Senses: The absolute and difference thresholds
What is the softest sound you can hear, the dimmest light you can see, the most diluted substance you can taste? Researchers in sensory psychology have performed many experiments over the years to answer these questions. Their research has established measures for the senses known as absolute thresholds. just as the threshold of a doorway is the dividing point between being outside a room and inside, the absolute threshold of a sense marks the difference between not being able to perceive a stimulus and being just barely able to perceive it. Psychologists have arbitrarily defined this absolute threshold as the minimum amount of sensory stimulation that can be detected 50% of the time.
If you are listening to music, the very fact that you can hear it means that the absolute threshold has been crossed. But how much must the volume be turned up or down for you to notice the difference? Or, if you are carrying some bags of groceries, how much weight must be added or taken away for you to be able to sense that your load is heavier or lighter? The difference threshold is a measure of the smallest increase or decrease in a physical stimulus that is required to produce the just noticeable difference (JND). The JND is the smallest change in sensation that a person is able to detect 50% of the time. If you were holding a 5-pound weight and 1 pound were added, you could easily notice the difference. But if you were holding 100 pounds and 1 additional pound were added, you could not sense the difference. Why not?
More than 150 years ago, researcher Ernst Weber (1785-1878) observed that the JND for all the senses depends on a proportion or percentage of change in a stimulus rather than on a fixed amount of change. This observation became known as Weber´s law. A weight you are holding must increase or decrease by 1/50, or 2%, for you to notice the difference; in contrast, if you were listening to music, you would notice a difference if a tone became slightly higher or lower in pitch by about only 0.33%. According to Weber´s law, the greater the original stimulus, the more it must be increased or decreased for the difference to be noticeable.
As you might suspect, the difference threshold is not the same for all the senses.