Neuroscience is the scientific study of the nervous system and its function. Since the nervous system can be studied at many levels—from the molecular structure of nerve cell membranes to the whole-brain functions involved in the highest of human mental activity—neuroscience is a multidisciplinary field. Neuroscientists might study the nervous systems of simple creatures (e.g., sea slugs), more complex animals (e.g., rats, cats, and monkeys), or human beings. At meetings of the Society for Neuroscience one finds over ten thousand scientists representing a wide variety of fields such as microbiology, histology, neuroanatomy, neurophysiology, physiological psychology, developmental psychology, neuropsychology, neuroradiology, neurology, psychiatry, and cognitive science.
The various domains of research in neuroscience might be grouped into the following topic areas:
- Cellular and molecular neurophysiology;
- Developmental neuroscience;
- Sensory and motor neuroscience;
- Regulatory neuroscience;
- Behavioral and cognitive neuroscience; and
- Clinical neuroscience.
After a brief review of the history of neuroscience, each of these areas will be described. This will be followed by a summary of current research on religious experience and brain function. Finally, the philosophical presuppositions of neuroscience will be briefly summarized.
The history of neuroscience
The ancient-to-modern history of neuroscience involved resolution of four major issues. The first issue was whether mental activity and control of behavior emanated from the brain or the heart. Next, there was the issue of whether the critical parts were the ventricles and cerebral fluids (the pneumatic theory) or the brain tissue itself. When it became clear that the various structures of the brain were the organs of thought and behavior, there was a controversy over localization of these functions in specific brain areas versus a holistic view of brain function. Finally, with respect to neural tissue, research in the late nineteenth and early twentieth centuries established that independent neural cells (neurons) are the basic functional units of the nervous system.
The work of the ancient Greek physician Hippocrates (460–375 b.c.e.) on epilepsy is among the most important ancient contributions to neuroscience. Hippocrates denied that epilepsy was a divine or sacred manifestation, arguing instead that it was a disease of the brain. He considered the brain to be the seat of all mental experience. The alternative view was advanced by Aristotle (384–322 b.c.e.), who is considered by many to be the greatest biologist of antiquity. Aristotle taught that the heart was the center of sensation, movement, and intelligence. Around the same time, important contributions were also being made in Alexandria. Herophilos and Erasistratos (third century b.c.e.) distinguished various brain structures, provided the first description of the ventricles, and associated intelligence with the greater number of convolutions in the cortex of the brain.
The progress of these ancient neuroscientists was extended in the work of Galen (129–199 c.e.) in Rome. Galen contributed important studies of the cranial nerves and the spinal cord. Using transections of the spinal cord, he determined that the spinal cord was an extension of the brain and the conduit of sensory and motor information. However, Galen's theory of brain function was pneumatic, focusing on the ventricles rather than brain tissue. During the medieval period, the theories of Galen became dogma and were transmitted without much modification into the Renaissance.
New advances in neuroscience during the Renaissance were triggered by a rediscovery of the work of the ancients, including descriptions of the original work of Galen. Progress during the Renaissance was made by Andreas Vesalius (1514–1564), who discredited the ventricular theory; René Descartes (1596–1650), who had the idea of the body as a machine and developed the concept of a reflex; and Thomas Willis (1612–1675), an anatomist who provided the most complete description to that date of the anatomy of the brain (and whose book was famously illustrated by Christopher Wren).
The progress in neuroscience achieved in the Renaissance was accelerated during the Enlightenment. Important progress during this period was made by Franz Joseph Gall (1758–1828) and Johann Casper Spurzheim (1776–1832). These investigators proposed that the gyri of the cortex were composed of cells that connected to the brain stem and spinal cord, and, therefore, the cortex could control movement via its connections to the spinal cord. The most famous aspect of the work of Gall and Spurzheim was phrenology, the study of the relationship between skull surface features and mental faculties. What is important about this work is that skull features were thought to reveal the size of underlying cortical gyri, thus phrenology was the first extensive theory of the cortical localization of cognitive functions.
Pierre Flourens (1794–1867) is credited with demolishing phrenology, while advocating the idea that all intellectual functions are coextensive within the cortex (a holist view). Flourens developed the research strategy of removing or lesioning parts of the brains of animals and observing consequent changes in their behavior. However, the localizationist view was kept alive by the work of Paul Broca (1796–1881) and Carl Wernicke (1848–1904), who identified the expressive and receptive language areas of the left cerebral hemisphere (respectively). Based on the discovery of the bioelectric nature of the function of muscles by Luigi Galvani (1737–1798), Gustav Theodor Fritsch (1838–1929) and Eduard Hitzig (1838–1907) demonstrated that the brain was electrically excitable, and that electrically stimulating different cortical locations produced different behavioral effects.
During the nineteenth and early twentieth centuries advances were also being made in understanding the microstructure and microfunction of the nervous system. Due to the availability of improved microscopes, Theodore Schwann (1810–1882) was able to discover and describe the fact that the organs and tissues of animals, including the brain, were made up of many individual cells. However, Schwann believed that the brain, unlike other organs, was made up of cells that were not separated by membranes, but rather were a continuously interconnected network. It took a lifetime of painstaking work by the Spanish neuroanatomist Santiago Ramón y Cajal (1852–1934) to demonstrate that each neuron is an independent and discontinuous cell. Charles Scott Sherrington (1857–1952) was the first to describe the synapse, the point of communication between neurons. The microstructure of the nervous system became dynamic with the description of the action potential by A.L. Hodgkin (1914–1998) and A. F. Huxley (1917–). Finally, the hypothesis (since substantiated) that the site of learning within the nervous system is the synapse was advanced in 1949 by Donald O. Hebb (1904–1985).
Cellular and molecular neurophysiology
Cellular and molecular neurophysiology is the study of the molecular structure and physiological functioning of nerve cells. The critical property of a neuron is that it has a membrane that is an electrochemical battery. There is a difference in electrical charge between the inside and outside of the cell created by an uneven distribution of sodium and potassium ions. Also critical to neural function are the processes by which this electrochemical potential can be disturbed so as to trigger an action potential that can be transmitted from one end of an axon to the other. Research has demonstrated that the electrical properties of neurons are based on voltage controlled ion channels within the nerve cell membrane. These channels open or close depending on surrounding voltage levels. Voltage controlled ion channels are the subject of intense study in molecular neuroscience since they are the basis of the resting potential, the excitability threshold of the membrane, the phenomena of the action potential and its transmission, and most of the critical events at the synapse, where information is transmitted from one neuron to another.
Much work in cellular neurophysiology is focused on events occurring at the synapse. In simple outline, it has been demonstrated that the arrival of the action potential at the end of the axon (the terminal button) causes calcium ions to enter the cell. This causes packets of a transmitter substance to release their contents into the extremely small space between neurons. The transmitter substance attaches to receptors on the post-synaptic neuron, causing ion gates to open, resulting in a slight electrical perturbation in this post-synaptic cell. Various other mechanisms have been found that clear activated receptors, take back excess transmitter substance, and recreate the transmitter substance packets.
Another active area of work in neurophysiology is the investigation of the ways that synaptic functions are modified by learning. Using the nervous system of a sea slug, it was demonstrated that learning involved changes in the conductive properties of synapses. Where in the human nervous system such synaptic changes can occur, and how they occur, are important issues in the study of human learning and memory. An important recent advance was the identification of the NMDA receptor that appears to be important in triggering the synaptic changes involved in learning. NMDA receptors are present on neurons within the hippocampus, a brain structure that is known to be important for some forms of memory.
Transmitter substances, and their respective types of post-synaptic receptors, come in a wide variety. Among the major transmitter substances identified are acetylcholine, dopamine, norepinephrine, serotonin, GABA, and glycine, but there are many more than this. There are, for example, at least four variants of dopamine. The field of neuropharmacology attempts to exploit this variety by finding drugs that act in a specific manner on synapses that use a particular neurotransmitter so that specific functions of the brain can be modulated. The drug Prozac, for example, inhibits the reuptake of transmitter substance within synapses that utilize serotonin, causing these particular synapses to be active for a longer period of time.
Developmental neuroscience studies the fetal and childhood development of the nervous system. There are stages in development of the brain and nervous system: development of cell identity, cell migration, axon growth and guidance, growth of dendrities and synaptic formation, differentiation of connections based on early experiences, and myelinization of axons.
Nerve cells differentiate from undifferentiated stem cells. Stem cells become precursor neuroblasts that eventually produce neurons, glial cells, or other cells within the nervous system. This differentiation process is based primarily upon interactions with neighboring cells. Because of the potency of the influences of neighboring cells in causing undifferentiated stem cells to become neurons, a great deal of research is being done in an attempt to introduce undifferentiated stem cells into the adult brain as a means of reinstituting developmental processes within areas of damaged neural tissue (e.g., stem cells into the spinal cord in individuals who are paralyzed from spinal cord injury).
Cells differentiate into neurons in the middle of the brain surrounding the neural tube and ventricles. However, these cells must migrate outward to form various brain structures. Once cells have migrated to their appropriate place in the nervous system, axons form and begin to grow toward distant targets. For example, neurons in the motor cortex may send axons all the way down into the spinal cord to synapse on motor neurons there. Mechanisms for stimulating and guiding cell migration and axonal growth are topics of intensive study. An example of a congenital brain abnormality related to a failure of appropriate axonal growth is agenesis of the corpus callosum, a condition in which the axons that are supposed to cross between the two cerebral hemispheres do not find their way and, instead, end up traveling toward the back of the brain.
Another important developmental process is the growth of dendrites and the formation of synapses. An interesting aspect of this process is the overexuberance of dendrite and synapse formation in the first two years of life, and the subsequent loss of dendrites and synapses. It is thought that this loss of dendrites and synapses represents brain differentiation based on experience, such that connections that get incorporated into information processing and memory circuits survive, and the others do not survive.
A final developmental process is the progressive increase in myelinization of axons that, in some systems (e.g., the interhemispheric axons of the corpus callosum), are still increasing in myelinization well into the second decade of life. The myelin sheath allows a neuron to transmit actions potentials more rapidly and efficiently. Thus, myelinization of axons contributes to increased cognitive processing speed and power.
Sensory and motor neuroscience
This domain of neuroscience studies the way sensory information (vision, hearing, etc.) is received, coded, and recognized by the nervous system, and the means by which the nervous system controls motor activity in service of both reflexive and purposive movement.
The largest volume of work in sensory neuroscience has involved vision. What is becoming clear from this research is that different properties of the visual signal are processed by separate brain areas. In the cortical area that first receives visual signals, there are different cellular systems for detecting light-dark boundaries and for coding color. As information is further processed, there are separate cortical areas for processing complex visual properties: the parietal lobe for visual guidance of movement, the inferior temporal lobe for object recognition, and a superior temporal area for spatial analysis. Similar processes occur in the processing of sound, touch, and pain. The existence of multiple visual processing areas raises an interesting question regarding how these various sensory properties are reconnected to create a unified percept. This problem is known as the binding problem, a problem that has yet to be solved.
Motor systems are studied by neuroscientists all the way from simple reflexes controlled by the spinal cord, to the voluntary control of skilled movement initiated and regulated by the motor cortex and various subcortical structures. One of the knotty issues in the study of motor activity is determining the modes by which spinal cord reflexes, more complex innate motor responses (such as eating, drinking, sleeping, fear, aggression, etc.), learned habitual behaviors, and conscious voluntary activity are all coordinated with each other and with important vestibular, propriocpetive, and visual sensory information.
Regulatory neuroscience studies widely distributed neural and hormonal systems by which the brain influences bodily systems both to insure homeostasis and to prepare the body for particular forms of response to the environment. These neural systems provide regulatory control of breathing, cardiac function, food intake and metabolism, water intake and retention, stress responses, reactions to pain, and sexual development and activity. Regulatory neuroscience overlaps with research in developmental neuroscience with respect to hormonal influences on growth, sexual differentiation, and brain development. The class of regulatory substances called neuromodulators lies somewhere between the direct neural control of bodily systems carried out by the autonomic nervous system, and the release of hormones into the blood stream by the brain's pituitary gland. Neuromodulators are substances that act like synaptic transmitters, but which are released into extracellular space bathing large areas of the brain in order to regulate the general level of activity in specific brain systems.
Another important phenomenon studied within regulatory neuroscience is the interactions between psychological states, brain function, and the activity of the immune system (called psychoneuroimmunology). This research focuses on a number of recently discovered ways by which the neural activity that constitutes certain psychological and affective states (such as responses to stress, general levels of depression or distress, or a sense of well-being) can affect the activity level of the immune system. This area of research is beginning to explain why the belief that one is receiving a beneficial treatment has such a ubiquitous and powerful positive effect on health and recovery from illness (i.e., the placebo effect).
Behavioral and cognitive neuroscience
One of the most significant scientific trends of the latter half of the twentieth century has been the joining of cognitive science and neuroscience into a field called cognitive neuroscience. This field studies the role of various neural systems in complex forms of thought and behavior such as attention, object recognition, spatial orientation, skilled motor activity, language production and comprehension, arithmetic, music, historical (episodic) memory, and the affective-cognitive aspects of social perception.
Methodological developments were an important catalyst for this merger. During the first twothirds of the twentieth century, methods for studying brain processes contributing to more complex cognition was limited to studies of changes in the behavior of animals created by lesions made in different areas of the brain, or elicited by electrical stimulation of various brain structures. It was also possible to record electrical activity from the depths of the brain of behaving animals. Investigation of human cognition was generally limited to study of individuals with various forms of brain damage, or to brain wave recordings from the scalp.
Technical advances in the methodologies available to neuroscience have remarkably enhanced the ability of investigators to study complex human cognition. Most notable has been the development of the various forms of neuroimaging: computer assisted tomography (CAT), magnetic resonance imaging (MRI), functional MRI (fMRI), positron emission tomography (PET), and single photon emission computed tomography (SPECT). These methods provide the cognitive neuroscientist with noninvasive ways of viewing the structure of the nervous system (CAT and MRI), or the relative level of functional activity of various brain areas (fMRI, PET, and SPECT) in an alive, awake, and mentally active human being.
One example of the kinds of studies that can be done with neuroimaging methods is a 1999 study that compared PET scans taken while Italian-, French-, and English-speaking dyslexic individuals were attempting to read. This research demonstrated that, despite the fact that reading disability took somewhat different forms in the different languages, there was nevertheless a consistent area of diminished brain activity during reading (in the left temporal lobe). This study illustrates the capacity of neuroimaging to reveal, non-invasively, characteristics of human brain processing that occur during a complex cognitive task, such as reading, in normal and cognitively impaired individuals.
Much of the early history of the field of neuroscience involved the study of individuals with brain damage or brain disease. Studies of patients with epilepsy, brain tumors, traumatic brain damage, or brain diseases (e.g., multiple sclerosis, Parkinson's disease, and Alzheimer's disease) have contributed much to new ideas and theories about how the brain works.
A few individual cases in clinical neurology have been particularly influential. For example, in 1861 Paul Broca (1824–1880) described a patient who had suddenly lost his speech, but was otherwise cognitively normal. The patient has been called "Tan" in the literature, since "tan" was the only word the patient could utter. Autopsy of the brain of this patient showed a lesion of the left frontal lobe, suggesting that this frontal area, subsequently called Broca's area, was important for expressive language. Similarly, a single case referred to in the literature by the initials "H. M." made a substantial contribution to the understanding of explicit, episodic memory. In 1953, H.M. underwent neurosurgical removal of the medial temporal lobes of both hemispheres to control seizures. The result was a profound amnesia in which old memories were preserved, but the ability to form new conscious, explicit memories was permanently lost. This case focused intensive neuroscientific study on the role of the hippocampus (a structure of the medial temporal lobe) in memory formation—research that is still ongoing.
The contribution of new ideas about brain function goes not only from clinical cases to basic neuroscience, but also from neuroscience to clinical medicine. All of the areas of neuroscience described above feed critical information into areas of investigation concerned with those human diseases and disorders of the nervous system that are diagnosed and treated by neurologists, neurosurgeons, psychiatrists, neuroendocrinologists, and neuropsychologists. An example of the impact of basic neuroscience on clinical medicine is the contribution made by studies of the synapse and synaptic transmitters to the development of new drugs that affect the nervous system (neuropharmacology) and behavior (psychopharmacology).
Neuroscientific study of religious experience and moral agency
Most persons identify as most uniquely spiritual their experiences of religious ecstasy and awe—those moments when one feels most transcendent or overwhelmed by the feeling of divine presence. However, the fact that such religious experiences can accompany epileptic seizures (or drug intoxication) has been recognized since ancient times. This observation caused many ancient cultures to associate epilepsy with possession by gods or demons.
There is a significant literature in modern neurology that suggests that in some cases of temporal lobe epileptic seizures, religious experiences result from the abnormal neural activity in the temporal lobes and limbic system. Consistent with the clinical data from temporal lobe epilepsy, investigators have shown that electromagnetic stimulation of the temporal lobes increases the likelihood of experiencing a "sense of presence," leading some investigators to speculate that abnormal temporal lobe activity is the neural basis of all religious experiences.
Other investigators studying the activity of the brain during religious experiences have suggested the importance of other brain areas. Andrew Newburg and Eugene d'Aquili have argued that the sense of diminishment of self and an awareness of oneness with god or the universe that is experienced by some during transcendental meditation or some forms of prayer is associated with diminished activity in the parietal lobes, rather than increased activity in the temporal lobes. These investigators interpret these results as indicating a neural correlate of an absence of the sense of self and the achievement of a sense of "absolute unitary consciousness."
These studies of brain activity during religious experiences at least make it clear that religious experiences (whether feelings of ecstasy, awe, or oneness) have correlates in brain functional states. What is as yet unclear is whether these functional brain states are unique to religious experiences or also occur in similar situations that the person would not report as religious. Is the religious attribution to the experience being studied a matter of the context in which the state occurs, or rather a matter of the particular brain state? Nevertheless, over the last two decades of the twentieth century, there has been increasing interest in the neuroscientific study of religious experiences, such that a new field has taken shape that is being called neurotheology.
There has also been considerable neuroscientific study of the processes involved in moral decision-making and moral behavior. A long history of cases from clinical neurology has pointed to the important role of the medial frontal cortex in inter-personally responsible action and moral behavior. Important work by Antonio Damasio has strongly suggested that deficits in these areas involve absence of the unconscious elicitation of negative and positive emotions in relationship to contemplated behaviors, and that the medial frontal cortex is important in triggering emotional reactions to contemplated actions. In a similar vein, fMRI studies of persons attempting to solve moral dilemmas have suggested that areas of the brain involved in emotion are activated to the degree that the particular moral dilemma would demand direct action toward another person.
Philosophy of neuroscience
As is evident in what is described above, neuroscience as a field is committed, to a greater or lesser degree, to four basic philosophical positions: empiricism, physicalism, reductionism, and determinism. Like all science, neuroscience is empiricist in attempting to learn what is true through systematic observations and experimentation. However, neuroscientists might differ regarding whether empiricism is the only contributor to knowledge of truth. Physicalism maintains that human (or animal) mind and behavior are the product of the physical activity of the nervous system. While some neuroscientists might have an extra-scientific commitment to body-soul or mind-body dualism, neuroscientific theory and research would not admit the concept of any nonmaterial entity. Reductionism refers to at least two different positions. Methodological reductionism is merely the idea of breaking more complex things into parts and studying the parts, such as studying changes in synaptic efficiency as a part of what happens in the brain during learning. Causal (or explanatory) reductionism presumes that the causes of any particular mental or behavioral event can be found in more and more elementary mechanisms, such that eventually all mental activity is explainable in a bottom-up manner by chemistry and physics. Some neuroscientists have begun to adopt the concept of non-reductive (or top-down) causal principles emerging within complex systems such as the brain, thus loosening the grip of causal reductionism on neuroscience. However, methodological reductionism is still a predominant principle within the field. Finally, determinism suggests that the physical state of the brain at one point in time is entirely determinative of the immediate future activity of the brain. Certainly, much research in the neurosciences proceeds as if current brain activity is predictive of future brain (mental) activity. However, neuroscientists differ with respect to the question of existence of conscious agency or free will. There is, as yet, no generally accepted theory as to how conscious agency might emerge from brain activity, or how such agency would create a nonreducible causal influence on the processes studied by molecular neuroscience.
See also Aristotle; Consciousness Studies; Determinism; Experience, Religious: Cognitive and Neurophysiological Aspects; Experience, Religious: Philosophical Aspects; Mind-body Theories; Mind-brain Interaction; Neural Darwinism; Neurophysiology; Neuropsychology; Neurotheology; Physicalism, Reductive and Nonreductive; Placebo Effect; Psychology
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