A group of psychiatric conditions, also known as mood disorders, that are characterized by disturbances of affect, emotion, thinking, and behavior. Affective or mood disorders are strong mental disturbances that interfere with daily life (Fig. 1). Depression is the most common of these disorders. About 10–20% of those affected by depression also experience manic episodes; hence, this condition is known as manic depression or bipolar affective disorder. The affective disorders are not distinct diseases; instead, they are psychiatric syndromes that probably have multiple or complex etiologies. See also: Bipolar disorder; Depression; Emotion; Mental disorders; Stress (psychology); Stress and depression

Any disorder of recognition. Agnosia is a rare neurological condition in which an individual fails to recognize and identify persons or objects (see illustration). Typically, agnosia is the result of damage to specific areas of the brain. Historically, agnosias were named by Sigmund Freud in 1891 to denote disturbances in the ability to recognize and name objects. Usually, the misrecognition involves one sensory modality, with primary sensation in that modality intact, and the ability to name the item after its presentation via another sensory modality; for example, the agnostic patient cannot identify a bell by sight, but can recognize it by its sound or touch. Brenda Milner and Hans-Lukas Teuber in 1968 defined agnosia as a "normal percept . . . stripped of its meaning." Oliver Sacks described a patient who failed to recognize his wife's face, mistaking it for a hat. See also: Brain; Nervous system (vertebrate); Neurobiology; Sensation

A significant, yet relatively selective loss of memory. Amnesia is a type of memory loss, often resulting from brain disease or injury (see illustration). The inability to remember can be temporary or permanent. In general, amnesia can be characterized along two dimensions with respect to its onset: an inability to remember events that occurred after the onset of amnesia is referred to as anterograde amnesia, and a deficit in remembering events that occurred prior to the onset of amnesia is referred to as retrograde amnesia. Amnesia can be due to a variety of causes, or etiologies, and it can be classified according to whether the cause is primarily neurological or psychological in origin. See also: Brain; Memory; Nervous system (vertebrate); Neurobiology; Psychology

A heterogeneous affective disorder involving a depressed mood and/or a loss of interest in daily activities. The most common form of affective disorder is depression (also termed clinical depression) [Fig. 1]. A major depressive episode is defined by a pervasively depressed or low mood (experienced most of the day over a period of two weeks or longer) and at least four associated symptoms affecting sleep, appetite, hedonic (pleasure) capacity, interest, and behavior. Major depressive episodes have several clinical forms. The lifetime rates of affective disorders are increasing, with an earlier age of onset in more recent generations. The onset of major depression most often occurs in individuals in their late 20s to mid-30s. See also: Affective disorders; Mental disorders

An integrative field of research that analyzes brain activity while experimental subjects make decisions. Neuroeconomics is a consilient discipline (that is, a discipline that combines the principles of other disciplines to produce a comprehensive analysis) that applies neuroscientific tools and methodologies to understand the decision making that occurs inside the brain (see illustration). Economics is the study of constrained decision making, and it uses both mathematical and statistical models of the decision goals and outcomes without considering the mechanisms leading to decisions. Neuroscience has focused primarily on cataloging mechanisms without considering the purpose of decisions. For this reason, neuroeconomics is a natural combination that draws from the best of, and extends, both fields. In addition, because the brains of all animals are economic, that is, they have limited resources to achieve necessary goals, neuroeconomic experiments are not limited to studies of human beings, but have also employed apes, monkeys, and rodents. See also: Brain; Decision analysis; Decision theory; Nervous system (vertebrate); Neurobiology

The study of the interactions among behavioral, neural and endocrine, and immune functions. It is perhaps the most recent convergence of disciplines that has evolved to achieve a more complete understanding of adaptive processes. At one time, the immune system was considered an independent agency of defense that protected the organism against foreign material (that is, proteins that were not part of one's “self”). Indeed, the immune system is capable of considerable self-regulation. However, converging data from the behavioral and brain sciences indicate that the brain plays a critical role in the regulation or modulation of immunity. This research indicates that the nervous and immune systems, the two most complex systems that have evolved for the maintenance of homeostasis, represent an integrated mechanism for the adaptation of the individual and the species. Thus, psychoneuroimmunology emphasizes the study of the functional significance of the relationship between these systems—not in place of, but in addition to, the more traditional analysis of the mechanisms governing the functions within a single system—and the significance of these interactions for health and disease. See also: Neuroimmunology