Understanding Psychology (10th Ed)

102 Chapter 
4 
Sensation and Perception
a change in stimulation has occurred. Thus, the difference threshold is the minimum 
change in stimulation required to detect the difference between two stimuli, and so 
it also is called a
 just noticeable difference 
(Nittrouer & Lowenstein, 2007).
The size of a stimulus that constitutes a just noticeable difference depends on 
the initial intensity of the stimulus. The relationship between changes in the original 
size of a stimulus and the degree to which a change will be noticed forms one of the 
basic laws of psychophysics: Weber’s law.
 Weber’s law 
( Weber is pronounced “VAY-
ber”) states that a just noticeable difference is a constant proportion of the intensity of 
an initial stimulus (rather than a constant amount).
For example, Weber found that the just noticeable difference for weight is 1:50. 
Consequently, it takes a 1-ounce increase in a 50-ounce weight to produce a notice-
able difference, and it would take a 10-ounce increase to produce a noticeable differ-
ence if the initial weight were 500 ounces. In both cases, the same proportional 
increase is necessary to produce a just noticeable difference—1:5 = 10:500. Similarly, 
the just noticeable difference distinguishing changes in loudness between sounds is 
larger for sounds that are initially loud than it is for sounds that are initially soft, 
but the proportional increase remains the same. 
Weber’s law helps explain why a person in a quiet room is more startled by the 
ringing of a telephone than is a person in an already noisy room. To produce the 
same amount of reaction in a noisy room, a telephone ring might have to approxi-
mate the loudness of cathedral bells. Similarly, when the moon is visible during the 
late afternoon, it appears relatively dim—yet against a dark night sky, it seems quite 
bright.
Sensory Adaptation: 
Turning Down Our Responses
You enter a movie theater, and the smell of popcorn is everywhere. A few minutes 
later, though, you barely notice the smell. The reason you become accustomed to the 
odor is sensory adaptation.