A-B-A design Experimental design in which participants first experience the baseline condition (A), then experience the experimental treatment (B), and then return to the baseline (A).
Abnormal psychology The area of psychological investigation concerned with understanding the nature of individual pathologies of mind, mood, and behavior.
Absolute threshold The minimum amount of physical energy needed to produce a reliable sensory experience; operationally defined as the stimulus level at which a sensory signal is detected half the time.
Accommodation According to Piaget, the process of restructuring or modifying cognitive structures so that new information can fit into them more easily; this process works in tandem with assimilation.
Acquisition The stage in a classical conditioning experiment during which the conditioned response is first elicited by the conditioned stimulus.
Action potential The nerve impulse activated in a neuron that travels down the axon and causes neurotransmitters to be released into a synapse.
Acute stress A transient state of arousal with typically clear onset and offset patterns.
Alzheimer's disease A chronic organic brain syndrome characterized by gradual loss of memory, decline in intellectual ability, and deterioration of personality.
Amacrine cells Cells that integrate information across the retina; rather than sending signals toward the brain, amacrine cells link bipolar cells to other bipolar cells and ganglion cells to other ganglion cells.
Ambiguity A perceptual object that may have more than "one interpretation.
Amnesia A failure of memory caused by physical injury, disease, drug use, or psychological trauma.
Addiction A condition in which the body requires a drug in order to function without physical and psychological reactions to its absence; often the outcome of tolerance and dependence.
Ageism Prejudice against older people, similar to racism and sexism in its negative stereotypes.
Aggression Behaviors that cause psychological or physical harm to another individual.