Absolute refractory period -  The period during which a neuron lies dormant after an action potential has been completed.

Absolute threshold -  The minimum amount of stimulation needed for a person to detect the stimulus 50 percent of the time.

Accommodation -  The process by which the shape of an eye’s lens adjusts to focus light from objects nearby or far away. Also: the modification of a schema as new information is incorporated.

Acetylcholine -  A neurotransmitter involved in muscle movement, attention, arousal, memory, and emotion.

Achievement motive -  An impulse to master challenges and reach a high standard of excellence.

Achievement tests -  An assessment that measures skills and knowledge that people have already learned.

Acronym -  A word made out of the first letters of several words.

Acrostic -  A sentence or phrase in which each word begins with a letter that acts as a memory cue.

Action potential -  A short-lived change in electric charge inside a neuron.

Activation-synthesis theory -  A theory proposing that neurons in the brain activate randomly during REM sleep.

Active listening -  A feature of client-centered th erapy that involves empathetic listening, by which the therapist echoes, restates, and clarifies what the client says.

Adaptation -  An inherited characteristic that increases in a population because it provides a survival or reproductive advantage.

Adaptive behaviors -  Behaviors that increase reproductive success.

Additive strategy -  The process of listing the attributes of each element of a decision, weighing them according to importance, adding them up, and determining which one is more appealing based on the result.

Adoption studies -  Studies in which researchers examine trait similarities between adopted children and their biological and adoptive parents to figure out whether that trait might be inherited.

Adrenal cortex -  The outer part of the adrenal glands, which secretes corticosteroids.

Adrenal medulla -  The inner part of the adrenal glands, which secretes catecholamines.

Adrenocorticotropic hormone (ACTH) -  A hormone released by the pituitary gland that stimulates release of corticosteroids from the adrenal cortex.

Afferent nerves -  Bundles of axons that carry information from muscles and sense organs to the central nervous system.

Afterimage -  A color we perceive after another color is removed.

Age of viability -  The point at which a fetus has some chance of surviving outside the mother if born prematurely.

Agonists -  Chemicals that mimic the action of a particular neurotransmitter.

Agoraphobia -  A disorder involving anxiety about situations from which escape would be difficult or embarrassing or places where there might be no help if a panic attack occurred.

Algorithm -  A step-by-step procedure that is guaranteed to solve a problem.

All-or-none law -  States that neurons fire to generate an action potential only if stimulation reaches a minimum threshold.

Alpha waves -  Type of brain waves present when a person is very relaxed or meditating.

Alternate-forms reliability -  The ability of a test to produce the same results when two different versions of it are given to the same group of people.

Ambiguous language -  Language that can be understood in several ways.

Amplitude -  The height of a wave.

Amygdala -  A part of the limbic system of the brain that is involved in regulating aggression and emotions, particularly fear.

Animism -  The belief that inanimate objects are alive.

Anorexia nervosa -  A disorder characterized by refusal to maintain a body weight in the normal range, intense fear about gaining weight, and highly distorted body image.

Antagonists -  Chemicals that block the action of a particular neurotransmitter.

Anterograde amnesia -  An inability to remember events that occurred after a brain injury or traumatic event.

Antisocial personality disorder -  A disorder characterized by a lack of conscience and lack of respect for other people’s rights, feelings, and needs, beginning by age fifteen.

Appraisal -  The process of evaluating an environmental challenge to determine whether resources are available for dealing with it.

Approach-approach conflict -  A conflict between two desirable alternatives.

Approach-avoidance conflict -  A conflict that arises when a situation has both positive and negative features.

Aptitude tests -  An assessment that predicts people’s future ability to acquire skills or knowledge.

Archetypes -  Images or thoughts that have the same meaning for all human beings.

Assimilation -  The broadening of an existing schema to include new information.

Atherosclerosis -  Hardening of arteries because of cholesterol deposits.

Attachment -  The close bond between babies and their caregivers.

Attachment styles -  Types of attachment, which include secure attachment, anxious-ambivalent attachment, and avoidant attachment.

Attitudes -  Evaluations people make about objects, ideas, events, or other people.

Attributions -  Inferences people make about the causes of events and behavior.

Atypical antipsychotic drugs -  A new class of antipsychotic drugs that are effective for treating negative and positive symptoms of schizophrenia. They target the neurotransmitters serotonin and dopamine.

Auditory nerve -  A nerve that sends impulses from the ear to the brain.

Automatic thoughts -  Self-defeating judgments people make about themselves.

Autonomic nervous system -  The part of the peripheral nervous system connected to the heart, blood vessels, glands, and smooth muscles.

Availability heuristic -  A rule-of-thumb strategy in which people estimate probability based on how quickly they remember relevant instances of an event.

Avoidance-avoidance conflict -  A conflict that arises when a choice must be made between two undesirable alternatives.

Avoidant personality disorder -  A disorder involving social withdrawal, low self-esteem, and extreme sensitivity to being evaluated negatively.

Aversion therapy -  A therapy in which a stimulus that evokes an unpleasant response is paired with a stimulus that evokes a maladaptive behavior.

Axon -  A fiber that extends from a neuron and sends signals to other neurons.