9.3 Differential sensitivity
Absolute thresholds represent only one type of threshold; one could also ask whether the subject can detect a difference between two stimuli. The threshold for detection of difference is called a difference threshold or difference limen (DL). The difference threshold is a measure of the smallest detectable difference between two stimuli. Basically it answers the psychophysical question: ‘How different must two stimuli (e.g. two weights, two colours, two sounds) be from each other in order to detect them as different stimuli?’
The difference threshold, like the absolute threshold described in the beginning of this section, is a derived statistical measure; it is the difference in magnitude between two stimuli, usually a standard (S) and a comparison stimulus (T), that is detected 50 per cent of the time. For example, if two tones of the same intensity are presented to a listener, the listener will generally report that they are equal in loudness. However, as the intensity of one of the tones is gradually increased, an intensity difference between the tones will be reached at which they will be judged different in 50 per cent of trials. The magnitude of this difference specifies the difference threshold; that is, the amount of change in a stimulus necessary to produce a just noticeable difference (JND) in sensation. If the magnitude of a stimulus, say a sound, is 100 dB SPL, and the sound has to be increased to 110 dB SPL in order to be perceived as different, then the JND equals 10 dB.
The measurement of a JND can be made using any of the classical methods discussed above. A psychometric function similar to that shown in Figure 33 is generated except that the horizontal axis is ΔI instead of I. ΔI is the increment in intensity that, when added to the stimulus intensity I, produces a JND or the smallest detectable increment. From the psychometric function, a threshold value, the difference threshold, can be deduced. This difference threshold is the ΔI appropriate to the standard (S) used. For a different S, a different psychometric function is generated, and a different ΔI derived. By measuring the ΔI for a large number of standards you can develop a function that describes how the JND changes for different levels of stimulation.