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validity The extent to which a test measures what it is supposed to measure. See also predictive validity.
validity scales Scales used within a measure (e.g., a personality test) designed to assess whether the examinee is trying to disguise their traits or their attitudes.
variability The degree to which scores in a frequency distribution depart from the central value. See also central tendency, measure of central tendency, standard deviation (SD), variance (V).
variable-interval (VI) schedule A pattern of rewards similar to a fixed-interval schedule, but with the interval used varying from reward to reward. Thus, on a variable-interval 1 schedule, reward will be available on average after one minute has gone by, but for some rewards the actual interval might be shorter or longer.
variable-ratio (VR) schedule A pattern of rewards similar to a fixed-ratio schedule, but with the ratio used varying from reward to reward. Thus, on a variable-ratio 5 schedule, reward will be available on average after five responses, but for some rewards the number of required responses might be fewer, and for others more.
variance (V) A measure of the variability of a frequency distribution. It is computed by finding the difference between each score and the mean (M), squaring the result, adding all these squared deviations, and dividing the sum by the number of cases. If N is the number of scores, then V = sum of (score – M)2/ N.
vasocongestion A contraction of the capillaries that squeezes blood away from that area. See also vasoconstriction.
vasoconstriction The constriction of blood vessels brought on by activation of the sympathetic branch of the autonomic nervous system. Vasoconstriction occurs in emergencies, when blood is diverted from the skin and internal organs to the muscles. It is also crucial in mammalian thermoregulation; in response to excessive cold, blood is diverted from the skin to reduce heat loss. See also vasodilatation.
vasodilatation The dilating of blood vessels brought on by activation of the parasympathetic division of the autonomic nervous system. Vasodilatation is one component of mammalian thermoregulation; in response to excessive heat, warm blood flows to the body's surface and results in heat loss by radiation. See also vasoconstriction.
vasovagal reaction A reaction from the parasympathetic branch of the nervous system in which the pulse slows, blood pressure drops, the muscles go slack, and the person can fall to the ground in a dead faint.
vegetative signs Physical manifestations that often accompany major depression, such as loss of appetite and weight loss, weakness, fatigue, poor bowel functioning, sleep disorders (most often early-morning awakenings), and loss of interest in sex.
ventral tegmental area (VTA) A region in the midbrain containing dopamine-releasing pathways thought to be involved in reward.
vestibular senses A set of receptors that provide information about the orientation and movements of the head, located in the semicircular canals and the vestibular sacs of the inner ear.
VI schedule See variable-interval (VI) schedule.
visible spectrum The range of wavelengths to which our visual system can respond, extending from about 400 (the wavelength usually perceived as the color violet) to 750 nanometers (the wavelength usually perceived as the color reddish orange).
visual imagery The capacity to form and use quasi-perceptual representations, often referred to as mental pictures, in the absence of the relevant visual input.
visual pigment The chemical inside a photoreceptor that, when exposed to light, changes form, thus releasing some energy and triggering a neural impulse. The chemical is then restored to its original form so that it becomes ready to respond to the next bit of incoming light.
visual search task A task in which observers are asked to hunt for a specified target within a field of stimuli. The identity of the target can be defined in various ways ("look for the red item," or "look for the green letter H"), and the background items can vary both in number and in identity.
visual segregation The step in perception that involves locating an object's boundary, so that the perceiver can discern where one object stops and the next begins.
VR schedule See variable-ratio (VR) schedule.
VTA See ventral tegmental area.
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