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basal ganglia In the extrapyramidal motor system, a set of subcortical structures in the cerebrum that send messages to the spinal cord through the midbrain to modulate various motor functions.
basal metabolic rate The speed at which organisms ordinarily "burn" food to maintain themselves; this rate is higher for endotherms than for ectotherms.
basic emotions According to some theorists, a small set of elemental, built-in emotions revealed by distinctive patterns of physiological reaction and facial expression.
basilar membrane See cochlea.
behavioral contrast A pattern of responding in which an organism seems to evaluate a reward relative to other rewards that are available or that have been available recently. For example, an animal might respond only weakly to a reward of two pellets if it recently received a reward of five pellets for some other response.
belongingness in learning The fact that the ease with which associations are formed depends on the items to be associated. This holds for classical conditioning in which some conditioned stimulus-unconditioned stimulus combinations are more effective than others (e.g., learned taste aversions) and for instrumental conditioning in which some response-reinforcer combinations work more easily than others (e.g., specific defense reactions in avoidance conditioning of species). See also biological constraints, equipotentiality.
benzodiazepines A class of medications used to combat anxiety; the class includes Valium, Xanax, and Klonopin.
beta rhythm A rhythmic pattern in the electrical activity of the brain, often observed when one is engaged in active thought.
between-subject comparisons Within an experiment, comparing one group of individuals to a different group; usually contrasted with within-subject comparisons.
Big Five A nickname often used to refer to five apparently crucial dimensions of personality: extroversion, neuroticism (or emotional instability), agreeableness, conscientiousness, and openness to experience. These five traits often emerge from factor analyses of trait terms.
binding problem The problem confronted by the brain of recombining the elements of a stimulus, once these elements have been separately analyzed by different neural systems.
binocular disparity An important cue for depth perception. Each eye obtains a different view of an object, the disparity becoming less pronounced the farther the object is from the observer.
biological constraints Principles governing what each species can learn easily and what it cannot learn at all. See also belongingness in learning.
biomedical model An approach to mental disorders that emphasizes somatogenic causes.
bipolar cells The intermediate neural cells in the eye that are stimulated by the receptors and excite the ganglion cells.
bipolar disorder A mood disorder in which the patient swings from one emotional extreme to another, experiencing both manic and depressive episodes. Formerly called manic-depressive psychosis.
bisexuality A sexual orientation in which a person has erotic and romantic feelings for both their own and the opposite sex.
blindsight The ability of a person with a lesion in the visual cortex to reach toward or guess at the orientation of objects projected on the part of the visual field that corresponds to this lesion, even though they report that they can see absolutely nothing in that part of their visual field.
blind spot The region of the retina that contains no visual receptors and therefore cannot produce visual sensations.
blocking effect An effect produced when two conditioned stimuli, A and B, are both presented together with the unconditioned stimulus (US). If stimulus A has previously been associated with the unconditioned stimulus while B has not, the formation of an association between stimulus B and the US will be impaired (i.e., blocked).
blood-brain barrier Specialized membranes that surround the blood vessels within the brain and that filter toxins and other harmful chemicals, ensuring brain cells a relatively pure blood supply.
bottom-up processes See top-down processes.
brightness A perceived dimension of visual stimuli — the extent to which they appear light or dark.
brightness contrast The perceiver's tendency to exaggerate the physical difference in the light intensities of two adjacent regions. As a result, a gray patch looks brighter on a black background, darker on a white background.
Broca's area A brain area in the frontal lobe crucial for language production. See also aphasia.
bulimia An eating disorder characterized by repeated binge-and-purge bouts. In contrast to anorexics, bulimics tend to be of roughly normal weight. See also anorexia nervosa.
bystander effect The phenomenon that underlies many examples of failing to help strangers in distress: the larger the group a person is in (or thinks he is in), the less likely he is to come to a stranger's assistance. One reason is diffusion of responsibility (no one thinks it is his responsibility to act).
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