Fear: The affective response to an actual current external danger which subsides with the elimination of the threatening condition.Conditioning, Classical: Learning that takes place when a conditioned stimulus is paired with an unconditioned stimulus.Freezing Reaction, Cataleptic: An induced response to threatening stimuli characterized by the cessation of body movements, except for those that are involved with BREATHING, and the maintenance of an immobile POSTURE.Conditioning (Psychology): A general term referring to the learning of some particular response.Amygdala: Almond-shaped group of basal nuclei anterior to the INFERIOR HORN OF THE LATERAL VENTRICLE of the TEMPORAL LOBE. The amygdala is part of the limbic system.Extinction, Psychological: The procedure of presenting the conditioned stimulus without REINFORCEMENT to an organism previously conditioned. It refers also to the diminution of a conditioned response resulting from this procedure.Electroshock: Induction of a stress reaction in experimental subjects by means of an electrical shock; applies to either convulsive or non-convulsive states.Galvanic Skin Response: A change in electrical resistance of the skin, occurring in emotion and in certain other conditions.Startle Reaction: A complex involuntary response to an unexpected strong stimulus usually auditory in nature.Memory: Complex mental function having four distinct phases: (1) memorizing or learning, (2) retention, (3) recall, and (4) recognition. Clinically, it is usually subdivided into immediate, recent, and remote memory.Avoidance Learning: A response to a cue that is instrumental in avoiding a noxious experience.Association Learning: The principle that items experienced together enter into a connection, so that one tends to reinstate the other.Phobic Disorders: Anxiety disorders in which the essential feature is persistent and irrational fear of a specific object, activity, or situation that the individual feels compelled to avoid. The individual recognizes the fear as excessive or unreasonable.Behavior, Animal: The observable response an animal makes to any situation.Anxiety: Feeling or emotion of dread, apprehension, and impending disaster but not disabling as with ANXIETY DISORDERS.Acoustic Stimulation: Use of sound to elicit a response in the nervous system.Dental Anxiety: Abnormal fear or dread of visiting the dentist for preventive care or therapy and unwarranted anxiety over dental procedures.Cues: Signals for an action; that specific portion of a perceptual field or pattern of stimuli to which a subject has learned to respond.Rats, Long-Evans: An outbred strain of rats developed in 1915 by crossing several Wistar Institute white females with a wild gray male. Inbred strains have been derived from this original outbred strain, including Long-Evans cinnamon rats (RATS, INBRED LEC) and Otsuka-Long-Evans-Tokushima Fatty rats (RATS, INBRED OLETF), which are models for Wilson's disease and non-insulin dependent diabetes mellitus, respectively.Cycloserine: Antibiotic substance produced by Streptomyces garyphalus.Learning: Relatively permanent change in behavior that is the result of past experience or practice. The concept includes the acquisition of knowledge.Analysis of Variance: A statistical technique that isolates and assesses the contributions of categorical independent variables to variation in the mean of a continuous dependent variable.Hippocampus: A curved elevation of GRAY MATTER extending the entire length of the floor of the TEMPORAL HORN of the LATERAL VENTRICLE (see also TEMPORAL LOBE). The hippocampus proper, subiculum, and DENTATE GYRUS constitute the hippocampal formation. Sometimes authors include the ENTORHINAL CORTEX in the hippocampal formation.Desensitization, Psychologic: A behavior therapy technique in which deep muscle relaxation is used to inhibit the effects of graded anxiety-evoking stimuli.Facial Expression: Observable changes of expression in the face in response to emotional stimuli.