The third major part of the neuron is the axon, coming out of the soma like a hose. The axon carries the output messages of a neuron (nerve impulses) along its length to its axon terminals (axon endings). There is only one axon per neuron, although it can branch into multiple axon terminals. In a typical neuron, the root end of the axon emerges out of the soma at a small swelling called the axon hillock. Between the axon hillock and the first segment of the axon is where the nerve impulse is first generated (see discussion of the action potential, the nerve impulse, that follows below).

Myelinated and Unmyelinated Axons Some axons have a glial cell covering known as the myelin sheath. The glial cells capable of producing myelin include oligodendrocytes of the CNS and the Schwann cells of the PNS. This fatty, insulating, myelin sheath has gaps in it, revealing the bare axon, at regular intervals along the axon's length. These bare spots along the length of a myelinated axon are called nodes, or nodes of Ranvier, after their discoverer. Figure 5.1.35.1.3\PageIndex{3}: Nodes of Ranvier. Nodes of Ranvier are gaps in the myelin sheath which covers myelinated axons. Nodes contain voltage-gated potassium (K+) and sodium (Na+) channels. Action potentials travel down the axon by "jumping"from one node to the next, speeding conduction of the action potential down the length of the axon toward the axon ending, also known as the axon bouton, axon button, or axon terminal. The function of the myelin sheath and the nodes is to speed up the rate at which nerve impulses travel down the length of the axon toward their destination, the axon terminal. In myelinated axons, the impulses sort of "jump" from node to node allowing the action potential to move more rapidly down the axon. This leaping of the nerve impulse (action potential) from node to node is called saltatory conduction, from the Latin "saltatore" which means to dance. Imagine the romantic image of the impulse dancing from node to node. More details on the electrical nature of this conduction will be addressed in the next section of this chapter. Not all axons are myelinated. Unmyelinated axons tend to be older in evolution and to be the smaller diameter axons (classified as C fibers based on their small diameters; large diameter myelinated axons are called A fibers). In unmyelinated axons, in order to move, the nerve impulse must be regenerated at every successive point along the axon. This takes time and slows the conduction of the nerve impulse (the action potential) down the length of the axon. Therefore, conduction of the action potential down the length of an unmyelinated axon is relatively slow. An example of unmyelinated C fibers are axons that are part of slow pain pathways. These pathways mediate the slower aching pain that follows tissue damage. The quick, sharp pain from an injury is mediated by larger diameter A fibers (axons). Not sure if figure below is needed.

Neuron Anatomy and The Synapse To understand neuron function, it is important to review the neuronal anatomy presented previously. Although there are some exceptions, neurons have three major structural parts - - the soma or cell body, dendrites (the "receivers" of the neuron), and the axon (the ouput end of the neuron). The entire neuron is bounded by a cell membrane, the neuronal membrane. The neuronal membrane of a neuron has channels or "doors" for ions (electrically charged atoms) which are able to pass through the membrane when specific channels are opened. Figure 5.1.15.1.1\PageIndex{1} shows this basic neuron anatomy. The soma or cell body contains organelles, such as ribosomes and mitochondria which are common to most types of cells in the body. These are involved in the basic metabolism of the cell. The soma also contains the nucleus, where the genes and chromosomes (containing DNA) are located. The second main part of the neuron are the dendrites, the information receivers of the neuron. Dendrites in some neurons can branch profusely (dendritic trees), expanding the region of the neuron that can receive inputs from other neurons. The receptor sites (or more technically, postsynaptic receptor sites because of their location on the receiving or postsynaptic neuron) which receive molecules of neurotransmitter are located on the dendrites (and, to a lesser degree, on the soma) of the receiving neuron. On the dendrites are small dendritic spines which are associated with the connections between neurons (the synapses) and can change shape rapidly when learning occurs. Notice that the spines are not the same thing as dendritic branches. In Figure 5.1.25.1.2\PageIndex{2}, a dendritic branch with dendritic spines is shown on the left in microscopic detail, and on the right, are dendritic trees of two types of neuron found in the retina. Spines are not visible in the images of dendritic trees on the right because the dendritic spines are too small, while dendritic branches comprising the dendritic trees can easily be seen (see caption for Figure 5.1.25.1.2\PageIndex{2}). Note: A detailed discussion of the structure of the synapse and synaptic communication will be covered later in this chapter.

In this section, we continue our exploration of neurons, the building blocks of the nervous system. We examine how they generate electrochemical signals, and how the billions of neurons in the nervous system communicate with one another, a process known as synaptic transmission. Before tackling these topics, we review and expand the basic anatomy and functioning of neurons covered in part in Chapter 4. A sound grasp of these facts provides the groundwork for understanding how neuron potentials are generated within neurons and how they combine to trigger synaptic transmission. As you read, remember that the voltages and chemical events we discuss in this section, operating in large populations of brain cells, somehow generate our perceptions, thoughts, emotions, and the entirety of our mental experience. To date, how this happens, how patterns of neuron potentials in brain circuits become conscious minds, remains the greatest mystery of all facing modern science.

Neuron Structure The main thing that makes neurons special and differentiates them from other cells in the body is that they have many extensions of their cell membranes, generally referred to as processes. Neurons are usually described as having one, and only one, axon—a fiber that emerges from the cell body and projects to target cells. That single axon can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse (also called an action potential), which is communicated to one or more cells. The other processes of the neuron are dendrites, which receive information from other neurons across specialized areas called synapses. The dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. Figure 4.1.54.1.5\PageIndex{5} shows the structure of a typical neuron. The main parts of a neuron are labeled in the figure and described below. Figure 4.1.54.1.5\PageIndex{5}: Neuron. A somatic motor neuron (in the CNS) with labeled structures: cell membrane, dendrites, cell body (soma), axon, axon hillock, node of Ranvier, myelin sheath, axon terminal, and synaptic end bulbs (also called axon terminal buttons). The oligodendrocyte shown to the side of the axon is a glial cell that forms the myelin sheath surrounding the axon. The cell body (or soma; soma = "body") is the part of a neuron that contains the nucleus (shown as an oval structure in the center of the cell body, but not labeled) and most of the major organelles. The cell body is usually quite compact, and may not be much wider than the nucleus. The cell membrane is the structure that surrounds all the surfaces of the cell (including the dendrites and axon) and separates the inside of the cell from the outside of the cell. Dendrites are thin structures that are extensions of the cell body. Their function is to receive messages (excitatory and inhibitory post-synaptic potentials, EPSPs/IPSPs- see the nervous system communication chapter) from other cells and carry them to the cell body. A neuron may have many dendrites, and each dendrite may branch repeatedly to form a dendrite “tree” with more than 1,000 dendritic branches. Dendritic spines (small extensions on the surface of the dendritic branches) further increase surface area for receiving messages, allowing a given neuron to communicate with thousands of other cells. The axon is a long, thin extension of the cell body. It transmits nerve impulses away from the cell body and toward other cells. The axon hillock is a small bulge found at the base of motor neuron axons. The nerve impulse (or action potential) starts from the axon hillock. The axon branches at the end, forming the axon terminal. Branches of the axon terminal end in axon terminal buttons (also called axon endings, synaptic end bulbs, synaptic buttons/boutons, bouton terminaux, etc.) These are the points where the message is transmitted to other cells (via the release of chemicals called neurotransmitters), often to the dendrites of other neurons. A small gap called a synapse (also called a synaptic gap or synaptic cleft) is located between the end of the axon terminal and the surface of the receiving cell. An axon may branch hundreds of times, but there is never more than one axon per neuron. Many axons (especially the long axons of nerves in the peripheral nervous system) are covered by sections of myelin (also called the myelin sheath). The myelin sheath is composed of lipid layers that surround the axon. Myelin is a very good electrical insulator, like the plastic or rubber that encases an electrical cord. Axons that are covered by sections of myelin are called myelinated, whereas axons without myelin sheaths are called unmyelinated. Regularly spaced gaps between sections of myelin occur along the axon. (The gaps are actually much further apart than is shown in the figure- it is necessary to shrink the distance to fit all the structures in a diagram!) These gaps are called nodes of Ranvier, and they allow the transmission of nerve impulses along the axon. Nerve impulses jump from node to node in a process called saltatory conduction, allowing nerve impulses to travel along the axon very rapidly. The oligodendrocyte shown in the figure is a glial cell that produces myelin sheaths in the central nervous system (brain and spinal cord)- see the Glia section below.

Neuron Structure The main thing that makes neurons special and differentiates them from other cells in the body is that they have many extensions of their cell membranes, generally referred to as processes. Neurons are usually described as having one, and only one, axon—a fiber that emerges from the cell body and projects to target cells. That single axon can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse (also called an action potential), which is communicated to one or more cells. The other processes of the neuron are dendrites, which receive information from other neurons across specialized areas called synapses. The dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. Figure 4.1.54.1.5\PageIndex{5} shows the structure of a typical neuron. The main parts of a neuron are labeled in the figure and described below. Figure 4.1.54.1.5\PageIndex{5}: Neuron. A somatic motor neuron (in the CNS) with labeled structures: cell membrane, dendrites, cell body (soma), axon, axon hillock, node of Ranvier, myelin sheath, axon terminal, and synaptic end bulbs (also called axon terminal buttons). The oligodendrocyte shown to the side of the axon is a glial cell that forms the myelin sheath surrounding the axon. The cell body (or soma; soma = "body") is the part of a neuron that contains the nucleus (shown as an oval structure in the center of the cell body, but not labeled) and most of the major organelles. The cell body is usually quite compact, and may not be much wider than the nucleus. The cell membrane is the structure that surrounds all the surfaces of the cell (including the dendrites and axon) and separates the inside of the cell from the outside of the cell. Dendrites are thin structures that are extensions of the cell body. Their function is to receive messages (excitatory and inhibitory post-synaptic potentials, EPSPs/IPSPs- see the nervous system communication chapter) from other cells and carry them to the cell body. A neuron may have many dendrites, and each dendrite may branch repeatedly to form a dendrite “tree” with more than 1,000 dendritic branches. Dendritic spines (small extensions on the surface of the dendritic branches) further increase surface area for receiving messages, allowing a given neuron to communicate with thousands of other cells. The axon is a long, thin extension of the cell body. It transmits nerve impulses away from the cell body and toward other cells. The axon hillock is a small bulge found at the base of motor neuron axons. The nerve impulse (or action potential) starts from the axon hillock. The axon branches at the end, forming the axon terminal. Branches of the axon terminal end in axon terminal buttons (also called axon endings, synaptic end bulbs, synaptic buttons/boutons, bouton terminaux, etc.) These are the points where the message is transmitted to other cells (via the release of chemicals called neurotransmitters), often to the dendrites of other neurons. A small gap called a synapse (also called a synaptic gap or synaptic cleft) is located between the end of the axon terminal and the surface of the receiving cell. An axon may branch hundreds of times, but there is never more than one axon per neuron. Many axons (especially the long axons of nerves in the peripheral nervous system) are covered by sections of myelin (also called the myelin sheath). The myelin sheath is composed of lipid layers that surround the axon. Myelin is a very good electrical insulator, like the plastic or rubber that encases an electrical cord. Axons that are covered by sections of myelin are called myelinated, whereas axons without myelin sheaths are called unmyelinated. Regularly spaced gaps between sections of myelin occur along the axon. (The gaps are actually much further apart than is shown in the figure- it is necessary to shrink the distance to fit all the structures in a diagram!) These gaps are called nodes of Ranvier, and they allow the transmission of nerve impulses along the axon. Nerve impulses jump from node to node in a process called saltatory conduction, allowing nerve impulses to travel along the axon very rapidly. The oligodendrocyte shown in the figure is a glial cell that produces myelin sheaths in the central nervous system (brain and spinal cord)- see the Glia section below.

Nervous Tissue Cell Types Nervous tissue is composed of two types of cells: neurons (also called nerve cells) and glia (also called glial cells or neuroglia), as shown in Figure 4.1.34.1.3\PageIndex{3}. Neurons are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells. Glia are known to play a supporting role for nervous tissue. Ongoing research pursues an expanded role that glial cells might play in signaling, but neurons are still considered the basis of this function. Neurons are important, but without glial support they would not be able to perform their function.

Summary The nervous system coordinates all of the body’s voluntary and involuntary actions by transmitting electrical and chemical signals to and from different parts of the body. The two main divisions of the nervous system are the central nervous system (CNS, the brain and the spinal cord), and the peripheral nervous system (PNS, all other nervous tissue in the body). Nervous tissue contains two major cell types, neurons and glial cells. Neurons are the cells responsible for communication through electrical signals. Glial cells are supporting cells, maintaining the environment around the neurons. The structures that differentiate neurons from other body cells are the extensions of their cell membranes, namely one axon that projects to target cells, and one or more dendrites, which receive information from other neurons across specialized areas called synapses. The axon propagates nerve impulses (action potentials), which are communicated to one or more cells. Neurons can be classified depending on their structure, function, or other characteristics. One structural classification is based on the number of processes the neuron has- one (unipolar), two (bipolar) or many (multipolar). One functional classification groups neurons into those that participate in sensation (sensory neurons), integration (interneurons) or motor (motor neurons) functions. Some other ways of classifying neurons include what they look like, where they are found, who found them, what they do, or what neurotransmitters they use. The nervous tissue in the brain and spinal cord consists of gray matter and white matter. Gray matter contains the cell bodies and dendrites of neurons and white matter contains myelinated axons. Typically, neurons cannot divide to form new neurons. Recent animal research indicates that some limited neurogenesis is possible, but the extent to which this applies to adult humans is unknown. Several types of glial cells are found in the nervous system, including astrocytes, oligodendrocytes, microglia, and ependymal cells in the CNS, and satellite cells and Schwann cells in the PNS. Astrocytes contribute to the blood-brain barrier that protects the brain. Oligodendrocytes and Schwann cells create the myelin that insulates many axons, allowing nerve impulses to travel along the axon very rapidly.

Overview of the Nervous System The nervous system, illustrated in Figure 4.1.24.1.2\PageIndex{2}, is the human organ system that coordinates all of the body’s voluntary and involuntary actions by transmitting electrical and chemical signals to and from different parts of the body. Specifically, the nervous system extracts information from the internal and external environments using sensory receptors. It then usually sends signals encoding this information to the brain, which processes the information to determine an appropriate response. Finally, the brain sends signals to muscles, organs, or glands to bring about the necessary response. The two main divisions of the nervous system are the central nervous system (CNS), consisting of the brain and the spinal cord, and the peripheral nervous system (PNS), which includes all other nervous tissue, such as ganglia and nerves, outside the brain and spinal cord. The CNS and PNS are covered in greater detail in separate sections. In the example above, your eyes detected the skateboarder, the information traveled to your brain, and your brain instructed your body to act to avoid a collision.

While the cellular and molecular mechanisms that influence on physical and mental health have long been a central focus of neuroscience, only in recent years has attention turned to the epigenetic mechanisms behind the dynamic changes in gene expression responsible for normal cognitive function and increased risk for mental illness. Ongoing research will show if this increased attention on epigenetics can be exploited in the development of new therapeutic options that may alter the traces that early environment leaves on the genome. However, as discussed in this module, the epigenome is not static and can be molded by developmental signals, environmental perturbations, and disease states, which present an experimental challenge in the search for epigenetic risk factors in psychological disorders (Rakyan, Down, Balding, & Beck, 2011). The combination of genetic association maps studies with epigenome-wide developmental studies may help identify novel molecular mechanisms to explain features of inheritance of personality traits and transform our understanding of the biological basis of psychology. Importantly, these epigenetic studies may lead to identification of novel therapeutic targets and enable the development of improved strategies for early diagnosis, prevention, and better treatment of psychological and behavioral disorders.

The primary epigenetic mark: DNA modification DNA methylation is the best-understood epigenetic modification influencing gene expression. DNA is composed of four types of naturally occurring nitrogenous bases: adenine (A), thymine (T), guanine (G), and cytosine (C). In mammalian genomes, DNA methylation occurs primarily at cytosine residues in the context of cytosines that are followed by guanines (CpG dinucleotides), to form 5-methylcytosine in a cell-specific pattern (Goll & Bestor, 2005; Law & Jacobsen, 2010; Suzuki & Bird, 2008). The enzymes that perform DNA methylation are called DNA methyltransferases (DNMTs), which catalyze the transfer of a methyl group to the cytosine (Adams, McKay, Craig, & Burdon, 1979). These enzymes are all expressed in the central nervous system and are dynamically regulated during development (Feng, Chang, Li, & Fan, 2005; Goto et al., 1994). The effect of DNA methylation on gene function varies depending on the period of development during which the methylation occurs and location of the methylated cytosine. Methylation of DNA in gene regulatory regions (promoter and enhancer regions) usually results in gene silencing and reduced gene expression (Ooi, O’Donnell, & Bestor, 2009; Suzuki & Bird, 2008; Sutter and Doerfler, 1980; Vardimon et al., 1982). This is a powerful regulatory mechanism that ensures that genes are expressed only when needed. Thus DNA methylation may broadly impact human brain development, and age-related misregulation of DNA methylation is associated with the molecular pathogenesis of neurodevelopmental disorders.

Early childhood is not only a period of physical growth; it is also a time of mental development related to changes in the anatomy, physiology, and chemistry of the nervous system that influence mental health throughout life. Cognitive abilities associated with learning and memory, reasoning, problem solving, and developing relationships continue to emerge during childhood. Brain development is more rapid during this critical or sensitive period than at any other, with more than 700 neural connections created each second. Herein, complex gene–environment interactions serve to increase the number of possible contacts between neurons, as they hone their adult synaptic properties and excitability. Many weak connections form to different neuronal targets; subsequently, they undergo remodeling in which most connections vanish and a few stable connections remain. These structural changes (or plasticity) may be crucial for the development of mature neural networks that support emotional, cognitive, and social behavior. The challenge for psychology has been to integrate findings from genetics and environmental (social, biological, chemical) factors into the study of personality and our understanding of the emergence of mental illness. These studies have demonstrated that common DNA sequence variation and rare mutations account for only a small fraction (1%–2%) of the total risk for inheritance of personality traits and mental disorders (Dick, Riley, & Kendler, 2010; Gershon, Alliey-Rodriguez, & Liu, 2011). Additionally, studies that have attempted to examine the mechanisms and conditions under which DNA sequence variation influences brain development and function have been confounded by complex cause-and-effect relationships (Petronis, 2010). Epigenetics has the potential to provide answers to these important questions. It refers to the transmission of observable characteristics (phenotype) in terms of gene expression in the absence of changes in DNA sequence (Waddington, 1942; Wolffe & Matzke, 1999). The advent of advanced techniques to study the distributions of regulators of gene expression throughout the genome led to the collective description of the “epigenome.” In contrast to the genome sequence, which is static and the same in almost all cells, the epigenome is highly dynamic, differing among cell types, tissues, and brain regions (Gregg et al., 2010). Recent studies have provided insights into epigenetic regulation of developmental pathways in response to a range of external environmental factors (Dolinoy, Weidman, & Jirtle, 2007). These environmental factors during early childhood and adolescence can cause changes in expression of genes conferring risk of mental health and chronic physical conditions. Thus, the examination of genetic–epigenetic–environment interactions from a developmental perspective may determine the nature of gene misregulation in psychological disorders. This module will provide an overview of the main components of the epigenome and review themes in recent epigenetic research that have relevance for psychology, to form the biological basis for the interplay between environmental signals and the genome in the regulation of individual differences in physiology, emotion, cognition, and behavior.

Some common questions about nature–nurture are, how susceptible is a trait to change, how malleable is it, and do we “have a choice” about it? These questions are much more complex than they may seem at first glance. Height seems like a trait firmly rooted in our nature and unchangeable, but the average height of many populations in Asia and Europe has increased significantly in the past 100 years, due to changes in diet and the alleviation of poverty. There are a few rare genes that have been found to have significant (almost always negative) effects, such as the single gene that causes Huntington’s disease, or the Apolipoprotein gene that causes early onset dementia in a small percentage of Alzheimer’s cases. Aside from these rare genes, however, the genetic impact on behavior is broken up over many genes, each with very small effects. In fact, the same is true of environmental effects. We know that extreme environmental hardship causes catastrophic effects for many behavioral outcomes, however, within the average range of environmental events, those responsible for differences (e.g., why some children in a suburban third-grade classroom perform better than others) are much more difficult to grasp. The difficulties with finding clear-cut solutions to nature–nurture problems bring us back to the other great questions about our relationship with the natural world: the mind-body problem and free will. Investigations into what we mean when we say we are aware of something reveal that consciousness is not simply the product of a particular area of the brain, nor does choice turn out to be an orderly activity that we can apply to some behaviors but not others. So it is with nature and nurture: What at first may seem to be a straightforward matter, able to be indexed with a single number, becomes more and more complicated the closer we look. It is tempting to predict that the more we understand the wide-ranging effects of genetic differences on all human characteristics—especially behavioral ones—our cultural, ethical, legal, and personal ways of thinking about ourselves will have to undergo profound changes in response. One of the most important things modern genetics has taught us is that almost all human behavior is too complex to be nailed down, even from the most complete genetic information, unless we’re looking at identical twins. The science of nature and nurture has demonstrated that genetic differences among people are vital to human moral equality, freedom, and self-determination, not opposed to them. As Mordecai Kaplan said about the role of the past in Jewish theology, genetics gets a vote, not a veto, in the determination of human behavior. We should indulge our fascination with nature–nurture while resisting the temptation to oversimplify it.

Another option for observing nature-nurture in humans involves twin studies. There are two types of twins: monozygotic (MZ) and dizygotic (DZ). Monozygotic twins, also called “identical” twins, result from a single zygote (fertilized egg) and have the same DNA. They are essentially clones. Dizygotic twins, also known as “fraternal” twins, develop from two zygotes and share 50% of their DNA. Fraternal twins are ordinary siblings who happen to have been born at the same time. To analyze nature–nurture using twins, we compare the similarity of MZ and DZ pairs. Sticking with the features of height and spoken language, let’s take a look at how nature and nurture apply: Identical twins, unsurprisingly, are almost perfectly similar for height. The heights of fraternal twins, however, are like any other sibling pairs: more similar to each other than to people from other families, but hardly identical. This contrast between twin types gives us a clue about the role genetics plays in determining height. Now consider spoken language. If one identical twin speaks Spanish at home, the co-twin with whom she is raised almost certainly does too. But the same would be true for a pair of fraternal twins raised together. In terms of spoken language, fraternal twins are just as similar as identical twins, so it appears that the genetic match of identical twins doesn’t make much difference. Twin and adoption studies are two instances of a much broader class of methods for observing nature-nurture called quantitative genetics, the scientific discipline in which similarities among individuals are analyzed based on how biologically related they are. We can do these studies with siblings and half-siblings, cousins, twins who have been separated at birth and raised separately (Bouchard, Lykken, McGue, & Segal, 1990; such twins are very rare and play a smaller role than is commonly believed in the science of nature–nurture), or with entire extended families (see Plomin, DeFries, Knopik, & Neiderhiser, 2012, for a complete introduction to research methods relevant to nature–nurture). For better or for worse, contentions about nature–nurture have intensified because quantitative genetics produces a number called a heritability coefficient, varying from 0 to 1, that is meant to provide a single measure of genetics’ influence of a trait. In a general way, a heritability coefficient measures how strongly differences among individuals are related to differences among their genes. But beware: Heritability coefficients, although simple to compute, are deceptively difficult to interpret. Nevertheless, numbers that provide simple answers to complicated questions tend to have a strong influence on the human imagination, and a great deal of time has been spent discussing whether the heritability of intelligence or personality or depression is equal to one number or another.

Overview There are three related problems at the intersection of philosophy and science that are fundamental to our understanding of our relationship to the natural world: the mind–body problem, the free will problem, and the nature–nurture problem. It seems that most people, even those without much knowledge of science or philosophy, have opinions about the answers to these questions that come simply from observing the world we live in. Our feelings about our relationship with the physical and biological world often seem incomplete. We are in control of our actions in some ways, but at the mercy of our bodies in others; it feels obvious that our consciousness is some kind of creation of our physical brains, at the same time we sense that our awareness must go beyond just the physical. This incomplete knowledge of our relationship with nature leaves us fascinated and a little obsessed, like a cat that climbs into a paper bag and then out again, over and over, mystified every time by a relationship between inner and outer that it can see but can’t quite understand. It may seem obvious that we are born with certain characteristics while others are acquired, and yet of the three great questions about humans’ relationship with the natural world, only nature–nurture gets referred to as a “debate.” In the history of psychology, no other question has caused so much controversy and offense: We are so concerned with nature–nurture because our very sense of moral character seems to depend on it. The problem is, most human characteristics aren’t usually as clear-cut as, for example height or instrument-mastery, affirming our nature–nurture expectations strongly one way or the other. In fact, even the great violinist might have some inborn qualities—perfect pitch, or long, nimble fingers—that support and reward his hard work. And the basketball player might have eaten a diet while growing up that promoted her genetic tendency for being tall. When we think about our own qualities, they seem under our control in some respects, yet beyond our control in others. And often the traits that don’t seem to have an obvious cause are the ones that concern us the most and are far more personally significant. What about how much we drink or worry? What about our honesty, or religiosity, or sexual orientation? They all come from that uncertain zone, neither fixed by nature nor totally under our own control.

Evolutionary Psychology and Cultural Influences In evolutionary psychology, culture also has a major effect on psychological adaptations. For example, status within one’s group is important in all cultures for achieving reproductive success, because higher status makes someone more attractive to mates. In individualistic cultures, such as the United States, status is heavily determined by individual accomplishments. But in more collectivist cultures, such as Japan, status is more heavily determined by contributions to the group and by that group’s success. For example, consider a group project. If you were to put in most of the effort on a successful group project, the culture in the United States reinforces the psychological adaptation to try to claim that success for yourself (because individual achievements are rewarded with higher status). However, the culture in Japan reinforces the psychological adaptation to attribute that success to the whole group (because collective achievements are rewarded with higher status). Evolutionary psychology, in short, does not predict rigid robotic-like “instincts.” That is, there isn’t one rule that works all the time. Rather, evolutionary psychology studies flexible, environmentally-connected and culturally-influenced adaptations that vary according to the situation. Psychological adaptations are hypothesized to be wide-ranging, and include food preferences, habitat preferences, mate preferences, and specialized fears. These psychological adaptations also include many traits that improve people's ability to live in groups, such as the desire to cooperate and make friends, or the inclination to spot and avoid frauds, punish rivals, establish status hierarchies, nurture children, and help genetic relatives. Research programs in evolutionary psychology develop and empirically test predictions about the nature of psychological adaptations.

Gene Selection Theory In modern evolutionary theory, all evolutionary processes boil down to an organism’s genes. Genes are the basic “units of heredity,” or the information that is passed along in DNA that tells the cells and molecules how to “build” the organism and how that organism should behave. Genes that are better able to encourage the organism to reproduce, and thus replicate themselves in the organism’s offspring, have an advantage over competing genes that are less able. For example, take female sloths: In order to attract a mate, they will scream as loudly as they can, to let potential mates know where they are in the thick jungle. Now, consider two types of genes in female sloths: one gene that allows them to scream extremely loudly, and another that only allows them to scream moderately loudly. In this case, the sloth with the gene that allows her to shout louder will attract more mates—increasing reproductive success—which ensures that her genes are more readily passed on than those of the quieter sloth.

Basics of Evolutionary Theory Evolution simply means change over time. Many think of evolution as the development of traits and behaviors that allow us to survive this “dog-eat-dog” world, like strong leg muscles to run fast, or fists to punch and defend ourselves. However, physical survival is only important if it eventually contributes to successful reproduction. That is, even if you live to be a 100-year-old, if you fail to mate and produce children, your genes will die with your body. Thus, reproductive success, not survival success, is the engine of evolution by natural selection. Every mating success by one person means the loss of a mating opportunity for another. Yet every living human being is an evolutionary success story. Each of us is descended from a long and unbroken line of ancestors who triumphed over others in the struggle to survive (at least long enough to mate) and reproduce. However, in order for our genes to endure over time—to survive harsh climates, to defeat predators—we have inherited adaptive, psychological processes designed to ensure success. At the broadest level, we can think of organisms, including humans, as having two large classes of adaptations—or traits and behaviors that evolved over time to increase our reproductive success. The first class of adaptations are called survival adaptations: mechanisms that helped our ancestors handle the “hostile forces of nature.” For example, in order to survive very hot temperatures, we developed sweat glands to cool ourselves. In order to survive very cold temperatures, we developed shivering mechanisms (the speedy contraction and expansion of muscles to produce warmth). Other examples of survival adaptations include developing a craving for fats and sugars, encouraging us to seek out particular foods rich in fats and sugars that keep us going longer during food shortages. Some threats, such as snakes, spiders, darkness, heights, and strangers, often produce fear in us, which encourages us to avoid them and thereby stay safe. These are also examples of survival adaptations. However, all of these adaptations are for physical survival, whereas the second class of adaptations are for reproduction, and help us compete for mates. These adaptations are described in an evolutionary theory proposed by Charles Darwin, called sexual selection theory.

Nonhuman Animal Subject Research One area of controversy regarding research techniques is the use of nonhuman animal subjects. One of the keys to behaving in an ethical manner is to ensure that one has given informed consent to be a subject in a study. Obviously, animals are unable to give consent. For this reason and others related to animal welfare, there are some who believe that researchers should not use nonhuman animal subjects in any case. There are others that advocate for using nonhuman animal subjects because nonhuman animal subjects many times will have distinct advantages over human subjects. Their nervous systems are frequently less complex than human systems, which facilitates the research. It is much easier to learn from a system with thousands of neurons compared to one with billions of neurons like humans. Also, nonhuman animals may have other desirable characteristics such as shorter life cycles, larger neurons, and translucent embryos. However, it is widely recognized that this research must proceed with explicit guidelines ensuring the safe treatment of the animals. For example, any research institution that will be conducting research using nonhuman animal subjects must have an Institutional Animal Care and Use Committee (IACUC). IACUCs review the proposed experiments to ensure an appropriate rationale for using nonhuman animals as subjects and ensure ethical treatment of those subjects. Furthermore, many researchers who work with nonhuman animal subjects adhere to the Three R's: Replacement, Reduction, and Refinement (Russell & Burch, 1959). Replacement suggests that researchers should seek to use inanimate systems as a replacement for nonhuman animal subjects whenever possible. Furthermore, replacement is also suggested to replace higher level organisms with lower level organisms whenever possible. The idea is that instead of choosing a primate to conduct the study, researchers are encouraged to use a lower level animal such as an invertebrate (a sea slug, for example) to conduct the study. Reduction refers to reducing the number of nonhuman animal subjects that will be used in the particular study. The idea here is that if a study can learn sufficient information from one nonhuman animal, then they should only use one. Finally, refinement is about how the nonhuman animals are cared for. The goal is to minimize discomfort that the subject experiences and to enhance the conditions that the subject experiences throughout their life. For a full discussion of the Three R's, see Tannenbaum and Bennett (2015). In conclusion, many researchers argue that what we have learned from nonhuman animal subjects has been invaluable. These studies have led to drug therapies for treating pain and other disorders; for instance, most drugs are studied using animals first, to ensure they are safe for humans. Animal nervous systems are used as models for the human nervous systems in many areas. Sea slugs (Aplysia californica) have been used to learn about neural networks involved in learning and memory. Cats have been studied to learn about how our brain's visual system is organized. Owls have been used to learn about sound localization in the auditory system. Indeed, research using nonhuman animal subjects has led to many important discoveries.

Ethics in Neuroscience Research Research has a very complicated history with respect to ethics. This is true when discussing our treatment of nonhuman animal subjects and our treatment of human subjects as well. Let’s start by discussing the ethical considerations for nonhuman animal subject research. Nonhuman Animal Subject Research One area of controversy regarding research techniques is the use of nonhuman animal subjects. One of the keys to behaving in an ethical manner is to ensure that one has given informed consent to be a subject in a study. Obviously, animals are unable to give consent. For this reason and others related to animal welfare, there are some who believe that researchers should not use nonhuman animal subjects in any case. There are others that advocate for using nonhuman animal subjects because nonhuman animal subjects many times will have distinct advantages over human subjects. Their nervous systems are frequently less complex than human systems, which facilitates the research. It is much easier to learn from a system with thousands of neurons compared to one with billions of neurons like humans. Also, nonhuman animals may have other desirable characteristics such as shorter life cycles, larger neurons, and translucent embryos. However, it is widely recognized that this research must proceed with explicit guidelines ensuring the safe treatment of the animals. For example, any research institution that will be conducting research using nonhuman animal subjects must have an Institutional Animal Care and Use Committee (IACUC). IACUCs review the proposed experiments to ensure an appropriate rationale for using nonhuman animals as subjects and ensure ethical treatment of those subjects. Furthermore, many researchers who work with nonhuman animal subjects adhere to the Three R's: Replacement, Reduction, and Refinement (Russell & Burch, 1959). Replacement suggests that researchers should seek to use inanimate systems as a replacement for nonhuman animal subjects whenever possible. Furthermore, replacement is also suggested to replace higher level organisms with lower level organisms whenever possible. The idea is that instead of choosing a primate to conduct the study, researchers are encouraged to use a lower level animal such as an invertebrate (a sea slug, for example) to conduct the study. Reduction refers to reducing the number of nonhuman animal subjects that will be used in the particular study. The idea here is that if a study can learn sufficient information from one nonhuman animal, then they should only use one. Finally, refinement is about how the nonhuman animals are cared for. The goal is to minimize discomfort that the subject experiences and to enhance the conditions that the subject experiences throughout their life. For a full discussion of the Three R's, see Tannenbaum and Bennett (2015). In conclusion, many researchers argue that what we have learned from nonhuman animal subjects has been invaluable. These studies have led to drug therapies for treating pain and other disorders; for instance, most drugs are studied using animals first, to ensure they are safe for humans. Animal nervous systems are used as models for the human nervous systems in many areas. Sea slugs (Aplysia californica) have been used to learn about neural networks involved in learning and memory. Cats have been studied to learn about how our brain's visual system is organized. Owls have been used to learn about sound localization in the auditory system. Indeed, research using nonhuman animal subjects has led to many important discoveries.

One Major Concern With Lesion/Surgery Studies One thing to remember about all studies of lesion or surgical patients is that the ability to generalize to the population during these studies may be questionable. It is important to keep in mind that that the reason these patients are studied is because they had some sort of issue with their brain. It is reasonable to wonder whether their brains are representative of “normal subjects,” that is, subjects who do not have lesions or other issues. For example, perhaps someone with epilepsy, after having years of seizures, has a different brain organization than someone without epilepsy. In that circumstance, what we learn from them in a split brain study may not be applicable to a non-epileptic population. References

One additional way to study the contributions of each hemisphere separately is through a procedure known as a Wada. In a Wada procedure, a barbiturate (a depressant drug used for various purposes including sedation) is used to put one half of the brain “to sleep” and then the contributions of the other hemisphere can be studied. Wada procedures are typically used for similar purposes as are cortical mapping techniques such as direct cortical stimulation. But, instead of mapping specific functions to specific areas (as with direct cortical stimulation), the Wada procedure maps functions to hemispheres. Usually, the Wada is used to identify which hemisphere is responsible for language processing and memory tasks. Although scientists know that language functions are usually in the left hemisphere, it is not always the case (particularly in left-handed individuals), so the Wada will help determine which hemisphere is dominant for language functions. For memory functions, both hemispheres play a significant role, but during the Wada, doctors are able to determine which hemisphere has stronger memory function. One Major Concern With Lesion/Surgery Studies One thing to remember about all studies of lesion or surgical patients is that the ability to generalize to the population during these studies may be questionable. It is important to keep in mind that that the reason these patients are studied is because they had some sort of issue with their brain. It is reasonable to wonder whether their brains are representative of “normal subjects,” that is, subjects who do not have lesions or other issues. For example, perhaps someone with epilepsy, after having years of seizures, has a different brain organization than someone without epilepsy. In that circumstance, what we learn from them in a split brain study may not be applicable to a non-epileptic population.

Another technique that is worth mentioning is transcranial magnetic stimulation (TMS). TMS is a noninvasive method that causes depolarization or hyperpolarization in neurons near the scalp. Depolarizations are increases in the electrical state of the neuron, while hyperpolarizations are decreases. In TMS, a coil of wire is placed just above the participant’s scalp (as shown in Figure 2.4.42.4.4\PageIndex{4}). When electricity flows through the coil, it produces a magnetic field. This magnetic field travels through the skull and scalp and affects neurons near the surface of the brain. When the magnetic field is rapidly turned on and off, a current is induced in the neurons, leading to depolarization or hyperpolarization, depending on the number of magnetic field pulses. Single- or paired-pulse TMS depolarizes site-specific neurons in the cortex, causing them to fire. If this method is used over certain brain areas involved with motor control, it can produce or block muscle activity, such as inducing a finger twitch or preventing someone from pressing a button. If used over brain areas involved with visual perception, it can produce sensations of flashes of light or impair visual processes. This has proved to be a valuable tool in studying the function and timing of specific processes such as the recognition of visual stimuli. Repetitive TMS produces effects that last longer than the initial stimulation. Depending on the intensity, coil orientation, and frequency, neural activity in the stimulated area may be either attenuated or amplified. Used in this manner, TMS is able to explore neural plasticity, which is the ability of connections between neurons to change. This has implications for treating psychological disorders, such as depression, as well as understanding long-term changes in neuronal excitability. Note that TMS is different from the previous techniques in that we are not taking images of what the brain is doing. TMS disrupts or stimulates the brain and actively changes what the brain is doing.

Using Indirect Functional Imaging Techniques to Study a Disorder: Autism Spectrum Disorder PET and fMRI studies of ASD have found different levels of neuronal activity in the amygdala and the hippocampus compared to subjects without ASD. These areas are notable because they are a part of the “social brain.” These studies have largely focused on patients with ASD when they are viewing faces. As the viewing of faces is a large part of socializing (for example, reading expressions and making eye contact) and socializing is one area where many autistic patients have issues, these studies help provide further information for doctors and researchers to use. (See Philip et al. (2012) for a review of the fMRI studies of ASD.) Transcranial Magnetic Stimulation Another technique that is worth mentioning is transcranial magnetic stimulation (TMS). TMS is a noninvasive method that causes depolarization or hyperpolarization in neurons near the scalp. Depolarizations are increases in the electrical state of the neuron, while hyperpolarizations are decreases. In TMS, a coil of wire is placed just above the participant’s scalp (as shown in Figure 2.4.42.4.4\PageIndex{4}). When electricity flows through the coil, it produces a magnetic field. This magnetic field travels through the skull and scalp and affects neurons near the surface of the brain. When the magnetic field is rapidly turned on and off, a current is induced in the neurons, leading to depolarization or hyperpolarization, depending on the number of magnetic field pulses. Single- or paired-pulse TMS depolarizes site-specific neurons in the cortex, causing them to fire. If this method is used over certain brain areas involved with motor control, it can produce or block muscle activity, such as inducing a finger twitch or preventing someone from pressing a button. If used over brain areas involved with visual perception, it can produce sensations of flashes of light or impair visual processes. This has proved to be a valuable tool in studying the function and timing of specific processes such as the recognition of visual stimuli. Repetitive TMS produces effects that last longer than the initial stimulation. Depending on the intensity, coil orientation, and frequency, neural activity in the stimulated area may be either attenuated or amplified. Used in this manner, TMS is able to explore neural plasticity, which is the ability of connections between neurons to change. This has implications for treating psychological disorders, such as depression, as well as understanding long-term changes in neuronal excitability. Note that TMS is different from the previous techniques in that we are not taking images of what the brain is doing. TMS disrupts or stimulates the brain and actively changes what the brain is doing.

Magnetoencephalography (MEG) is another technique for noninvasively measuring neural activity. The flow of electrical charge (the current) associated with neural activity produces very weak magnetic fields that can be detected by sensors placed near the participant’s scalp. Figure 2.3.32.3.3\PageIndex{3} depicts a subject in an MEG machine. The number of sensors used varies from a few to several hundred. Due to the fact that the magnetic fields of interest are so small, special rooms that are shielded from magnetic fields in the environment are needed in order to avoid contamination of the signal being measured. MEG has the same excellent temporal resolution as EEG. Additionally, MEG is not as susceptible to distortions from the skull and scalp. Magnetic fields are able to pass through the hard and soft tissue relatively unchanged, thus providing better spatial resolution than EEG. MEG analytic strategies are nearly identical to those used in EEG. However, the MEG recording apparatus is much more expensive than EEG, so MEG is much less widely available.

Given that this electrical activity must travel through the skull and scalp before reaching the electrodes, localization of activity is less precise when measuring from the scalp, but it can still be within several millimeters when localizing activity that is near the scalp. While EEG is lacking with respect to spatial resolution, one major advantage of EEG is its temporal resolution. Data can be recorded thousands of times per second, allowing researchers to document events that happen in less than a millisecond. EEG analyses typically investigate the change in amplitude (wave height) or frequency (number of waves per unit of time) components of the recorded EEG on an ongoing basis or averaged over dozens of trials (see Figure 2.3.22.3.2\PageIndex{2}). The EEG has been used extensively in the study of sleep. When you hear references to "brain waves", those are references to information obtained using EEG. Figure 2.3.22.3.2\PageIndex{2}: Example of EEG analysis output. Panel A represents changes in the relative strength of different frequencies in the EEG data over time. Panel B represents changes in the amplitude in the instantaneous EEG voltage over time. Noba Psychophysiological Methods in Neuroscience. CC BY SA-NC 4.0 International License. MEG Magnetoencephalography (MEG) is another technique for noninvasively measuring neural activity. The flow of electrical charge (the current) associated with neural activity produces very weak magnetic fields that can be detected by sensors placed near the participant’s scalp. Figure 2.3.32.3.3\PageIndex{3} depicts a subject in an MEG machine. The number of sensors used varies from a few to several hundred. Due to the fact that the magnetic fields of interest are so small, special rooms that are shielded from magnetic fields in the environment are needed in order to avoid contamination of the signal being measured. MEG has the same excellent temporal resolution as EEG. Additionally, MEG is not as susceptible to distortions from the skull and scalp. Magnetic fields are able to pass through the hard and soft tissue relatively unchanged, thus providing better spatial resolution than EEG. MEG analytic strategies are nearly identical to those used in EEG. However, the MEG recording apparatus is much more expensive than EEG, so MEG is much less widely available.

Magnetic resonance imaging (MRI) is a noninvasive medical imaging technique based on a phenomenon of nuclear physics discovered in the 1930s, in which matter exposed to magnetic fields and radio waves was found to emit radio signals. In 1970, a physician and researcher named Raymond Damadian noticed that malignant (cancerous) tissue gave off different signals than normal body tissue. He applied for a patent for the first MRI scanning device, which was in use clinically by the early 1980s. The early MRI scanners were crude, but advances in digital computing and electronics led to their advancement over any other technique for precise imaging, especially to discover tumors. MRI also has the major advantage of not exposing patients to radiation. Drawbacks of MRI scans include their much higher cost, and patient discomfort with the procedure. The MRI scanner subjects the patient to such powerful electromagnets that the scan room must be shielded. The patient must be enclosed in a metal tube-like device for the duration of the scan, sometimes as long as thirty minutes, which can be uncomfortable and impractical for ill patients. The device is also so noisy that, even with earplugs, patients can become anxious or even fearful. These problems have been overcome somewhat with the development of “open” MRI scanning, which does not require the patient to be entirely enclosed in the metal tube. Figure 2.2.42.2.4\PageIndex{4} shows an MRI machine with a platform for the patient to lie on. Patients with iron-containing metallic implants (internal sutures, some prosthetic devices, and so on) cannot undergo MRI scanning because it can dislodge these implants.

Magnetic resonance imaging (MRI) is a noninvasive medical imaging technique based on a phenomenon of nuclear physics discovered in the 1930s, in which matter exposed to magnetic fields and radio waves was found to emit radio signals. In 1970, a physician and researcher named Raymond Damadian noticed that malignant (cancerous) tissue gave off different signals than normal body tissue. He applied for a patent for the first MRI scanning device, which was in use clinically by the early 1980s. The early MRI scanners were crude, but advances in digital computing and electronics led to their advancement over any other technique for precise imaging, especially to discover tumors. MRI also has the major advantage of not exposing patients to radiation. Drawbacks of MRI scans include their much higher cost, and patient discomfort with the procedure. The MRI scanner subjects the patient to such powerful electromagnets that the scan room must be shielded. The patient must be enclosed in a metal tube-like device for the duration of the scan, sometimes as long as thirty minutes, which can be uncomfortable and impractical for ill patients. The device is also so noisy that, even with earplugs, patients can become anxious or even fearful. These problems have been overcome somewhat with the development of “open” MRI scanning, which does not require the patient to be entirely enclosed in the metal tube. Figure 2.2.42.2.4\PageIndex{4} shows an MRI machine with a platform for the patient to lie on. Patients with iron-containing metallic implants (internal sutures, some prosthetic devices, and so on) cannot undergo MRI scanning because it can dislodge these implants.

Temporal Versus Spatial Resolution Within functional imaging techniques, researchers are frequently focused on one of two questions. They may ask “When does this activity occur?” Or “Where does this activity occur?” Some techniques are better for answering one of these questions, whereas other techniques are better for answering the other question. We describe how well a technique can determine when the activity has occurred as temporal resolution. For example, was the brain region activity occurring sometime in the last hour, the last minute, the last second, or within milliseconds? While some techniques are excellent at determining precisely when the activity occurred and other techniques are quite terrible at it. Additionally, we can describe how well a technique can determine where the activity has occurred as spatial resolution. For example, did the activity occur in the temporal lobe somewhere or can we narrow that down to a specific gyrus (ridge) or sulcus (groove) of the cerebral cortex? If it occurred on a particular gyrus can we narrow it down to a particular portion of that gyrus? As with temporal resolution, some techniques are excellent at determining precisely where the activity occurred whereas other techniques are less accurate.

Functional Imaging Many researchers are also interested in how the brain works. Some studies begin with the scientific question of “what does this part do?” Or more commonly, “Where in the brain does this happen?” Functional imaging techniques allow researchers to learn about the brain activity during various tasks by creating images based on the electrical activity or the absorption of various substances that occurs while a subject is engaging in a task. Such techniques can be used, for example, to visualize the parts of the brain that respond when we're exposed to stimuli that upset us or make us happy. Temporal Versus Spatial Resolution Within functional imaging techniques, researchers are frequently focused on one of two questions. They may ask “When does this activity occur?” Or “Where does this activity occur?” Some techniques are better for answering one of these questions, whereas other techniques are better for answering the other question. We describe how well a technique can determine when the activity has occurred as temporal resolution. For example, was the brain region activity occurring sometime in the last hour, the last minute, the last second, or within milliseconds? While some techniques are excellent at determining precisely when the activity occurred and other techniques are quite terrible at it. Additionally, we can describe how well a technique can determine where the activity has occurred as spatial resolution. For example, did the activity occur in the temporal lobe somewhere or can we narrow that down to a specific gyrus (ridge) or sulcus (groove) of the cerebral cortex? If it occurred on a particular gyrus can we narrow it down to a particular portion of that gyrus? As with temporal resolution, some techniques are excellent at determining precisely where the activity occurred whereas other techniques are less accurate.

A Scientific Model of the Universe:  Two Basic Assumptions The modern scientific view of the world assumes that there are at least two fundamental properties of the universe.  The first assumption common to most scientists is that the entire universe is material or physical, composed exclusively of matter (including so-called dark matter) and energy (including electromagnetic energies such as light, heat, ultraviolet, and various other radiant energies, as well as other physical energies and forces such as electrical, gravitational, nuclear, and so forth).  The entire universe is governed by physical laws.  This view of the universe is called materialism or physicalism--the view that everything that exists in the universe consists of matter, energy, and other physical forces and processes.  Most important to biopsychology is the application of this principle to psychology and psychological processes.  If everything in the universe is physical, then applied to psychology, including biopsychology, this means that the mind, our mental processes and subjective mental experiences, must also be entirely physical processes in an entirely material brain.  Using this fundamental assumption of the modern scientific view of the universe, this means that the mind is entirely material, dependent upon the physical activities of an entirely material organ, the brain.  On this view, the mind is what the brain does, and the brain and its processes are completely physical, material, just matter and energy in highly specific and organized form.  This does not mean that all matter is conscious, nor does it mean that the mind is just energy.  The key idea is that the matter and energy must be organized in a particular way.  That is, a mind, consciousness, can only emerge from matter, energy, and physical processes if they are organized in a very specific and complex form--that form that we know as a brain and its physical operations.  Where did brains come from and how did they acquire the specific organization of matter and energy needed to make a conscious mind within them?  The scientific answer is evolution. Although this is the view among most biological psychologists, there are a few who believe, like many students do, that the brain, along with the rest of life, was created by a divine being and that therefore the mind has divine origins.  Typically, this belief is accompanied by the assumption that the mind is not physical, but that it is akin to the soul and the soul is believed to be non-material.  Belief that the mind is non-material and therefore independent of the physical brain and its physical processes is known as mind-body dualism or mind-brain dualism, which literally means that the mind and the functioning of the brain (assumed to be entirely physical) are two (dual) separate processes, completely independent of one another.  The origin of dualism is often attributed to the 17th century French philosopher and mathematician, Rene Descartes. If this view were true, then we would expect that brain damage would have no effect on the mind.  However, brain damage does affect the mind and the specific location of the damage produces more or less specific, fairly predictable, effects on the mind, modifying the mind and behavior in various ways.  Examples of this are coma due to head injury; the effects of Parkinson's disease on movement after the disease damages areas of the brain known as the basal ganglia; changes in personality and emotion due to injury to the front of the brain, specifically the frontal lobes; memory loss in Alzheimer's Disease; and so on.  Though you don't have to accept the assumption of physicalism when studying the brain if your religious beliefs are contrary to the idea, nevertheless it is important that you be aware of the assumption of physicalism/materialism that most biological psychologists accept, at least as a working hypothesis, if not a philosophical position, as they do their brain research. The second major assumption among most scientists is determinism--the belief that all events in the universe have prior causes and that these causes are external to the human will.  This implies that humans do not have free will.  Instead human behavior is caused by events external to us such as our upbringing, our social and cultural environment, by our brain structure and functioning, and by our genes and our evolution as a species.  In some versions of this viewpoint, since we do not have control over many of the factors in our environment, our genes, and our evolution as a species, our brain function and thus our behavior is actually controlled by causes outside of our control.  On this view, free will is an illusion that arises from our awareness of our mental processes as we make choices based on our selection of various behavioral options that we see open to us, but what we often fail to realize is that those choices are determined by many factors beyond our awareness and control (Koenigshofer, 2010, 2016).  Free will vs. determinism is an issue that is far from being resolved and remains controversial even among scientists, including biological psychologists.  Investigation by biological psychologists of the brain processes involved in choice and decision-making is ongoing and may eventually shed light on this difficult issue.  Again, it is not necessary for you to be a determinist to study the brain, but it is important for you to be aware of the doctrine of determinism as you consider the implications of brain research as you progress through this textbook and your course in biological psychology.

A Scientific Model of the Universe:  Two Basic Assumptions The modern scientific view of the world assumes that there are at least two fundamental properties of the universe.  The first assumption common to most scientists is that the entire universe is material or physical, composed exclusively of matter (including so-called dark matter) and energy (including electromagnetic energies such as light, heat, ultraviolet, and various other radiant energies, as well as other physical energies and forces such as electrical, gravitational, nuclear, and so forth).  The entire universe is governed by physical laws.  This view of the universe is called materialism or physicalism--the view that everything that exists in the universe consists of matter, energy, and other physical forces and processes.  Most important to biopsychology is the application of this principle to psychology and psychological processes.  If everything in the universe is physical, then applied to psychology, including biopsychology, this means that the mind, our mental processes and subjective mental experiences, must also be entirely physical processes in an entirely material brain.  Using this fundamental assumption of the modern scientific view of the universe, this means that the mind is entirely material, dependent upon the physical activities of an entirely material organ, the brain.  On this view, the mind is what the brain does, and the brain and its processes are completely physical, material, just matter and energy in highly specific and organized form.  This does not mean that all matter is conscious, nor does it mean that the mind is just energy.  The key idea is that the matter and energy must be organized in a particular way.  That is, a mind, consciousness, can only emerge from matter, energy, and physical processes if they are organized in a very specific and complex form--that form that we know as a brain and its physical operations.  Where did brains come from and how did they acquire the specific organization of matter and energy needed to make a conscious mind within them?  The scientific answer is evolution. Although this is the view among most biological psychologists, there are a few who believe, like many students do, that the brain, along with the rest of life, was created by a divine being and that therefore the mind has divine origins.  Typically, this belief is accompanied by the assumption that the mind is not physical, but that it is akin to the soul and the soul is believed to be non-material.  Belief that the mind is non-material and therefore independent of the physical brain and its physical processes is known as mind-body dualism or mind-brain dualism, which literally means that the mind and the functioning of the brain (assumed to be entirely physical) are two (dual) separate processes, completely independent of one another.  The origin of dualism is often attributed to the 17th century French philosopher and mathematician, Rene Descartes. If this view were true, then we would expect that brain damage would have no effect on the mind.  However, brain damage does affect the mind and the specific location of the damage produces more or less specific, fairly predictable, effects on the mind, modifying the mind and behavior in various ways.  Examples of this are coma due to head injury; the effects of Parkinson's disease on movement after the disease damages areas of the brain known as the basal ganglia; changes in personality and emotion due to injury to the front of the brain, specifically the frontal lobes; memory loss in Alzheimer's Disease; and so on.  Though you don't have to accept the assumption of physicalism when studying the brain if your religious beliefs are contrary to the idea, nevertheless it is important that you be aware of the assumption of physicalism/materialism that most biological psychologists accept, at least as a working hypothesis, if not a philosophical position, as they do their brain research. The second major assumption among most scientists is determinism--the belief that all events in the universe have prior causes and that these causes are external to the human will.  This implies that humans do not have free will.  Instead human behavior is caused by events external to us such as our upbringing, our social and cultural environment, by our brain structure and functioning, and by our genes and our evolution as a species.  In some versions of this viewpoint, since we do not have control over many of the factors in our environment, our genes, and our evolution as a species, our brain function and thus our behavior is actually controlled by causes outside of our control.  On this view, free will is an illusion that arises from our awareness of our mental processes as we make choices based on our selection of various behavioral options that we see open to us, but what we often fail to realize is that those choices are determined by many factors beyond our awareness and control (Koenigshofer, 2010, 2016).  Free will vs. determinism is an issue that is far from being resolved and remains controversial even among scientists, including biological psychologists.  Investigation by biological psychologists of the brain processes involved in choice and decision-making is ongoing and may eventually shed light on this difficult issue.  Again, it is not necessary for you to be a determinist to study the brain, but it is important for you to be aware of the doctrine of determinism as you consider the implications of brain research as you progress through this textbook and your course in biological psychology.

The Brain The most important organ controlling our behavior and mental processes is the brain. Therefore, biopsychologists are especially interested in studying the brain, its neurochemical makeup, and how it produces behavior and mental processes (Wickens, 2021). Figure 1.1.11.1.1\PageIndex{1}:  Photo of the left side of a human brain.  Front is at the left.  The massive cerebral cortex hides many subcortical brain structures beneath.  Note the folds on the surface of the cerebral cortex.  This folding increases in mammal species with increasing complexity of the brain of the species and is thought to originate from the "cramming" of more cortical tissue into the skull over evolutionary time. (Image from Wikimedia Commons; File:Human brain NIH.png; https://commons.wikimedia.org/wiki/F..._brain_NIH.png; image is from NIH and is in the public domain.).   Modern technology through neuroimaging techniques has given us the ability to look at living human brain structure and functioning in real time. Neuroimaging tools, such as functional magnetic resonance imaging (fMRI) scans, are often used to observe which areas of the brain are active during particular tasks in order to help psychologists understand the link between brain and behavior. Figure 1.1.21.1.2\PageIndex{2}: Brain Imaging Techniques (Copyright; author via source) Different brain-imaging techniques provide scientists with insight into different aspects of how the human brain functions. Three types of scans include (left to right) PET scan (positron emission tomography), CT scan (computed tomography), and fMRI (functional magnetic resonance imaging). (credit “left”: modification of work by Health and Human Services Department, National Institutes of Health; credit “center": modification of work by "Aceofhearts1968"/Wikimedia Commons; credit “right”: modification of work by Kim J, Matthews NL, Park S.) Magnetic resonance imaging (MRI) scans of the head are often used to help psychologists understand the links between brain and behavior.  As will be discussed later in more detail, different tools provide different types of information. A functional MRI, for example, provides information regarding brain activity while an MRI provides only information about structure.   Figure 1.1.31.1.3\PageIndex{3}: MRI of Human Brain (public domain; via Wikimedia Commons)

Biopsychology - The Interaction of Biology and Psychology Psychology is the scientific study of behavior and mental processes in animals and humans. Modern psychology attempts to explain behavior and the mind from a wide range of perspectives. One branch of this discipline is biopsychology which is specifically interested in the biological causes of behavior and mental processes.  Biopsychology is also referred to as biological psychology, behavioral neuroscience, physiological psychology, neuropsychology, and psychobiology. The focus of biopsychology is on the application of the principles of biology to the study of physiological, genetic, evolutionary, and developmental mechanisms of behavior in humans and other animals. It is a branch of psychology that concentrates on the role of biological factors, such as the central and peripheral nervous systems, neurotransmitters, hormones, genes, and evolution on behavior and mental processes. Biological psychologists are interested in measuring biological, physiological, or genetic variables in an attempt to relate them to psychological or behavioral variables. Because all behavior is controlled by the central nervous system (brain and spinal cord), biopsychologists seek to understand how the brain functions in order to understand behavior and mental activities. Key areas of focus within the field include sensation and perception; motivated behavior (such as hunger, thirst, and sex); control of movement; learning and memory; sleep and biological rhythms; and emotion. With advances in research methods, more complex topics such as language, reasoning, decision making, intelligence, and consciousness are now being studied intensely by biological psychologists.

Biopsychology is the study of biological mechanisms of behavior and mental processes.  It examines the role of the nervous system, particularly the brain, in explaining behavior and the mind. This section defines biopsychology, critically examines a common myth about the brain, and briefly surveys some of the primary areas of research interest in biopsychology.