is that damage to the prefrontal cortex impairs people’s capacity for cognitive control. These individuals are much less inhibited from performing primitive, often socially inappropriate behaviors, such as urinating in public or appearing not fully clothed. They also lose the ability to coordinate ordinarily routine behaviors, even though they can still execute each part of a sequence. For example, doctors observed one patient making a cup of coffee with sugar by pouring the coffee into a cup, stirring, and then adding sugar. This person obviously knew each individual bit of behavior but failed to execute them in the proper order.
This pattern of disturbance is often referred to as a dysexecutive syndrome, and it corresponds well with what we have learned from animals with damage to the prefrontal cortex. Like Phineas Gage, they become distractible, exhibit inappropriately aggressive or sexual behavior, and at times persist in seemingly useless actions. Studying animals with lesions of the prefrontal cortex has led to ideas about what functions different parts of this brain structure may serve. One notion is that the lower part of the prefrontal cortex (just over the eye sockets) is primarily responsible for inhibiting unwanted behaviors, whereas parts higher in the frontal lobes (behind the upper part of the forehead) are more involved in remembering goals and coordinating behavior. While this may be basically correct in humans as well, research has begun to suggest more complex possibilities.
The recent invention of methods for studying the brain activity of normal human individuals as they perform specific tasks has dramatically accelerated research in this area. Two of the most popular methods are functional magnetic resonance imaging (fMRI) and the recording of event-related electrical potentials (ERPs). These techniques have made clear that like most mental functions, inhibitions and control involve many brain regions, not just the prefrontal cortex. Although the prefrontal cortex is regularly activated in tasks that rely on inhibition and control, other parts of the brain also become active.
This finding has raised a fundamental question: Are there brain structures dedicated to inhibition and control, or do those functions arise from the interaction of many structures, no one of which can be credited exclusively as the “inhibitory” or “control” center of the brain? Although our brains are like computers in many respects, one dramatic way in which this is not true is that unlike most computers, brains do not have a central processing unit (CPU). If you remove the CPU from your desktop computer, it will grind to a complete halt and become only a rather expensive way of warming your office. In contrast, selective damage to the brain rarely if ever results in a complete loss of function. Rather, a person loses certain faculties (for example, vision, hearing, high-level reasoning), but often not completely, and most other abilities may remain. It appears that functions are distributed across the brain. This appears to be true for inhibition and control as well.
The CEO
One way to understand how control might be distributed across different structures in the brain is to consider an analogy between the brain and a large modern corporation, such as a car manufacturer, with many departments. The Research and Development (R&D) team might have come up with a great new design for a more efficient engine, or the folks in Market Research may have determined that there is a growing need for a particular type of vehicle. In an efficient corporation, one unit will transmit information directly to another rather than send everything to a CEO (or CPU) for redistribution. The R&D team consults with the market researchers to see if there would be any interest in the new design, or the marketing folks check with R&D about whether they can actually produce a car that would meet a market niche. As plans begin to take shape, other units weigh in on safety, costs, advertising and promotion, and so on. Eventually, as the plan takes shape, the CEO may be responsible for the final decision whether or not to go forward. But the full richness of the plan, its evaluation, and its execution involve many, if not all parts of the company. That is, even though we think of the CEO as the central executive of a company, planning and executing any new endeavor actually relies on interactions between a variety of different units within the company. Modern theories of how the brain functions are remarkably similar to this model.
Brain Tracks
Under one current view of how the brain executes its control function, the pattern of neural activity in the prefrontal cortex influences how information flows in the other parts of the brain that are responsible for actually carrying out a task. Another analogy might help. We can think of the frontal cortex as a switch operator for a complex system of railroad tracks. The brain is like a set of tracks (pathways) connecting various origins (for example, brain areas responding to particular sensory stimuli) to specific destinations (brain areas controlling behavioral responses, the retrieval of a memory, and so on). The operator’s goal is to move the trains (neural activity carrying information about the sensory event) from each origin to its proper destination as efficiently as possible, avoiding collisions.
When the track is clear (that is, a train can speed from origin to destination without risk of running into any others), then no intervention is needed. Similarly, a behavior that is reflexive, or automatic, does not need the assistance of the frontal cortex. However, if two trains must cross the same length of track, then the railroad needs some coordination to guide them safely to their destinations. Neural activity in the prefrontal cortex can be thought of as a map that determines which pattern of “tracks” is used to solve the task. Note that this function need not be restricted to associating sensory stimuli to behavioral responses; it applies equally well to “routing” internal mental processes, such as the retrieval of a memory and the expression of an emotion.
This view accords well with the pattern of disturbances we see when a person’s frontal lobes are damaged. If switch operators do not do their job, then the trains of the brain start to bump into each other, causing delays, confusion, and disorganized functioning. However, not all is lost. Trains that run on their own tracks, or simply happen not to be on a course that will cause them to collide with others, will run fine. This is the case for people with frontal damage. They retain the capacity for many behaviors (for example, pouring coffee, knowing to add sugar, knowing that it is relevant to stir), even if they cannot execute them in a fully coordinated fashion.
Note that a critical function of the switch operator is to know exactly when to throw the switch: too early and a train will not make it onto the right track; too late and a collision may occur. Recent research has suggested that the neurotransmitter dopamine may play this critical role in the brain— telling the prefrontal cortex when to update its plans and switch the tracks in other parts of the brain. This may explain how disturbances in dopamine—thought to be a central player in psychiatric diseases such as schizophrenia—could lead to impaired frontal lobe function and corresponding deterioration of higher-level functioning.
The “brain track” analogy provides a useful view of how control may be exercised in the brain, but what does it tell us about inhibition? One of the easiest ways to see inhibition at work is the Stroop task, often used in laboratories. In this task, subjects are asked to name the color in which a word is printed. The twist is that the word itself names a color. To see how it works, try taking the Stroop test yourself; it is on page 13 of the brain primer, which is located after page xxxii. Take a moment to identify the colors of the words printed there.
You probably felt an instant of hesitation and confusion, perhaps even a sense that you were about to name different colors from the colors of the words themselves. When we see a word, we are more used to reading it than to naming the color in which it is printed. To prove this, show the words to friends, but don’t tell them what to do: just ask them to respond however they wish. Almost certainly they will read the words and say, for example, “green” for the one printed in red. Reading the word is our “default,” or prepotent, response. The reason you had trouble naming the color is that you had to suppress, or inhibit, this response in order to answer the question posed.
Some investigators believe that inhibition is a dedicated function of the prefrontal cortex, particularly the lower part. In the Stroop task, according to this model, the prefrontal cortex directly suppresses our tendency to read the word. However, another possibility—closer to that afforded by our “train track” analogy—is that the prefrontal cortex does not actually suppress word reading; rather, it gives extra “juice” to the effort to name the color. Since you cannot say “red” and “green” at the same time, these two responses are in competition. Without the assistance of the prefrontal cortex, the habitual word-reading response would win out (that is, saying “green”). However, with the assistance of the prefrontal cortex (switching the tracks so that color naming has the advantage), the color-naming response gets the edge and wins the competition. From this perspective, inhibition is a more distributed function, occurring at a more local level—that is, between the two verbal responses—and the role of the prefrontal cortex is simply to help favor the weaker one when that is needed.
The Influential Referee
We’ve described the prefrontal cortex as the managerial CEO and as the alert accidentpreventing railroad switch operator. But this remarkable brain system is even more versatile; it is also a subtle influencer. To offer one final analogy, consider the two brain responses as if they were two boxers, and the prefrontal cortex as a referee who hopes to influence the fight. The view that the prefrontal cortex plays a central role in inhibition suggests that the referee tips the fight by slugging one of the boxers himself. Or, alternatively, the referee gives a bit of extra latitude to one of the boxers, who exploits this advantage to knock out the other. As yet, we do not know for sure which image is a better metaphor for how inhibition is really carried out in the brain. However, this example shows how a function such as inhibition, which on the surface may appear centralized, could in fact be carried out in a more distributed way, involving close coordination between the prefrontal cortex and other areas of the brain engaged in performing a particular task. While we have begun to gain insights into the brain mechanisms underlying inhibition and control, and how they can go awry because of injury or disease, we are not yet at a point where we can have a meaningful impact on these functions—either to enhance normal function or to repair damage. With deeper understanding however, and with the remarkable pace at which relevant technologies are developing, there is no question that we are moving toward this goal.
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