The Making of the Mind: The Neuroscience of Human Nature (4 page)

Further insight may ultimately come from comparisons of the human genome with the chimpanzee genome. An exciting lead identified regions of the human genome that have dramatically changed from the chimpanzee genome while being conserved without alteration among other mammals in general. These regions are potentially interesting hot spots. Referred to by geneticists as human accelerated regions, they total forty-nine in number (HAR1–49). The one that has changed the most in relation to the chimpanzee genome, HAR1, is part of an RNA gene that is expressed during human cortical development.
²⁵
The finding is especially intriguing because the RNA gene (HAR1F) is expressed while the brain is developing in the womb from seven to nineteen weeks after gestation. This time window is known to be a period when cortical neurons are migrating into position to determine the structure of the six layers of human neocortex. Perhaps it will someday be possible through further neurodevelopmental research to know the details about how the human cortex differs from the nonhuman cortex. It is anticipated that massive cortical expansion, coupled with relatively modest reorganization in the neural networks underlying five key parts of the modern ensemble, will prove vital to the uniqueness of the human brain.

So far, the emphasis has been on how the human brain might differ from nonhuman species. The chapter will now turn to the remarkable commonalities between human and nonhuman brains. Not only do all primates follow the same basic plan for the brain, but so do all vertebrates. The hindbrain, midbrain, and forebrain organization is a constant. At the level of the individual neuron, a similar constancy across life forms can be seen. The pioneering studies of how nerve impulses are conducted in the mammalian nervous system used the neurons of squid, horseshoe crabs, and frogs. The most basic nervous system—a simple net of neurons—is found in the jellyfish. Yet the same electrochemical mechanisms at work in the jellyfish are also at work in the human brain.
²⁶

Besides being able to communicate nerve impulses from one cell to the next, all primates, all mammals in general, and even reptiles, must send signals from the brain to the body via the spinal cord and, conversely, receive messages in the brain from the body. The most primitive parts of the brain, then, are in the metencephalon and myelencephalon of the hindbrain, with direct connections to the spinal cord. A key function of the hindbrain is neural control of the lungs to breathe and the heart to circulate blood, essential life support mechanisms.
²⁷
These, as well as many other essential functions, are just as necessary for life in fish and amphibians as they are for life in reptiles and mammals. It is as if nature conserved brain components that worked well—from individual units such as neurons to complex structures such as brainstems—and reused them in other species.

The mesencephalon or midbrain includes a nucleus of cells known as the
substantia nigra
, which produces a neurotransmitter called dopamine. A loss of the functioning of these neurons results in the motor control difficulties of Parkinson's disease.
²⁸
Dopamine is also a central neurotransmitter in the reward circuit of the brain, which plays a major role in drug addiction, as will be seen in
chapter 6
.

Above the brainstem in the forebrain, however, significant differences appear in different classes of the animal kingdom. Mammals are characterized by more complex forebrains compared with reptiles. The diencephalon of the forebrain is the interior, lower region that lies on top of the brainstem and includes the thalamus and a smaller structure, the hypothalamus, which connects with the most primitive portion of the telencephalon, known as the limbic cortex or rhinencephalon. This primitive structure makes up virtually the entire forebrain of the crocodile. As Richard Thompson clarified, “No negative inferences about the function of the limbic forebrain or of crocodiles is meant, for in addition to being vicious, the crocodile is an intact functioning organism responsive to sensory stimulation and engaging in a variety of behaviors: feeding, fleeing, fighting, and reproduction.”
²⁹
The hypothalamus is linked with several limbic structures important in memory (hippocampus) and emotion (amygdala) that will be discussed in chapters
6
and
7
. Together they constitute the limbic system that plays a central role in our feelings of pleasure, happiness, anger, fear, sadness, and other human emotions.

The other part of the diencephalon, the thalamus, relays signals to higher regions of the brain in the most advanced layer of the telencephalon, the cerebral hemispheres.
³⁰
These inputs to the cerebral hemispheres from the thalamus regulate the brain's state of wakefulness and activity level from signals originating in the brain stem. They also convey the inputs from all our peripheral sensory systems, such as vision, taste, and touch, to the sensory regions of the telencephalon; these are located in the outer neural covering of the cerebral hemispheres called the neocortex. The neocortex can be divided into four general regions. The frontal lobe is the most anterior region behind the eyes and forehead and extending back to a dividing central fissure. Just posterior to the central fissure is the parietal lobe. Behind the parietal lobe at the very posterior end of the cerebral hemispheres is the occipital lobe. The lateral fissure divides off the temporal lobe—near the ears—as the region inferior to the frontal and parietal lobes. Much of what will be discussed in later chapters regarding the ensemble hypothesis makes reference to these specific regions of the cerebral cortex.

The neocortical regions are much larger in the forebrain of human beings compared with other mammalian species. Paul MacLean theorized that the human brain is a composite of the expanded neocortical areas and phylogenetically older regions of the limbic system and the diencephalon.
³¹
Of special importance, he proposed a conception of the limbic system as a brain network shared in common by all mammals and presumably inherited from the very earliest mammals in evolutionary history. The hippocampus, amygdala, and other structures constitute a border surrounding the diencephalon and were referred to as the limbic system. The emotional feelings provided by the limbic system aided the survival and reproduction of individuals. On top of this older limbic structure is the neocortex, which enables the brain to take in visual, auditory, and other perceptual information about the environment to make reasoned, unemotional decisions that benefit survival. In humans, the neocortex is so large and well-developed that it can support reasoning, problem solving, language, and even literacy in the form of reading and writing.
³²
In terms of the argument here, the five parts of the ensemble hypothesis resulted primarily from the expansion and reorganization of the neocortical forebrain.

As knowledge in the cognitive social neuroscience progressed, Maclean's
view that the limbic system was dedicated to emotional processing proved too limited. It became clear that a key part of the limbic system, the hippocampus, plays an important role in the cognitive function of memory storage in nonhumans as well as in humans. It also became clear that the human experience of emotion depends on activation of regions in the neocortex as well as on limbic structures. Still, Maclean's hypothesis appropriately stressed the composite nature of the human brain. As Joseph LeDoux summarized it in the
Annual Review of Neuroscience
: “In particular, the notion that emotions involve relatively primitive circuits that are conserved throughout mammalian evolution seems right on target,” as is the idea that “cognitive processes might involve other circuits and might function relatively independently of emotional circuits, at least in some circumstances.”
³³
Maclean also accurately described the massive expansion and reorganization of the forebrain as the neural underpinning of the modern human mind.

OVERVIEW

The following chapters address the unique mental capacities—such as executive function, language, and mental time travel—that make for a beautiful mind, not in only a few creative geniuses, but in us all. These capacities enable our most uplifting and laudable characteristics as human beings, our morality and spirituality. Yet they also mediate the most shocking evil. We alone are capable of murdering—with intent and malice—entire populations of other human beings. Our capacity for genocide perplexingly coexists with our moral capacity to know that such acts are wrong. Each such defining feature, the good and the atrocious, sets human beings apart from all other inhabitants of earth.

In
chapter 2
, the advanced social intelligence of human beings is considered. Because of its power, human beings moved beyond the glacially slow form of change found in natural selection and modification by biological descent through genetic inheritance. Changes in the human mind over the history of modern human beings have instead occurred through the rapid process of cultural evolution. Unlike any other species, the human brain, from the moment of birth, is immersed in a complex culture that profoundly shapes the functioning of the mind. The human mind cannot be reduced to a genetically predisposed brain structure because the culture in which we are raised is so rich and important to mental development. The human brain includes mechanisms for understanding the mental intentions of others and for learning from the behavior of others through imitation and direct instruction. These mechanisms—interacting with advanced working memory—paved the way for the cooperative invention of new artifacts and the social transmission of culture.

In
chapter 3
, the development of an advanced form of working memory is considered. The key ingredient is the improvement of the executive functions of working memory that allow for human beings to plan ahead, to inhibit impulses, and to delay gratification. However, other changes also occurred in working memory as an interaction with the advent of language. A specific store for verbal information was added in addition to temporary storage components for visual and spatial information. These changes together provided
for a flexibility in human behavior and the capacity to innovate and plan new ways of doing things. Without these changes, the “human revolution” in creating cultural artifacts would never have started.

In
chapter 4
, symbolic thought and its extension in language is addressed. Of the symbols that inhabit our cultural mind, words are the most mysterious. Arbitrary sounds emitted by the human vocal tract stand for objects and events in the physical world and abstract ideas in the world of thought. By stringing words together, speakers of the same language can communicate everything and anything. This is possible only because the left hemisphere of the human brain has become specialized for the production and comprehension of speech. This distinctive part of the ensemble accounts for why there has been only limited success in attempts to teach chimpanzees language. That we can communicate about abstract ideas in essentially an infinite variety of ways is rightly viewed as fundamental to the divide between human and ape mentality.

As described in
chapter 5
, a fourth part of the ensemble emerged from an interiorized form of language—the inner voice—combined with an ability to make causal inferences. Instead of using language to communicate with others, it migrated inward as a medium for silent thought. The inner voice capitalized on our advanced working memory to allow for a running narrative
of the everyday experiences that enter our conscious awareness. Trains of connected thought can run, sustained by our verbal working memory. Moreover, this inner voice teamed up with a capacity to infer the causes of events even when they are hidden and not perceptually obvious. This combination of interiorized language and causal inference provided the human mind with a new cognitive tool to interpret conscious experiences. This interpretive function of the left hemisphere has been discovered in research with patients who had undergone surgery to separate the left and right hemispheres as a means of controlling severe epileptic seizures.

The human mind is the only known exception to the unidirectional flow of time in nature, as will be explained in
chapter 6
. To recollect the what, when, and where of past events implies our capacity for mental, if not physical, time travel. Not only can the mind go back in time to relive past experience, but it can also use the same brain network for moving forward in time to imagine future events. Much of what makes the human mind so unique stems directly from our recollections and replays of the past and our fantasies and apprehensions about the future. The hazards as well as the benefits of mental time travel are outlined in the chapter.

In sum, chapters
2
through
6
specify each of the five parts of the ensemble that distinguishes the modern human mind. In the
Descent of Man and Selection in Relation to Sex
, Charles Darwin stressed the underlying continuity of the human mind with the nonhuman as follows:

There can be no doubt that the difference between the mind of the lowest man and that of the highest animal is immense. An anthropomorphous ape, if he could take a dispassionate view of his own case, would admit that, though he could form an artful plan to plunder a garden—though he could use stones for fighting or for breaking open nuts—yet that the thought of fashioning a stone into a tool was quite beyond his scope. Still less, as he would admit, could he follow metaphysical reasoning, or solve a mathematical problem, or reflect on God, or admire a grand natural scene…. Nevertheless, the difference, great as it is, certainly is one of degree and not of kind.
³⁴

According to the ensemble hypothesis, Darwin was wrong here. The immense difference between the modern human mind and any nonhuman
mind is in fact a matter of kind and not degree. Darwin was certainly aware that language by itself seemed to represent a discontinuity, for he went on to state: “If it could be proved that certain high mental powers were absolutely peculiar to man, which seems extremely doubtful, it is not improbable that these qualities are…mainly the result of the continued use of a perfect language.”
³⁵
He, of course, was unaware of the findings of the cognitive social neurosciences that began to accumulate about one hundred years after he first published
The Descent of Man
in 1871. The continuity thesis of Darwin in the realm of the mind—if not the body—failed to appreciate the summative effects of five different parts of the modern mental ensemble. Further, and this is fatal for Darwin's view that we differ only as a matter of degree from apes, it failed to recognize the nonlinear interactions of these parts. The pairwise and higher order interactions of advanced working memory, advanced social intelligence, the left-hemisphere interpreter, and mental time travel, as well as symbolic thought embodied in language, produce a qualitatively different kind of mind.