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The use of behavioral methods to measure the sensory capacities of animals and nonverbal humans such as infants. The observations are analogous to those obtained from adult human subjects who are asked to observe appropriate stimuli and report what they see. The behavioral methods differ primarily in that stimulus-response associations are established by means other than verbal instructions, either as unlearned reflexes or through conditioning. Any sense or species may be studied, but most work has been done on the vision and hearing of primates, cats, pigeons, and rats. Typical investigations determine (1) the absolute threshold (the minimum intensity needed to elicit a standard response); (2) the difference threshold (the minimum change in a stimulus needed to elicit a standard response); and (3) points of apparent equality (values of stimuli that elicit no response because a change in one aspect compensates for a change in another). A few investigations have determined stimulus scales that express quantitative relations between the physical stimulus and the perceptual effect over a range of stimulus values. These various measures provide a picture of the sensory function, such as visual acuity, color sensitivity, loudness or pitch perception, and odor discrimination. Efficiency and sensitivity to the sensory function of interest are the major factors that govern the choice of method in behavioral psychophysics.
Reflex methods are the most convenient since no training is required. An example is the preferential looking response in infants. Without training, infants spend more time looking at patterned stimuli than at blank fields. Two stimulus fields are placed before the infant, and the relative time spent looking at each is determined. A preference for one pattern according to this measure indicates that the infant can detect a difference between the two.
Unconditioned reflex methods are limited to sensory functions for which appropriate reflexes may be found; also, they usually impose severe limits on the specific stimulus conditions that may be used. Conditioning methods add considerable flexibility. In Pavlovian conditioning, the stimulus of interest becomes the conditioned stimulus through its association with a stimulus that elicits a clear-cut reflexive response (the unconditioned stimulus).
Operant conditioning offers still more flexibility. Typically the subject is rewarded for making an indicator response in the presence of one stimulus value; reward is withheld (or another response rewarded) in the presence of other stimulus values. The responses are selected to suit the species and the stimulus under study.
Interest in the development of sensory function has spurred the use of behavioral methods in infants and young children, while the effects of controlled sensory input during development have been extensively monitored in nonhuman subjects. Applications to the prevention and control of sensory disorders also are increasing. For example, a number of toxicants, drugs, and environmental stressors have been related to sensory disorders; the methods of behavioral psychophysics are used to follow the development of these disorders under controlled conditions, and to uncover potential preventive and therapeutic measures. See Psychology, Sensation