The Scope and Aims of Biological Psychology
Psychology is both a biological and a social science, as Chapter 1 points out. The present chapter emphasizes the biological aspects of psychology, just as Chapter 3 emphasizes the social aspects. Both aspects must be taken into account for a comprehensive view of psychology, and at some points in this chapter we note interactions between biological and social variables in determining behavior and experience. The relevance of the biological approach for many fields of psychology is demonstrated by the fact that biological bases of behavior are mentioned in most of the following chapters.
The field that studies biological aspects of psychology and biological contributions to psychology has been given several names. Originally it was called ‘physiological psychology’ when ‘physiology’ was used to designate what we now call biomedical sciences. As specialized fields broke off from physiology—including biochemistry, immunology, and neurochemistry—and as new fields were introduced—including genetics and endocrinology—the designation ‘physiology’ became too narrow, so the more inclusive term ‘biological psychology’ became more appropriate. Other terms sometimes used for this field include ‘physiology and behavior’ and ‘behavioral neuroscience’.
The development of neuroscience as an integrated but multidisciplinary field in the last quarter of the twentieth century benefited biological psychology (behavioral neuroscience). Through the progress of neuroscience, biological psychology strengthened its ties and interactions with neighboring sciences and advanced its program of research and applications.
Biological psychology is concerned with both similarities and differences among individuals. The anthropologist Clyde Kluckhohn observed that each person is in some ways like all other people, in some ways like some other people, and in some ways like no other person. We can extend this observation to the much broader range of animal life. In some ways each person is like all other animals (e.g., we all need to ingest complex organic nutrients), in some ways like all other vertebrates (e.g., having a spinal column), in some ways like all other mammals (e.g., nursing as a baby), and in some ways like all other primates (e.g., having a hand with an opposable thumb and a relatively large, complex brain). These similarities and differences make it instructive to compare species and individuals in regard to behavior and biology, as we will note in some examples. This is also a theme of Chapter 19.
The Scope of Biological Psychology
The scope of biological psychology is indicated by the following list of some of the main fields it encompasses. For each major field, we state a question to indicate its relevance. Some of these topics will be discussed later in this chapter or taken up in other chapters.
1. Biological foundations of behavior. What is the biological ‘machinery’ on which behavior depends?
- Functional neuroanatomy: The nervous system and behavior. What are the main structures of the nervous system and how is each involved specifically in behavior?
- Neurophysiology: Generation, conduction, transmission, and integration of neural signals. How is information coded, transmitted and processed in the nervous system?
- Psychopharmacology: Neurotransmitters, drugs and behavior. How do neurons transmit signals chemically, and how can this be affected by drugs?
- Psychoneuroendocrinology: Hormones and behavior. How do hormones affect reproductive parental and social behavior?
- Psychoneuroimmunology: The immune system and behavior. How does the immune system interact with the endocrine and nervous systems in determining behavior?
2. Evolution and development of the nervous system. How does an evolutionary approach help in understanding behavior?
- Evolution of brain and behavior. How are brain differences among species related to differences in their behavioral capacities?
- Lifespan development of the brain and behavior. How are brain development and change over the life span related to behavior?
3. Sensory processes and perception.
- General principles of sensory processing. What principles of functioning and biological mechanisms hold for all sensory modalities?
- Processing in the different sensory modalities: Touch and pain, hearing, vestibular perception, taste and smell, vision. What are some of the structural adaptations that enable different sensory receptors to respond to different kinds of stimuli?
4. Movements and actions. How are mental or cognitive processes translated into behavior?
5. Motivation and regulation of behavior.
- Sex: Evolutionary, hormonal, and neural bases. How do sex hormones help to organize the early development of the nervous system?
- Homeostasis: Active regulation of internal states. What mechanisms help to keep bodily conditions relatively constant but also responsive to calls for activity?
- Biological rhythms, sleep, and dreaming. How are various bodily functions, such as hormone levels, neural activity, and muscle tonus, regulated during wake-fulness and various states of sleep, including dreaming?
- Affection and affiliation. How are affection and social attachment affected by levels of certain hormones?
6. Emotions and mental disorders.
- Emotions, aggression, and stress. What regions of the brain are particularly involved in emotions?
- Psychopathology: Biological bases of behavior disorders. How are certain mental disorders related to abnormalities of brain chemistry and anatomy?
- Biological perspectives on learning and memory. How are changes in learning and memory over the life span related to changes in the nervous system?
- Neural mechanisms of learning and memory. What regions of the brain are involved in conditioning?
- Language and cognition. What brain regions are particularly involved in language? What treatments can aid recovery of language following a stroke?
Biological Psychology has been a Major Part of Modern Psychology from the Start
Biological psychology has been a major part of psychology from the start of modern psychology in Europe and North America in the nineteenth century. For a brief history of biological psychology, see Rosenzweig (2000). Biological psychology remains a major field of research and instruction in the main English-speaking countries, and in some other countries. There are several current textbooks in the field; some are listed in the Resource References of this chapter. Reviews of advances in biological psychology appear in such publications as the Annual Review of Psychology, Annual Review of Neuroscience, Trends in Neuroscience, Trends in Cognitive Science, Behavioural and Brain Sciences, and Current Opinions in Neurobiology.
Relations between Biological and Social Psychology
The influences of both genetics and experience on behavior and mental functions (nature and nurture) are discussed in Chapter 3 and we will add a bit to that discussion here. The authors of Chapter 3 note the long-standing historical contest between proponents of social and biological perspectives. Stating that no one now denies either that human behavior has biological underpinnings or that it is influenced by environmental events, they claim that this is about as far as agreement goes. Here we take a further step, indicating how the biological approach is helping to understand the means and mechanisms by which some of the social/experiential factors influence behavior.
For studying and understanding many kinds of behavior, biological and social perspectives offer complementary avenues. In other cases, biological and social factors continuously interact and affect each other. We will give examples of both kinds.
As Chapter 3 emphasizes, concrete social behaviors are largely learned. A major field of biological psychology is investigating the biological mechanisms of learning and memory storage—how these processes occur, where in the nervous system they occur, how and why different kinds of learning vary over the life span. Some of these questions about learning and memory are taken up later in Section 4.4, and they are treated more fully in Chapter 8. Biological research complements social and cognitive research on learning and memory.
Emotional aspects of behavior are important in social psychology. Biological research and theorizing about emotion go back to the nineteenth century and continue actively today, as Chapter 12 reports. Biological research on emotion complements social and cognitive research on emotion.
In some cases social behavior is determined, in part, by biological factors. For example, social attachment and affection are determined in part by levels of the neuropeptide hormones oxytocin and vasopressin, which are secreted by neurons of parts of the hypothalamus. These neuropeptides are also involved in many aspects of reproductive and parental behavior; this is true in humans and other mammalian species (Carter, 1998). Recent research indicates that not only the concentration of vasopressin but also the distribution of vasopressin receptors among brain regions helps to determine social behaviors (Young, Nilsen, Waymire, MacGregor, & Insel, 1999). A rodent species, the prairie vole, known for its fidelity and sociability, was found to have a gene that determines where and when the vasopressin receptor is turned on; this gene is not present in two promiscuous and less social vole species. The investigators then inserted the prairie vole gene into house mice, which are not highly social and are promiscuous. When the genetically altered mice were given vasopressin, the males significantly increased their contact with females, a response not seen in normal male house mice. Impressed that a single gene can alter social behavior, the investigators now plan to study variation of the distribution of the vasopressin receptor among humans, especially because most psychiatric disorders are characterized by abnormal social attachments.
In other cases, biological and social factors continuously interact and affect each other in an ongoing series of events as behavior unfolds.
For example, the level of testosterone in a person’ circulation affects his dominance behavior and aggression in social groups. The dominance may be exhibited in a great variety of social settings, ranging from playing chess to physical aggression. In several studies in humans and primates, the level of testosterone correlates positively with the degree of dominance and with the amount of aggression exhibited. Winning a contest, whether a game of chess or a boxing match, raises the level of testosterone; losing a contest lowers the level of testosterone. The altered level of testosterone in turn is one of the factors that determine the display of dominance and aggression in the social group, and this has further repercussions. Thus, at any moment the level of testosterone is determined, in part, by recent dominance-submissive social experience, and the level of testosterone determines, in part, the degree of dominance and aggression. Of course, social/cultural factors also help to determine the frequency of aggression; as Chapter 3 points out, cross-cultural differences in rates of aggression exist that cannot be correlated with hormonal levels, and ways of expressing aggression and dominance are determined in part by sociocultural factors.
The neurotransmitter serotonin is another of several biological factors correlated with the magnitude of aggression, but in this case the correlation is negative; that is, the greater the aggression, the lower the concentration of serotonin. Serotonin seems to help suppress aggression. Serotonin concentration also varies with social experience; levels of serotonin decline as the individual’ position in a social group rises. For further discussion and references to the involvement of testosterone and serotonin in dominance and aggression, see Rosenzweig, Leiman, and Breedlove (1999, pp. 423-424).
Bodily Systems Basic to Behavior
Understanding the operation of the body is essential for research on biological foundations of psychology. While different organs were speculated to be the site of the mind or soul in ancient times, it is now well established that the nervous system is critical in coordinating behavior and mental functions by receiving information from various parts of the body, processing incoming and stored information, and sending orders to muscles and glands. The nervous system does not accomplish its task alone; it works together with the endocrine system and the immune system.
The Nervous System
The nervous system is composed of neurons, about a trillion of them in the case of the human body, and extensively varied in morphology and function. A typical neuron contains a cell body, or soma, that subserves the maintenance and integration functions, and also branches, including many dendrites, which are involved in receiving signals, and an axon, which is involved in transmitting signals. Communication in the nervous system is highly convergent and divergent. A neuron receives multiple inputs at its dendrites or soma, and sends output through its axon to multiple target sites. The axon terminals form connections with dendrites or soma of the recipient neurons by specialized structures called synapses. The small gap (20 nanometers) between the presynaptic terminals and postsynaptic sites requires signal transmission to be mediated by different chemicals called neurotransmitters. Direct electrical transmission across certain synapses also exists at some sites in the nervous system of invertebrates and even vertebrates.
While attention has been focused on neurons in discussing the biological bases of mental function, the nervous system contains as many, or probably even more, glial cells, which are essential for the vitality and functioning of neurons. They regulate the chemical environment of neurons, remove debris and repair damage after injury. Certain types of glial cells (Schwann cells or oligodendrocytes) wrap their processes around some axons to form a myelin sheath, which not only provides protection but also increases the signal conduction velocity of the myelinated axon. Degeneration of the myelin sheath due to autoimmune diseases may lead to conduction failure and as a consequence result in disorders of motor behavior such as multiple sclerosis.
Although organisms are capable of reacting to stimuli of various types (visual, auditory, tactile, etc.), the nervous system transduces (codes) all the sorts of stimulus energies as a single form of signal: alteration in electrical potentials across the neuronal membrane. The neuron actively maintains a resting potential of about 70 mV, the interior of the neuron being electrically negative in relation to the exterior. The transduction of stimuli into neural signals depends on exchange of electrically charged ions through various channels (or gates) in the neuronal membrane controlling the permeability of ions. Lowering the membrane potential of an axon to its threshold sets off an action potential. This signal is all-or-none, self-propagating and non-decremental, which results from sequential activation and deactivation of voltage-dependent sodium and potassium channels. Some properties of neuronal coding are related to behavior of ion channels. For example, because the action potential is all-or-none, its amplitude cannot code the stimulus intensity. However, some nerves code the intensity of stimuli by their firing rate (the frequency of action potentials). After an action potential there is a brief absolutely refractory period and then a relatively refractory period, during which the membrane is hyperpolarized (more negative inside than in the resting state) because the rectifier potassium channels remain open. A stronger stimulus is needed to bring the hyperpolarized membrane potential to the firing threshold. The higher the stimulus intensity, the sooner the hyperpolarization will be overcome. Thus a strong stimulus generates more frequent firing of action potentials than does a weak stimulus.
Neural impulses reaching axon terminals open calcium channels leading to a series of biochemical events and eventually causing the presynaptic terminal to release neurotransmitter molecules, which cross the synaptic cleft and bind to specific protein molecules called receptors located on the postsynaptic membrane. The binding alters the permeability of ion channels directly in so-called ionotropic receptors or indirectly through a second messenger system in so-called metabotropic receptors. Both result in excitatory or inhibitory effects on the post-synaptic neuron. It is critical to note that mental activity is related to inhibition as well as to excitation of neurons. For example, visual experience is induced by light impinging on the retina causing hyperpolarization and thus inhibition of the rod receptors; the rods are normally depolarized due to opening of some positive ion channels in darkness. Likewise, generation and modulation of some behaviors are due to inhibition of neurons that exert tonic inhibition on the motor pathway. The entire output of the cerebellum is inhibitory. It is also critical to note that activation of a single input rarely fires the postsynaptic neuron. Instead, firing of the post-synaptic neuron is decided by spatial summation of excitatory and inhibitory inputs from various sources and temporal summation of inputs impinging within a time limit. The convergence of multiple synaptic activity in determining firing of a neuron is the basis of the fact that processing of stimuli, as well as responses to them, is often modulated by various factors such as arousal, attention, emotion, or motivation.
The human nervous system contains a central and a peripheral division. The central nervous system (CNS) contains the brain and spinal cord, and the peripheral division contains cranial and spinal nerves innervating peripheral targets including sensory receptors, muscles, or visceral organs. Those nerves carrying information from and to the viscera and involved in control of their activity form the autonomic nervous system (ANS) which is further divided into the sympathetic and parasympathetic branches. The activity of the ANS has been implicated in motivational and emotional functions, as discussed in Chapters 11 and 12, respectively. The spinal cord processes information from sensory receptors, delivers motor orders to the muscles, and serves as a relay between the brain and the periphery. The brain contains a central core known as the brain stem including the medulla, pons, midbrain, thalamus, and hypothalamus. Around the brain stem are the basal ganglia and limbic system. All these structures are largely covered by the cerebral cortex, which is most developed in the human being; it can be further divided into the frontal, temporal, parietal, and occipital lobes in each hemisphere. While the two sides of the central nervous system are grossly symmetrical, functional and structural asymmetries are demonstrated in the cerebral cortex and other brain loci. Cells in the cerebral cortex group into layers while those in other structures group into aggregations called nuclei. Different subdivisions are involved in different functions, as is discussed in related chapters.
Sensory, motor, and mental functions are represented at many levels of the nervous system. Different types of information processing may be undertaken by neural circuits of distinct levels. One type of operation is to combine information from both sides of the body to derive a third dimension of perception that is not naturally reflected in the surface of the receptor organ. An example is localizing auditory or visual stimuli in the environment. Sound localization depends on the difference in intensity, time of arrival, or phase of sound waves as they impinge the two eardrums. Stereopsis (visual depth perception) depends on the fact that the image of a stimulus farther or nearer than the fixation point falls on non-corresponding positions of the two retinas (retinal disparity). The auditory localization mechanisms have been studied especially in owls and bats, animals that depend of auditory localization to prey in the dark. Some auditory neurons in the brain stem receive convergent inputs from the two cochleae. These neurons are able to detect sounds from specific locations which create a particular temporal delay in action potentials from the two ears (Carr & Konishi, 1990). In the visual system, the convergence of bilateral information does not take place until the primary visual cortex. However, in higher order visual cortical areas, there are regions in which neurons specialize in detecting visual depth (Poggio & Poggio, 1984).
In general, the nervous system appears to observe a division of labor. Various properties involved in a behavioral function rely on different groups of neurons within each level. This principle is best attested by the multiple visual representation areas in the cerebral cortex. Information concerning the form, color, motion, and position properties engages different cells in the primary visual cortex (area V1); this information is then dispatched to specific higher-order regions of the visual cortex. Information concerning form and color flows through a ventral stream of neural pathways to the inferotemporal cortex (the ‘what’ pathway), whereas that concerning motion and position flows dorsally to the parietal lobe (the ‘where’ pathway) (Mishkin & Ungerleider, 1982). It also appears that the amount of neural tissue devoted to representing a function is related to the fineness of discrimination or control in this function. This ‘more is better’ principle is well demonstrated by the large magnification factor in the foveal as compared with the peripheral regions of the primary visual cortex.
While the gross distribution of various functions in different parts of the CNS is largely determined by evolutionary and genetic factors, it is now well established that the fine structure of these representations is by no means static and varies from individual to individual. The anatomy of neural tissue subserving a function is subjected to fine-tuning by experience. For example, the cortical representational area for a specific finger can be enlarged by repeated use in a discriminative task or shrunk by depriving it of sensory input (Buonomano & Merzenich, 1998). This experience-dependent plasticity of the nervous system may serve as part of the biological basis of personality or idiosyncratic individual differences.
The Endocrine System
The endocrine system contains several endocrine glands, which are involved in regulation of a constellation of physiological and behavioral functions such as glucose utilization, conservation of fluid and ions, and facilitating sexual maturation and behavior. The endocrine system secretes hormones into the blood stream, through which they are carried to all parts of the body. Thus, in contrast to the swiftness of signal transmission and the specificity of target activation in the nervous system, signals in the endocrine system travel at a much slower velocity and activate a much wider array of targets.
To affect the target tissue, hormones must bind to specialized receptor molecules. Many hormones bind to receptors on the membrane of target cells, just as neurotransmitters do, and they activate a second messenger biochemical cascade to alter metabolic functions of the cell. But steroid hormones (including those released from the adrenal cortex and sex glands) enter the cells by penetrating through the membrane and binding to receptors in the cytoplasm. The hormone—receptor complex is transferred into the nucleus. In the nucleus, the complex is able to regulate expression of specific genes, which in turn direct protein synthesis in the cytoplasm and hence change cellular functions. Thus, while nerve impulses induce a relative short-latency and brief-duration response, hormones often induce a long-latency and long-lasting response. Therefore, hormones can prime the peripheral tissue and create a suitable internal milieu appropriate for later brisk action of neural signals.
Activity of the endocrine system is under hierarchical control, which provides one example of interactions between the endocrine and the neural systems. Neurons in certain nuclei of the hypothalamus secrete various releasing hormones into the median eminence, whence they are sent by the portal vein system to stimulate the anterior lobe of the pituitary gland to secrete different tropic hormones. These in turn regulate the activity of the adrenal glands, the gonads and the thyroid gland. Steroid hormones secreted by the peripheral endocrine glands or the pituitary enter the CNS and bind to receptors in the hypothalamus or elsewhere in the brain to exert an inhibitory feedback control on the secretion of various releasing or tropic hormones.
It is important to note that the action of hormones is not limited to regulating endocrine activity; they also participate in a constellation of physiological and behavioral functions. Receptors for hormones are found in widespread brain areas and are activated by hormones secreted from peripheral or central endocrine cells or neurons in numerous brain regions that also contain peptide or hypothalamic hormones. In many cases these peptide hormones, and other neuropeptides as well, coexist with traditional neurotransmitters in the nerve terminals and are co-released with them for transmission or modulation purposes. Hormones exert various kinds of effect on the nervous system. For example, sex hormones have been shown to have an organizational effect and an activational effect on sex functions. The former refers to the fact that early in development, the presence or absence of the male sex hormones switches the growth of the brain structures into a male or female mode and fosters the development of peripheral sex organs and secondary sexual characteristics. The activational effect refers to the fact that as the organism reaches sexual maturation, the release of sex hormones primes sexual behavior.
It should be mentioned that some hormones have other actions in addition to endocrine function. For example, infusion of corticotropin-releasing hormone (CRH) into the cerebral ventricles causes a wide spectrum of physiological and behavioral effects, including increased peripheral sympathetic outflow, inhibition of gastric motility and secretion, inhibition of feeding and sexual behavior, increased grooming, defensive withdrawal, and startle behavior. Such effects are present after hypo-physectomy or adrenalectomy and hence independent of the pituitary—adrenal axis. These responses resemble those that occur under the state of fear or anxiety, leading to the suggestion that CRH may act as a central anxiogenic (anxiety-inducing) mediator (Dunn & Berridge, 1990). As mentioned in Chapter 8, several stress-released hormones such as adrenaline, glucocorticoid, adrenocorticotropin (ACTH), vasopressin, and cholecystokinin influence memory in various aversive or appetitive learning tasks. Post-training administration of these hormones can enhance later retention. The effect appears not to depend on endocrine target tissue because some fragments or analogs of these hormones devoid of endocrine activity are also effective. It has been proposed that the stress-released hormones may act as endogenous memory modulators to facilitate storage of stressful experience (McGaugh, 1989). A stress-released hormone capable of coping with the stress during confrontation but also engraving the lesson in memory storage would increase survival and be highly adaptive from an evolutionary perspective. However, not all effects of stress-released hormones are beneficial. High levels of glucocorticoid in the peripheral tissue caused by prolonged stress may induce a constellation of harmful effects, which Hans Selye called the exhaustion phase of the general adaptation syndrome (Selye, 1956). In addition, high levels of glucocorticoid may activate certain neural receptors in the hippocampus and increase the vulnerability of hippocampal neurons to possible insults in daily life and thus accelerate aging (Sapolsky, 1992).
The Immune System
More and more evidence indicates intimate interactions between mental activity and the immune system. This system defends an organism from harmful foreign biological substances introduced into the body. So-called natural killer cells, members of the immune system, roam around the body to destroy infected cells or cancer cells. In addition, cells infected by viruses may secrete a peptide named interferon to suppress further reproduction of the virus. These are nonspecific actions of the immune system. Immune responses may also attack specific targets. Cells located in the thymus gland, bone marrow, spleen, and lymph nodes induce chemically mediated and cell-mediated immune reactions in detection of invading microorganisms. The B-lymphocytes, which originate in the bone marrow, release immunoglobulins into the body fluid; these act as antibodies that bind specifically to the unique protein (antigen) on the membrane of invaders and result in destruction of the invading cells. On the other hand, the T-lymphocytes, which originate in the thymus, have antibody attached on their membrane; they directly attack the invading microorganisms and destroy them or enhance their destruction by other cells. Cells in the immune system communicate with cytokines such as interleukins; these chemicals are synthesized and released by white blood cells to detect soluble toxic products from invading microorganisms and cause other white cells to proliferate. These cytokines may also provide information to other body systems as explained in the next paragraph.
Stress or psychological reactions to it have profound effects on immunoactivity. Emotional disturbance due to depression or highly stressful life events, such as school examinations or marital disruption, compromises the ability of the immune system and increases susceptibility to infectious diseases, autoimmune disorders, cardiac disease, and cancer. Animal studies have shown that enhancement or suppression of immune activity can be elicited by conditioned stimuli through a classical conditioning procedure (Ader & Cohen, 1993). These and other data suggest that there are three-way interactions among the neural, endocrine, and immune systems. The central nervous system influences the immune response through the autonomic nervous system and hypothalamic—pituitary axis. Sympathetic noradrenergic fibers innervate the thymus and spleen and affect functions of the immune cells. Receptors for neuropeptides are present on the membrane of these cells. Opiate blockers antagonize the suppression of the natural killer cells induced by intermittent shock stress, suggesting involvement of the endogenous opioid system. CRH and glucocorticoid have a profound immunosuppression effect. As noted in Chapter 12 on emotion, amygdaloid neurons are activated by emotional stimuli. Thus, through projections from the central amygdaloid nuclei to the paraventricular nucleus, stress could enhance release of CRH, which eventually releases glucocorticoid to suppress immune activity. Such effects may provide a biological mechanism by which social factors affect physiological conditions, such as vulnerability to contagious diseases or carcinogenic agents.
Conversely, cytokines released by immune cells or the CNS may act on the brain to affect neural activity (Rothwell & Hopkins, 1995). They bind to hypothalamic neurons, through which they influence secretion of various hormones, including increased release of ACTH, prolactin, and growth hormones but decreased release of luteinizing hormones. Behaviorally, the most conspicuous effect of cytokines given centrally or peripherally is to induce fever, and this appears to be mediated by release of CRH. In addition, cytokines may increase the amount of slow-wave sleep. These effects are part of the normal reaction to infection. It has therefore been proposed that the immune system may serve as a sensory channel informing the CNS of our internal chemical environment.
Methodology in Biological Psychology
Major aspects of methodology in research in biological psychology will first be presented and discussed. Then we will illustrate these considerations by reviewing briefly the search for the neural mechanisms of classical (Pavlovian) conditioning.
Biological Psychology Studies Both Humans and Nonhuman Animals
Biological psychology manipulates and observes psychological variables just as do all other fields of psychology, and thus employs in general the methodology of psychology depicted in Chapter 2. However, to delineate the somatic contribution to behavior and mental functions, biological psychology needs to draw evidence from studies combining methods of psychology with those of other disciplines such as neuroscience, pharmacology, immunology, and endocrinology. These methods allow the investigator to probe into relevant bodily systems. Further, in contrast to many other fields of psychology that focus their research mainly on human beings, biological psychology investigates both human and nonhuman species. Study of animal subjects is justified for two reasons. First of all, from an evolutionary perspective, the bodily systems subserving mental activity, like any other biological features, are under the pressure of natural selection and have evolved over a long period of time as a consequence of adaptation to the environment. Behavioral functions have also evolved, as suggested by Charles Darwin (1872) in his book, The Expression of the Emotions in Man and Animals. Therefore, as stated in Section 4.1, common components may underlie mental functions of both humans and other species in terms of their structural bases or operational mechanisms. Investigating the brain function of animals can therefore contribute significantly to our understanding of that of humans. This expectation is substantiated by numerous findings suggesting shared biological bases of human and nonhuman behavior illustrated in various chapters of this book. A few examples from the rich literature in this area include parallel processing of different visual properties (e.g., hue, shape, and movement) in separate brain regions, hormonal influences in sexual development, and engagement of the amygdala by emotional arousal.
The second reason for employing animal studies is purely methodological. For one thing, studies of development or lifespan changes in behavior and neural mechanisms are more practical to conduct in animals whose life spans are much shorter than our own. For another, invasive manipulation or assessment of brain functions cannot be performed in human subjects except under special circumstances. In rare cases, human bodily systems may be perturbed by natural causes or probed experimentally for therapeutic purposes (such as stimulation during surgical rectification of epilepsy); the ensuing behavioral symptoms may suggest relationships between structure and function. In the history of biological psychology, neurological findings have often served as good starting points in the history of biological psychology. However, follow-up animal studies are necessary to formally test and establish such inferences because damage produced by nature is seldom as well defined as experimental ablation or manipulation in animals. Various types of somatic or behavioral intervention can be applied to animals in a humane manner and also precisely in terms of time and location to unravel the relationships between body and behavior. Nonetheless, recently developed techniques such as functional brain imaging and magnetic brain stimulation allow us to measure or alter directly human brain activity and close the gaps in our knowledge between brain functions in humans and other species.
Even though mental functions across species may indeed vary with evolutionary status, the correspondence of particular sets of behavior from one species to another is hard to establish. When the neural mechanism of mental functions is inferred from animal studies, animal behavior is often used in the sense of a model for the mental function concerned. For example, imprinting behavior may serve as a model for filial attachment, and various types of aggression induced by different kinds of stimuli can model aspects of anger. The model system bears similarities to what is being modeled, but the two need not be identical. Such similarities could be based on different grounds. Some of them are based on apparent similarity in behavior, such as self drug-administration by animals as a model of addictive behavior. Some of them bear an essential theoretical component of the modeled function, as mastering certain tasks indicates declarative learning. Of course, inferred similarities must always be cross-checked by independent methods and by attempts to invalidate them.
Biological Psychology Uses a Variety of Approaches
To delineate the relationships between the bodily systems and mental functions, biological psychologists may adopt different approaches including somatic intervention, behavioral intervention, and correlation.
In somatic intervention, the investigator manipulates specific parts of a bodily system and assesses the consequence on mental/behavioral functions. The general strategy of manipulating a bodily system is to activate or suppress its function. Activation of a neural structure can be accomplished by applying weak electric currents of appropriate intensity or by local infusion of drugs stimulating receptors inducing excitatory postsynaptic potentials (EPSP) or blocking receptors inducing inhibitory postsynaptic potentials (IPSP). Local infusion of drugs blocking EPSP or inducing IPSP will have opposite effects. Pharmacological manipulation of the biochemical cascade linked to receptors also serves a similar end and allows further dissection of the intracellular events critical for behavior. Permanent suppression of a neural function can be achieved by destruction of the tissue with various types of lesion. However, administration of local anesthetics or drugs that block sodium channels can cause temporary and reversible lesions of the affected tissue.
Interactions between the somatic system and mental function are by no means unidirectional; behavioral experience may alter structural or functional properties of the neural, hormonal, and immune systems. Such effects are typically revealed by behavioral intervention in which behavioral or mental functions are manipulated and consequences in bodily systems are detected. Changes in the brain may be anatomical, physiological, or neurochemical. Anatomical changes include increase in axonal branches, dendritic arborization, or synaptic formation after exposure to an enriched environment or after certain types of learning experience. Physiological changes may be expressed as increase of evoked responses to input stimuli or alteration in membrane electrotonic properties tested either in vivo or in vitro. Neurochemical changes may include increases or decreases of cellular energy utilization, receptor binding, enzyme activity, release or reuptake in neurochemicals or hormones, protein synthesis, or gene expression.
Studies in biological psychology may also adopt the correlational approach in which behavioral and somatic variables are measured simultaneously in each of a group of subjects, so that the extent to which the two measures co-vary can be measured. For example, the neuro-physiological activity of a specific structure may correlate with behavioral performance during acquisition in a new learning task. In another case, the level of androgen or glucocorticoid may correlate with the social hierarchical status of monkeys in a colony. Structural or functional abnormalities may correlate with the severity of mental dysfunction in schizophrenic patients.
The interventional approaches help to find cause-and-effect relationships between the independent and the dependent variables. In contrast, the correlational approach cannot determine any causal relationship between the two measures. However, this by no means reduces its importance in biological psychology research, because the presence of a correlation between two measures suggests future intervention studies, and lack of correlation may help to limit the domain for further investigation. Even when a causal relationship between two variables has been established, correlational studies still provide additional information. Based on the evidence that disrupting the function of a somatic structure alters a specific type of behavior, one can conclude that the somatic structure exerts critical influence on that behavior, but there is no guarantee that the structure is normally engaged when the behavior is performed in nature. However, if some somatic measures in that structure co-vary with the behavioral performance under natural conditions, it is more likely that the structure is inherently engaged in carrying out the behavior. Therefore, discovery of somatic mechanisms underlying mental functions relies on convergent evidence from studies using different approaches in biological psychology research.
Some Issues Involved in Different Approaches and Methods
How to reach a specific interpretation for the effect of a treatment is one of the major concerns in biological psychology research. Consider, for example, the use of lesions to evaluate the role of a structure in a function. It appears to be straightforward to ablate the structure and observe the effect. However, removing a block of tissue or making electrolytic or radio frequency lesions destroys not only the soma but also fibers of passage from other regions. Better specificity may be achieved by employing excitotoxins that damage only cell bodies but spare the fibers of passage. Neurotoxins invading particular types of neural elements may cause selective depletion of specific transmitters. For example, 6-hydroxydopamine (6-OHDA) is often used to deplete central dopamine for examination of its role in motivational or motor behavior. Likewise, side effects of a pharmacological agent often, if not always, prevent a clear interpretation of specific influences on behavior. Specificity of pharmacological studies may be achieved by administration of several drugs sharing the same intended action but with different side effects, or by counteracting the agonist effect with specific antagonists.
To employ treatments with specific influences on the nervous system does not necessarily guarantee a clear-cut interpretation of the finding. Two further sources of complication should be noted. One comes from the fact that the bodily system is dynamic. As noted in Section 4.3, structures in the nervous system are intimately interconnected with a multitude of convergent and divergent fibers and are highly reactive. Perturbation at one point causes not only local changes but also changes in adjacent or even remote areas. For example, lesions or chemical depletion may cause a constellation of compensatory reactions in the remaining system and the resultant behavioral deficit could be attributed either to loss of the ablated components or reactive changes in the surviving components. In the case of depletion, replacement of the depleted substance may differentiate the two possibilities, which are really hard to resolve in lesion studies. Stimulation of neural tissue by electrical or pharmacological means may affect not only activity of the stimulated area but also that of connected regions. Thus, caution must be used in assigning any effect, and hence the function inferred from the presence of such an effect, directly to the manipulated structure.
The second complication comes from the fact that behavior is subject to multiple influences, and quite different underlying causes may result in the same or similar changes in explicit behavior. To take studies of learning and memory as an example, treatments given prior to learning tasks may affect acquisition performance by changing sensory-motor or motivational processes instead of learning ability. Thus, influences on performance should be ruled out before a conclusion on learning per se can be reached. The post-training treatment regimen may avoid such confounding, and effects thus produced could be due to influence on storage processing of the learned information. Sometimes, behavior may involve different phases of processing, each of which may extend over a period of time. Treatments that cause permanent and irreversible effects on the nervous system may not be able to dissect the neural structures engaged at different stages of the process. For example, permanent lesions made post-training in a structure cannot distinguish whether it affects memory formation or memory retrieval. Yet reversible and temporary lesions administered at different phases may well serve such a purpose.
Another major concern in methodology is how to choose, from a whole array of available techniques, the appropriate ones that can detect or affect the relevant somatic events, which vary tremendously in both the spatial and temporal dimensions. As noted in Section 4.3, the basic structural and functional unit of the nervous system is the neuron. Yet mental activity may be related to episodes occurring at subordinate levels, such as the synapse, intra-cellular organ-elles, and specific molecules, or at superordinate levels such as neural networks, maps, systems, or even the entire CNS. The dimensions of these structures vary from 10∼7 to 102mm. Different kinds of signal arise in these structures and may last for different periods of time varying from milliseconds, in the case of action potentials, to a whole life span, in the cases of increases or decreases of neural elements or the consequence of gene mutation. Methods presently available have constraints in both the spatial and temporal dimensions within which they can best function. For example, recording single unit activity accurately follows the signals from an individual neuron with a temporal resolution of milliseconds, but this technique is inadequate to detect the slower activity changes over a larger population of neurons. On the contrary, event-related potentials or the electroencephalogram (EEG) reflect massive field potential changes from an extensive region, but are unable to locate or distinguish the relative contribution of neural elements within that region. Figure 4.4 shows the spatial and temporal dimensions of neural processes appropriate for several methods.
It should be noted that the strength of a technique may not be an inherent property but should be judged in relation to the dimensions of the phenomenon to be studied. Improvement of techniques in a reductionistic direction (say, from EEG to unit activity to patch-clamp recording of a single channel) has contributed much to the advancement of our knowledge of neural basis of behavior. However, Bullock (1993) has rightly reminded the field that some aspects of brain function may be imbedded in the integrative aspects of neural activity, which should be brought out by methods with appropriate spatial and temporal scales.
Biological Psychology Draws Conclusions from Convergent Evidence
Based on the above discussion on methodology, it is clear that biological psychology has to rely on evidence from various lines of studies to establish relationships between the somatic system and mental function. The reasons can be recapitulated as follows.
- The neural substrate underlying a mental function may involve mechanisms from the molecular to the system levels and engage somatic events with different durations. No method could cover the whole spectrum of neural events. Moreover, an apparently unitary behavioral or mental function may include sub-processes or substages, each of which may have different neural substrates. Thus, studies using different methods yield findings on different facets of the relation, and only integration of all findings can generate the global picture.
- To achieve specificity of the treatment and reach valid interpretation of the observed effect, convergent evidence from studies using different methods is required.
- Studies adopting interventional approaches and the correlational approach may be complementary to substantiate the assertion that a particular somatic process is normally engaged in a mental function under natural conditions.
- Biological psychologists are interested in the extent of generalization of neural mechanisms underlying specific behavioral functions from one species to another, including humans.
The Neural Bases of Classical Conditioning
The search for neural bases of classical conditioning may be used to illustrate the dependence on convergent evidence in biological psychology. Ivan P. Pavlov established the behavioral principles of conditioning in a series of publications that began near the end of the nineteenth century (e.g., Pavlov, 1906). He later attempted to account for conditioning in terms of hypothetical irradiation of electrical activity across the surface of the cerebral cortex, but the irradiation was inferred from the behavior and was not based on direct observations and did not provide a satisfactory explanation. Later workers brought newer techniques to the search for the mechanisms and neural circuits involved in conditioning.
Psychologist/neuroscientist Richard F. Thompson and his colleagues, as well as other groups, have studied extensively the neural circuitry involved in eyelid conditioning in rabbits (for review, see Thompson & Kim, 1996). The task involves pairing of a tone (the conditioned stimulus, CS) with an air-puff (the unconditioned stimulus, US) administered to the eye; the air-puff causes the unconditioned response (UR)—blinking of the eye or closure of the eyelid. An early correlational study showed good correlation between activity of neurons in the hippocampus and the performance of the conditioned response (CR): the activity increased during acquisition only if the CS and US were paired and predicted the appearance and maturation of the CR. A behavioral intervention study showed that training experience enhanced synaptic efficacy in the hippocampus: after eyeblink conditioning, stimulation applied to the perforant path (an input to the hippocampus) of trained animals evoked larger monosynaptic population spikes in dentate granule cells. Enhanced responses in pyramidal neurons persisted even in the hippocampal slices taken from trained animals and assessed in vitro, suggesting that the changes had become intrinsic to the hippocampus. Similar neuronal activity model was also detected in the cerebellum. Unit activity changes in the deep cerebellar nuclei—especially the interpositus nucleus—also preceded and predicted the performance of the CR.
The roles of these two structures were distinguished by somatic intervention studies. Lesions of the cerebellum or the interpositus nucleus abolished acquisition in simple delayed classical conditioning but had no effect on performance of the UR, which ruled out involvement of performance factors and suggested a causal role of this structure in the task. In contrast, hippocampal lesions had little effect on acquisition or expression of CR in the simple delayed classical eyelid conditioning, suggesting that activity in the hippocampus did not bear any necessary role in CR performance. Yet hippocampal lesions did abolish learning in more complex paradigms such as trace conditioning and conditional or reversal discrimination learning, suggesting that the hippocampus subserves a different role than the cerebellum. The trace conditioned response has been compared to human declarative memory because awareness of the CS—US temporal contingency is a prerequisite for successful acquisition in trace conditioning (Clark & Squire, 1998).
The circuit and mechanism involved in the conditioned reflex engaging either structure was further delineated by use of a constellation of methods. For example, in the cerebellar circuitry, temporary and reversible lesions were used to distinguish between structures mediating plasticity and those mediating the output from the plastic site, which could not be properly dissociated by permanent lesion studies. Inhibitory chemical agents or local cooling were applied to either the anterior interpositus nucleus, which presumably is a major site of plasticity, or to its efferent sites such as the superior cerebellar peduncle and the red nucleus. The general findings were that suppression of the interpositus nucleus during training blocked performance in acquisition and the animals showed no trace of retention after the blockade was lifted and no saving if subjected to retraining. On the other hand, suppressing sites efferent to the interpositus nucleus during training blocked performance in acquisition, but the animals showed excellent performance in testing once the suppression was lifted, suggesting that the suppressed site was not essential for plasticity to occur.
Long-term depression (LTD) occurring in the cerebellar Purkinje cells has been proposed as a mechanism for plasticity underlying simple classical conditioning. LTD involves activation of glutamate metabotropic receptors. A recent study utilizing the newly developed gene-knock-out technique produced mice who did not express a specific type of metabotropic glutamate receptors (mGluR1). The results showed that these mutant mice were deficient both in LTD and eyeblink conditioning (Aiba et al., 1994), suggesting a correlation between a neural mechanism and behavior.
The involvement of the cerebellum and hippocampus in classical conditioning has also been investigated in human beings. Studies using positron emission tomography (PET) found that eyeblink conditioning in humans increased glucose metabolism or local cerebral blood flow of the cerebellar and hippocampal areas, among other regions, consistent with the findings from rabbits (Timmann et al., 1996). Furthermore, patients with unilateral cerebellar pathology were not able to acquire a conditioned eyeblink response (Bracha, Zhao, Wunderlich, Morrissy, & Bloedel, 1997). In contrast, amnesic patients with damage in the hippocampal area could acquire an eyeblink conditioned response in a delayed conditioning paradigm but failed to do so in a trace conditioning paradigm (Clark & Squire, 1998), again replicating the findings in nonhuman species. Thus, all the results from human and nonhuman studies employing various techniques converge in indicating that the cerebellum plays a critical role in mediating classical conditioning of certain somatic reflexes.
Motor Behavior: Reflexes, Movements and Actions
The behavior of people and other animals displays a great variety, ranging from relatively simple reflexes to coordinated movements and extending to complex acts or action patterns. Usually we are interested in acts or action patterns—complex, sequential behaviors, frequently oriented towards a goal—but even reflexes can tell us much about the principles of behavior and its bodily mechanisms.
An additional reason for taking up motor behavior here is that most of the chapters in this book omit this topic, simply assuming that mental or cognitive processes will be translated into behavior or into inhibition of behavior. Some psychologists and investigators in related disciplines, however, have made progress in studying how activity of muscles and glands is evoked and coordinated.
In the seventeenth century, the philosopher René Descartes noted that some responses appear to occur automatically to certain stimuli. He supposed that the neural energy of the stimulus is conducted to the spinal cord and there reflected (whence the term ‘reflex’) back out to the muscles, eliciting the response. At that time, the difference between sensory nerves and motor nerves had not yet been recognized. This was achieved early in the nineteenth century by Charles Bell and François Magendie, and it gave rise to the concept of the reflex arc. Late in the nineteenth and early in the twentieth century, investigators such as Charles Sherrington studied the laws of reflex action. Sherrington and Ivan Pavlov believed that behavior could be understood as a chain of successive reflexes, but this view was later shown to be over-simple. Rather, much behavior, ranging from locomotion to speech, appears to be organized by plans or programs that are established before acts occur, rather than each movement being triggered by the previous one. For example, walking is governed in part by a program integrated at the level of the spinal cord, as shown in studies with animals in which the spinal cord has been separated surgically from the brain (Brown, 1911). Not only can such animals walk on a treadmill, but they can continue to do so after transection of the dorsal roots deprives them of sensory input. Thus, walking appears to rely on an intrinsic motor program rather than being a sensory-instigated reflex.
In the case of speech, older explanations in terms of sensory-response units, each triggering the next, have given way to explanations in terms of plans in which each speech unit is placed in a larger pattern. Sometimes units are misplaced, although the pattern is preserved: ‘Our queer old dean’, said English clergyman William Spooner, when he meant, ‘Our dear old queen’. (Spooner was so prone to mix up the order of sounds in his sentences that this type of error is called a spoonerism.) Such mistakes reveal a plan: the speaker is anticipating a later sound and executing it too soon. A chain of reflexes would not be subject to such errors.
Even in terms of reflexes, behavior is more complicated than originally supposed. For one thing, reflexes can be modified and acquired through training, and the concept of conditioned reflexes has been important, beginning early in the twentieth century, as described earlier in this chapter and more fully in Chapter 6. For another, there is selective potentiation of reflexes and of the underlying neural circuits.
Selective potentiation refers to the fact that, at any given time, the activity of certain neural circuits is enhanced, whereas activity of other circuits is inhibited. For an example of operation of this principle, consider how an animal is able to walk over rough, uneven terrain. Walking involves two phases of movement in each leg: (1) the swing phase when the limb is off the ground and swinging forward; this is initiated by the flexion reflex, and (2) the stance phase, providing support and propulsion, involving the extension reflex. Normally, these two phases are evoked in succession by a generator circuit in the spinal cord, but what happens when an obstacle is encountered? For example, what happens if during movement a tap is delivered to the front of the paw? This is the part of the foot that is most likely to encounter something that might trip the animal or push its foot out from under it.
Experiments on this have been done with cats in which the spinal cord was separated surgically from the brain. Forssberg, Grillner, and Rossignol (1975) found that the same stimulus to the front of the foot could evoke either flexion or extension, depending on the phase of the leg movement when the tap was delivered. In the swing phase, the tap elicits flexion in all the joints of the leg; this lifts the leg and may allow it to clear an obstacle that might otherwise trip the cat. If the same tap is delivered as the cat is starting the stance phase, the stimulation elicits or strengthens the extension reflex, so a moving object that might have swept the cat’ foot out from under it is less likely to do so. Thus, a given stimulus does not by itself determine a reflex response; the response is determined also by the state of the animal at the time of stimulation. The principle of selective potentiation can also be applied to some questions of motivation discussed in Chapter 11.
As well as selective potentiation, there is selective inhibition. A familiar example is the inhibition of the limb and neck muscles during sleep. Sometimes this inhibition does not exactly coincide with sleep. Many people have had the unpleasant experience of feeling paralyzed just before going to sleep or just after waking; a survey of the general population in Germany and Italy showed that about 6% have had this experience (Ohayon, Zully, Guilleminault, & Smirne, 1999). There is also the pathological condition known as cataplexy in which a person, although remaining conscious, suddenly loses muscle tone and falls to the ground in moments of emotion, especially laughter. While investigating cataplexy, researchers in the Netherlands found that muscle tension in the legs of normal subjects declined markedly during laughter (Overeem, Lammers, & van Dijk, 1999). This may explain why expressions like ‘weak with laughter’ occur in many languages.
Chronic selective inhibition of the facial musculature is well known in Parkinson’ disease, and recently it has been reported for patients with schizophrenia (Kring, 1999). Compared with nonpatients, schizophrenia patients exhibit few outward signs of emotion, although recordings from facial muscles reveal very small, subtle facial activity characteristic of different emotions. In response to emotional stimuli, schizophrenia patients report experiencing as much emotion as nonpatients. Thus the social interactions of the patients are impaired by their lack of normal facial responses. Here is another case where biological condition affects social relations.
Varieties of Motor Behavior
Table 4.1 presents a classification in which motor behavior is arranged in a hierarchy, with simple reflexes at the top, proceeding next to more complex movements, and then to more complex acts, ranging from locomotion to acquired skills. Even the reflexes can be modified by conditioning, as we have seen, to response to new or altered stimuli. The examples toward the bottom of the table rely less and less on innate reflex arcs.
Levels of Control of Motor Behavior
The control of motor behavior is implemented at several different levels of the nervous system, depicted in simplified fashion in Figure 4.5. Motor neurons in the ventral horn of the spinal cord or the brain stem send out axons to innervate muscles of the body and face regions. These fibers generally release acetylcholine which acts on nicotinic receptors. A motor neuron along with its innervated muscle fibers constitutes a motor unit. For example, muscles for eye movements have an innervation ratio (axons to muscle fibers) of roughly one to three, while those in the leg have a ratio of one to several hundreds. The fewer the number of muscle fibers a motor neuron innervates, the finer is the motor control. Motor neurons that innervate the same muscle aggregate into nuclei or columns and may extend over several spinal segments. Spinal motor neurons receive inputs from skin, joints, tendons, and muscles. Such somato-sensory information is capable of activating spinal reflexes for maintaining posture (the stretch reflex), protection from injury (the flexion reflex), and so forth. While these reflexes are mediated through intra- or inter-segmental circuitry linked by excitatory or inhibitory synapses within the spinal cord, they are nonetheless subjected to modulation by experience and other psychological factors. This is accomplished by supraspinal inputs conveying signals to control locomotion, orientation, posture and balance, species-specific behavior, and skilled motor acts. However, the spinal cord should not be viewed as merely a relay device that translates the central or peripheral inputs into motor acts. Evidence indicates that in vertebrates, spinal circuits subserve the function of a central pattern generator that engenders rhythmic motor activity for locomotion (Grillner et al., 1995). Spinal motor neurons thus coordinate their intrinsic patterns of activity with extrinsic information from various sources and serve as the final common pathway of the motor system.
|Table 4.1 A classification of movements and acts|
Direct central inputs to spinal motor neurons arise from the cerebral cortex and some brainstem nuclei. These descending fiber tracts exert excitatory and inhibitory influences on the motor neurons. The cortical area subserving motor functions is mainly located in the frontal lobe. The primary motor cortex shows somatotopic organization; that is, motor control of adjacent body parts is mapped into adjacent regions of the primary motor cortex, and the amount of cortical tissue devoted to a specific body part reflects its fineness of motor control; thus the motor representation of parts involved in tool manipulation (hands and fingers) and speech (lips and tongue) are disproportionately huge. The corticospinal tract (also called the pyramidal tract due to its wedge shape in the medulla) arises from pyramidal neurons in the motor cortex. This tract contains large-diameter and fast-conducting axons and is involved in control of fine movements in arms, hands, and fingers. Damaging this tract in monkeys impairs manual dexterity but without significant effects on locomotion and reaching (Lawrence & Kuypers, 1968a). Fibers from the brain stem nuclei projecting to spinal motor neurons belong to the extrapyramidal system, and damage in this system in monkeys leads to deficits in locomotion and posture (Lawrence & Kuypers, 1968b).
Behavioral neuroscientists began to understand how the motor cortex commands movements by monitoring cortical neuron discharges in monkeys performing specific motor acts. Activity of single neurons did not bear a clear-cut relationship with some types of movements. In a series of studies, Georgopoulos (1997) trained monkeys to make arm movements in reaching to eight possible target directions elicited by signals in those directions; he recorded neuronal firing in the primary motor cortex simultaneously with the movement. Plotting the discharge rates of specific neurons against various directions of movements revealed a direction-tuning curve in the response of many motor neurons. That is, a neuron altered its firing rate according to the direction of the reach. Each neuron fired most frequently at one of the directions, and slowed down as the deviation from this best direction increased. However, the movement was not determined solely by the neuron coding the movement direction as its best direction. Instead, a population of neurons was activated shortly prior to and during the movement, and the vector sum of population activities correlated best with the direction of movement. In a follow-up study to distinguish whether such activity is motor-related or sensory-related, Georgopoulos and collaborators trained the monkey to perform a delayed reaching task. A signal 90 degrees away from the intended movement appeared and soon turned off. The monkey had to wait for a period after the signal disappeared and then reach in the intended direction. Recording from a population of cortical neurons showed the summation vector gradually shifting from the signal direction to the reaching direction during the waiting period, similar to a mental rotation of movement.
The nature of neural coding for mental features has been an issue fundamental to research in biological psychology. The so-called ‘single neuron doctrine’ suggests that activity of single neurons is capable of forming unitary mental representations, whereas other theories insist that representation of mental activity relies on ensembles of neurons. While both views have strength and weakness in guiding research and interpreting findings, the results of Georgopoulos give one of the best examples that traits of behavior (direction of movement in this case) are represented by collective activities in a population of neurons.
Some brain structures do not project directly to the spinal cord but nonetheless exert a profound influence on motor functions; one set is the basal ganglia. The basal ganglia contain several distinct but interconnected parts. This structure receives inputs from most of the cerebral cortex and the thalamus as well as dopaminergic projections from the brainstem. Output of the basal ganglia, flowing through its complicated internal circuitries, is funneled to the frontal cortex and brainstem. The basal ganglia circuitry appears to modulate limb and eye movement. It may be involved in representation of serial order of learned and innate motor sequences. In tasks involving sequential movements, basal ganglia neurons fired 100 ms or more in advance of the activity of muscles (Kermadi & Joseph, 1995). Neuronal discharges in the supplementary motor area, a major target of the basal ganglia, have been shown to be selective to sequences of movements with a specific order (Tanji & Shima, 1994). Parkinson’ disease involves degeneration of dopaminergic fibers projecting to the basal ganglia, disrupting functional balance in the basal ganglia circuitry and resulting in movement difficulties. Patients with obsessive—compulsive disorder are preoccupied with recurrent stereotypic sequence of behavior or thoughts. Recent evidence shows that they have functional abnormality in, among others, the basal ganglia (Saba, Dastur, Keshaven, & Katerji, 1998).
We mentioned the cerebellum in the last part of Section 4.4 in regard to its role in conditioning, but the importance of the cerebellum in regard to motor control was recognized even earlier, because injuries to the cerebellum cause impairments in motor coordination. Various parts of the cerebellum receive cerebral cortical and other central inputs through pontine nuclei as well as peripheral inputs from the spinal cord and vestibular nuclei. Inputs to the cerebellum are carried by the mossy fibers and climbing fibers. These inputs exert convergent excitatory effects on Purkinje neurons, which are the major output neurons of the cerebellar cortex. Axons of the Purkinje cells form inhibitory synapses onto the deep cerebellar nuclei, which in turn project to the motor cortex and other frontal regions and to motor neurons in the brain stem and spinal cord. The access of the cerebellum to an intricate set of information and to both pyramidal and extrapyramidal motor systems as well as its orderly intrinsic organization have invited speculation on the function it subserves and the mechanism to accomplish the function. In the Marr and Albus model (Albus, 1971), the cerebellar Purkinje cell with its two inputs functions as a learning-machine capable of detecting and reducing error from a preset criterion. The vestibulo-ocular reflex (VOR) refers to swift movement of the eyeball in a direction opposite to head turning in order to maintain a stable visual image. The involvement of the cerebellum in experiential modification of VOR supports this idea (Ito, 1998). This learning ability may play an important role in a function generally attributed to the cerebellum: coordination of motor programming or timing for rapid or automatic multi-joint complicated movement. Growing evidence has indicated that the cerebellum is involved not only in motor learning but also in other higher cognitive functions such as language and verbal working memory (Desmond & Fiez, 1998).
To students in biological psychology, the most challenging question concerning the study of the motor system is how the brain transfers emotion, motivation, visual signals, verbal instructions, as well as self-generated thoughts including plans, intention, and will into motor behavior. Recent evidence begins to address this issue, yet the picture is far from clear. In rats and cats, the midbrain periaqueductal gray (PAG) appears to mediate some reactions elicited by emotional stimuli (Bandler & Shipley, 1994). Part of its input derives from the limbic cortex, hypothalamus, and amygdala, all of which are implicated in emotional functions (see Chapter 12). Stimulating different parts of the PAG arouses various kinds of species-specific defensive behavior. Evidence has shown that fibers projecting from the central amygdaloid nucleus to the central gray mediate the freezing response during fear conditioning (LeDoux, 1995).
Another critical structure in this regard is the basal ganglia. A dorsal part of the basal ganglia (dorsal striatum) appears to be critical for associating an operant response to a specific stimulus reinforced by rewards (McDonald & White, 1993). Much attention has recently been paid to the nucleus accumbens lying at the ventral side of the basal ganglia. This structure receives multi-modality sensory information from the limbic system. It is innervated by brainstem dopaminergic fibers, the activity of which has been implicated in expectancy of reward (Schultz, 1998). The nucleus accumbens gains access to motor function by way of the frontal cortex and the extrapyramidal system. Manipulating functions of the nucleus accumbens has been shown to alter performance in instrumental learning tasks including those using addictive drugs as reward (Kalivas & Nakamura, 1999), suggesting the importance of this structure in interfacing motivational and motor circuits (Morgenson, Jones, & Yim, 1980).
How thoughts lead to acts remains largely a mystery. The finding that readiness potentials recorded from humans performing voluntary acts preceded subjective awareness of intending to move by 350 to 400 ms (Libet, 1985) remains most intriguing to anyone contemplating the relationship between intention and movement. Most, if not all, of motor behavior acts on or reacts to stimuli in the environment. Thus, the motor system must work intimately with the sensory systems. The primary motor cortex receives direct projections from the somato-sensory cortex. Motor neurons responsible for moving a specific part of the body receive information from sensory neurons responsive to stimulation in that part of the body. These fibers may provide rapid feedback during manipulation and account for the findings that a tactile signal is 75 ms faster in eliciting a hand movement than a visual signal (Evarts, 1974). The visual system nonetheless provides critical information in guiding motor acts. In reaching, saccadic eye movements and head turning precede the hand movement. If the eyes and head are prevented from moving, the reaching movement misses the target. The premotor cortex is a cortical region that lies just rostral to the primary motor cortex; it participates in preparing externally guided sequential movements. The dorsal part of this region receives information from the parietal cortex (Wise, Boussaoud, Johnson, & Caminiti, 1997), which receives the dorsal stream of visual information flow involved in detecting spatial cues. It is interesting to note that neurons in some sectors of the parietal cortex and frontal cortex show receptive fields of both visual and somatosensory stimuli. In some neurons of these regions, the receptive field to visual cues was shown to be modified by hand movements (Graziano & Gross, 1995). Reaching cannot rely totally on the retinotopic coordinate to locate the target. As the head or body turns to different directions, images of objects located at different positions in the space may fall on the same spot of the retina. Yet visual responses of some neurons in the parietal lobe could be modified by the position of gaze (Andersen, Bracewell, Barash, Gnadt, & Fogassi, 1990): neuronal discharge to a stimulus at the same retinotopic position was differentially amplified as a monkey fixated its eyes at different locations in space. Such amplification varying as a function of the gaze position allows transformation of a retinotopic coordinate into a head-centered or body-centered reference and facilitates reaching movement. Thus the parietal region may be an important region in coordinating sensorimotor function in an extrapersonal space.
Motor behavior should not be viewed only as the output of mental activity. It also sometimes contributes to enrich mental functions. Active manipulation of an object with the hands and fingers generally aids our perception of the shape and surface texture of the object. Furthermore, organisms from human infants to many other species show a natural tendency to imitate motor behavior of others. Recently some neurons in the rostral part of the ventral premotor cortex in monkeys were found to discharge not only when the monkey grasped or manipulated objects, but also when the monkey observed the same acts performed by another individual. It is proposed that these so-called ‘mirror neurons’ enable an organism to detect and understand the mental states of conspecifics. Such a function might be related to forming a ‘theory of mind’, which is essential for development of high order cognitive social interactions (Gallese & Goldman, 1998). A similar mirror system for recognition of gesture has also been detected in humans by positron emission tomography focusing on a region including Broca’ area. The presence of such an observation/execution matching function in a brain area critical for language has invited speculation on its role in the evolution of the human communication system (Rizzolatti & Arbib, 1998).
Some Main Principles of Biological Psychology
Material in this and other chapters of the book illustrate some main principles of biological psychology. We conclude and summarize this chapter by listing these principles here.
Communication among different parts and systems of the body occurs by means of three kinds of messages—neural, hormonal (endocrine), and immune system messages—and combinations of these kinds of messages also occur.
Most neural, hormonal, and immunological communication involves specialized receptor molecules that respond to only particular messenger molecules. Some receptor molecules are located in cell membranes and others are located inside the cells.
There are both excitatory and inhibitory messages in the neural, hormonal, and immune systems.
The neural, hormonal, and immune systems show cyclical changes in activity ranging in length from less than a day, about a day (circadian), to about a year (circannual) or more.
The neural, hormonal, and immune systems are all subject to modification by experience and learning.
The brain changes throughout life, as a function of both endogenous (intrinsic) and exogenous (extrinsic) factors and influences.
The nervous system is composed of separate cells that are distinct structurally, metabolically, and functionally.
The same stimulus input is represented at several levels and locations in neural sensory systems, and different kinds of analyses are made at the different levels and locations.
Behavioral interventions cause both behavioral and somatic effects, and somatic interventions cause both somatic and behavioral effects.
Social stimuli affect some neural and hormonal systems, and some neural and hormonal activities affect social behavior.
There is a basic cascade of neurochemical events that, continued to different lengths, can lead to a whole series of different effects, from neural conduction and behavioral activity to formation of long-term memory. Followed for a few steps, it ensures neural conduction and synaptic transmission; followed further it yields prolonged alteration in neural function that underlies phenomena such as habituation; continuation to a later stage yields memory storage and long-term potentiation; further continuation can lead to structural changes. Thus the neuro-chemical cascade deals with processes that span the time range from milliseconds to years.
Normally the entire brain is active all of the time; specific behavioral and cognitive activities increase or decrease the levels and patterns of activity of particular regions of the brain.
Different neural events differ widely in their spatial and temporal extents, so studying them requires techniques with different spatial and temporal resolutions. The discovery of new techniques and combinations of techniques is making it possible to study with increasing adequacy and accuracy the bodily processes that underlie behavior.
Specific mental functions usually involve coordination of neural activities among multiple brain sites and are regulated by multiple neuro-chemical systems in the brain.
Biological psychology and neuroscience study not only the processes and characteristics that hold for groups of persons or animals but also those that make individuals unique.
Stable percepts and personality traits are based on brain processes that are dynamic in terms of synaptic connections and biochemical processes.
An evolutionary perspective is essential to biological psychology. It offers two different but complementary emphases: (1) continuity of behavior and biological processes among species because of common ancestry, and (2) species-specific differences in behavior and biology that have evolved in adaptation to different environments.
An evolutionary approach often suggests hypotheses that may account for behaviors and their mechanisms, but these, like other hypotheses, must be tested thoroughly against other hypotheses before they are accepted or rejected.
Research in biological psychology can be applied to problems of human health and well-being, and attempts to make such applications also benefit research by providing tests of the adequacy of current findings and conclusions.