The Making of the Mind: The Neuroscience of Human Nature (7 page)

The phonological loop is by far the most intensively researched and best understood component of working memory. Neuroimaging studies have been able to identify the regions of the left frontal lobe that support the loop's function in storing and rehearsing verbal material. Broca's area (in the prefrontal cortex) and areas in the premotor and supplementary motor cortex are involved, as is another posterior region in the left parietal lobe.
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The frontal areas support the rehearsal of phonological representations that are actually stored in and retrieved from a region in the parietal lobe. This network, then, activates multiple regions in the left hemisphere of the brain to facilitate the learning of language. Although the learning of new words
may be the reason the loop proved adaptive, its value to language use did not stop there. Cognitive research has also demonstrated its role in silent reading and in the production of speech and written language. As will be seen in
chapter 5
, the phonological loop is not just for communicating via language. Rather, the inner voice made possible by the phonological loop led to a profoundly important addition to the human mind when it became a medium for silent verbal thought.

HUMAN EXECUTIVE ATTENTION

Besides the unique power of a phonological loop, the mind within us is capable of solving novel problems, delaying gratification, and inhibiting unwanted impulses. Although we may not always succeed in these endeavors, our brain provides us with the potential for success through its massive prefrontal cortex. Using again the metaphor of a house, extra square footage can mean larger rooms or additional rooms, as seen with the addition of a phonological loop. Both of these images seem applicable to the executive functions endowed by the prefrontal lobes of human beings. However, another metaphorical twist is also instructive—the extra space within each room would allow for more furnishings. The executive suite of the human brain is indeed impeccably furnished in the form of superior neural interconnectedness. A striking fact of reorganization at the level of neural circuitry emergent in
Homo sapiens
is the myelination of the subcortical white matter of the brain. Myelin insulates the axons of neurons, allowing for much faster transmission of neural impulses. Comparative neuroscience has clearly documented that the “relative volume of white matter underlying prefrontal association cortices is larger in humans than in great apes…compatible with the idea that neural connectivity has increased in the human brain.”
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The different lobes of the human neocortex do not mature at the same rate.
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Myelination occurs earliest in the primary motor and sensory cortices but is most prolonged in the prefrontal cortex. Following the same principle, the formation of new synapses is delayed in the prefrontal cortex; the peak synaptic density occurs at about fifteen months of age instead of much earlier for the sensory auditory cortex in the temporal lobe. Unneeded synapses
are then eliminated in these neocortical regions slowly over a period of years. Yet again, it is the prefrontal cortex that takes its time with this pruning. Whereas the auditory cortex has ended synapse elimination by age twelve, the prefrontal cortex is still removing synapses to improve the effectiveness and efficiency of its operations into the years of mid-adolescence. In fact, the executive functions of the prefrontal cortex are still developing during the years of young adulthood.

To illustrate the late development of the prefrontal cortex, consider the results of a neuroimaging study using magnetic resonance imaging (MRI) that contrasted the brains of adolescents who were twelve to sixteen years of age with those of young adults aged twenty-three to thirty years.
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The images showed very little difference in the occipital, parietal, and temporal lobes. It was only in the frontal lobes that postadolescent maturation was obvious. The young adults had maturational changes in the frontal lobes that were from two to five times as great as those detected in other parts of the brain. There was a reduction in gray matter in the young adults compared with the adolescents that “probably reflect[ed] increased myelination in peripheral regions of the cortex that may improve cognitive processing in adulthood…for such functions as response inhibition, emotional regulation, planning and organization.”
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Thus, in the modern human brain the prefrontal cortex, as well as the frontal lobe as a whole, grows to a massive size. They appear to be no larger than one would expect given our body size and genetic similarity to the great apes, but such relative comparisons should not obscure their absolute magnitudes. The human brain ranges from 239 to 330 cubic centimeters in the volume of the frontal lobe as a whole, with the prefrontal area accounting for between 43 to 54 cm
³
—these values dwarf those of a chimpanzee.
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Furthermore, the human frontal lobe undergoes a remarkably long period of maturation that extends even into young adulthood. The synaptic pruning and myelination that occur over the first twenty or so years of life equip the human brain with a level and sophistication of executive control of working memory found in no other species. We human beings possess exceptional capacities for inhibiting impulses and delaying gratification as well as the ability to plan solutions to novel problems. Although human beings reveal individual differences in these
abilities, the fact that we on average excel in such executive function must not be lost. Similarly, human failures to inhibit impulses, delay gratification, or solve novel problems are all too common. But such failings are often a source of regret and shame precisely for the reason that we recognize our inherent capacity to do better.

Cognitive neuroscience has identified a special network of executive attention that is mediated by structures in the frontal lobe.
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It allows for the executive control of one's thoughts and behavior. The lateral prefrontal cortex, the subcortical structures of the basal ganglia, and the anterior region of the cingulate gyrus constitute the network for executive attention. This network allows the manipulation of information held temporarily in the storage areas of working memory. It is also used in the conscious effort to retrieve a fact or event from its residence in long-term memory and bring it into awareness for use in working memory. Although this network of attention is of particular importance in human cognition, the brain houses two other networks of attention that all mammals depend on for survival. One is the alerting network that ensures the brain remains in an awake, alert state of vigilance. The other is the orienting network that makes certain the head and eyes look in the direction of a novel visual stimulus and covertly focuses attention on a particular region of space even when the head and eyes are already in position. It is easy to see how the alerting and orienting networks were essential for any prey species that must avoid a predator.

At the heart of executive attention is an ability to deal with conflict. The brain is processing information in multiple systems all at the same time. To the extent that these systems lead to different needs, motivations, and responses at any given moment, there is a competition for the control of behavior. The anterior cingulate gyrus lies embedded within the interior of the frontal lobe. The cingulate gyrus extends the length of the brain from the frontal lobe to the parietal lobe, lying just above the white fiber tract known as the corpus callosum, which connects the left cortical hemisphere with the right cortical hemisphere. It is part of the limbic system that constitutes the forebrain of all mammals and is known to play an important role in emotion. In fact, the section of the anterior cingulate that is closest to the front of the brain, curling downward in the ventral direction, is closely linked with the amygdala. The
amygdala works together with this emotional center of the anterior cingulate in the conscious awareness of fear.
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The other section of the anterior cingulate lies in a dorsal position just behind the emotional center, and it resolves cognitive conflicts. For example, if one habitually follows a certain route in driving home from work each evening, then the left and right turns can be made almost automatically. A driver can easily listen to the radio or daydream about her day or plans for the evening and the responses needed to get home are carried out effortlessly. However, suppose that before leaving from work the driver decides to stop at the grocery store to pick up something for dinner. This goal now sets up a cognitive conflict at that point in the route that calls for making a turn in the direction of the store instead of the habitual turn in the direction of home. Neuroimaging studies show activation of the cognitive center of the anterior cingulate gyrus in dealing with this response conflict. The automatic response must be inhibited and a response appropriate for the temporary goal must instead be selected. Cognitive neuroscientists study such conflict resolution with a laboratory task that simulates the same kind of situation. It is called the Stroop task. In its most common form, this requires reading the ink color of words that spell out primary colors, such as
red
,
blue
,
green
,
black
, and
yellow
. Word reading—at least in literate adults—is habitual and easily done automatically. A conflicting goal is established by deviously printing the color words in the wrong color ink—for example,
red
might appear in green ink. Because the goal is to read the ink color, one must inhibit the habitual response and select instead a response in conflict with it—say, green for the word
red
printed in green ink. When this task is done rapidly, it is very easy to make errors or to at least slow down considerably to make certain the correct response is made. The conflict situation not only slows responses and increases errors; it also requires greater effort to control behavior.
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Exactly what are the mechanisms that underlie the human capacity to control one's thoughts and behaviors? Three specific mechanisms have been identified that are carried out by the network of executive attention.
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One is updating the contents of working memory by rapidly adding or deleting information depending on its immediate relevance. As new information bombards us from the environment or as new information is retrieved from long-term
memory, the old contents of working memory must be discarded to make room for the new. Monitoring what should be discarded and what should be retained in the transient stores of working memory is thus a major part of successful updating. A second mechanism is shifting attention between two or more tasks or current goals. Because we often must keep in mind more than one goal or even attempt to do more than one task at a time, there must be an effective mechanism for shifting attention back and forth rapidly and flexibly. The third mechanism is inhibition. Through effortful control, inhibition allows the overriding of an automatic or dominant response in favor of one that is currently most appropriate. These are distinctly different mechanisms of cognitive control, but it is true that individuals who are good at one also tend to do well at the other two. Individuals who score well in updating, shifting, and inhibiting are more successful in regulating their thoughts and behaviors and resisting impulses and maladaptive urges. For example, they are more successful in “staying faithful to romantic partners…and successfully implementing dieting and exercising intentions.”
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Although the network of executive attention takes a long time to develop fully, the third year of life brings with it major gains in updating, shifting, and inhibiting.
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Toddlers at the age of twenty-four months are highly prone to perseveration—repeating a previous response, even though it is no longer appropriate. This tendency to keep repeating a behavior that is no longer appropriate is characteristic of adults who have suffered a brain injury to the frontal lobes. Both the toddlers and the frontal-lobe patients have difficulty with inhibiting a previous response, shifting attention to a new goal, and updating the contents of working memory appropriately.

Yet even after the third year it is apparent that executive control is still a work in progress. For example, the failure to inhibit inappropriate behavior is a hallmark of two-to-three-year-old children. The Simon Says game neatly shows just how difficult it is for such young children to inhibit a response until the brain has had nearly a full fourth year to develop. The game is played by performing an action when instructed by a toy bear but inhibiting the response when it is requested by a toy elephant. In fact, until the maturation of the third year of postnatal brain development is completed, the Simon Says game is simply beyond the executive attention capacity of the child.
The findings with Simon Says reveal that “children up to 40 months were unable to carry out the inhibition instruction at better than a chance level, and they performed just as rapidly in making incorrect responses to the elephant as they did in making correct responses to the bear. Although children could repeat the instruction, they did not seem able to use it to control their own behavior.”
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By the age of forty-eight months they performed it correctly almost every time.

An individual's ability to delay gratification is also related to differences in the functioning of executive attention. Waiting for a reward to come later requires the inhibition of taking immediate action to take the reward now. For example, consider a college student who is asked to go to a party early in the evening when he has plans to study for two hours before going. Resisting the impulse for the immediate social and emotional reward of joining friends at a party is necessary to obtain the larger reward of learning, completing assignments that will eventually need to be done, and preparing for an exam. The fun of the party will be added on top of these scholastic rewards. Human beings differ in their ability to delay immediate gratification, but the important point is that we, as a species, are capable of doing so. The default mode of the mammalian brain is to do whatever results in an immediate reward.