an experimental method for assessing the perceptual capabilities of nonverbal individuals (e.g., human infants, nonhuman animals). Infants will preferentially fixate a “more interesting” stimulus when it is presented at the same time with a “less interesting” stimulus, but only if the stimuli can be distinguished from one another. To minimize bias, on each trial the investigator is positioned so that he or she can observe the infant and make a judgment about which stimulus the infant fixates, but the stimuli themselves are visible only to the infant. To assess visual acuity, for example, on the first trial a coarse acuity grating is paired with a homogeneous gray stimulus of the same mean luminance. The infant preferentially looks at the grating. On successive trials, the spatial frequency of the grating is increased (the bars are made narrower) and the position of the grating versus the homogeneous field is randomized. When
the grating can no longer be discriminated by the infant, the likelihood that the grating will be chosen for fixation will drop to chance. [developed by Robert L. Fantz]