tween its base and the horizon can indicate what size an object is and can be the basis for size constancy. Size constancy refers to perceiving an object of a certain size, such as a six-foot-tall man, as being six feet tall regardless of whether he is close to the viewer or far away from the viewer.
Another invariant is time-to-contact. People and animals
can accurately judge when they will have contact with an-
other, but they are not always accurate at predicting speed. According to a 1981 study by researchers Davis Lee and Paul Reddish, birds fold their wings at a precise time just before reaching the water. However, in a 1990 study, William Schif and Rivka Oldak found that men and women judge time to
arrival diferently—fndings few people would likely dispute. A diference threshold examines how diferent two stimuli must be before an animal can detect that they are diferent at least 50 percent of the time. For example, a person might feel two weights and be asked to indicate whether they are the same or diferent. Te minimum amount of weight necessary in order to perceive a difer- ence 50 percent of the time is known as the diference threshold or the just notice- able diference (JND).
In the nineteenth century, psychologist Ernst Weber developed a law for all stimuli relating physical intensity to magnitude perception. It stated that the change in the intensity of a stimulus (for example, from a quieter to louder sound) divided by the intensity of the stimulus (the loudness of a sound) equals a constant. Later, Gustav Fechner modifed Weber’s law slightly by expressing the relationships in terms of logarithms. As a result, the law is sometimes called the Weber–Fechner law. dp = k dS
In the equation, dp refers to the diferential change in perception, dS refers to the diferential increase in stimulus, S refers to the stimulus, and k is the constant factor. Experiments have...