a technique for evaluating the perceptual abilities of non-speaking human infants and animals. Infants will preferentially focus a more interesting stimulant whenever it is shown simultaneously to a less interesting stimulant, but only if the stimulants can be differentiated from each other. To limit bias, on every trial the investigator is positioned so that they can view the infant and make a decision about which stimulant the infant focuses on, but the stimulants themselves are observable only to the infant.