Classical Psychophysical Theory
The application of the methods described in Chapter 3 yields a quantity which is expressed in physical units and called the threshold. The concept of the threshold as an index of absolute and differential sensitivity has been extremely useful in the study of sensory systems. Through the use of this quantity, investigators have been able to discover the stimuli to which our sensory systems are the most and the least sensitive. But psychophysicists have not restricted their work to this descriptive level: they have also gone beyond this level to propose theories concerning the underlying mechanisms of sensory thresholds. Each theory was proposed to account for empirical data obtained in psychophysical experiments. The theorists hoped to describe the neurophysiological or psychological processes within the observer which may have determined the observer's behavior. The validity of each theory must be evaluated by determining the degree to which precise quantitative deductions from the theory are confirmed by experimental data.
Early threshold theories were based upon the assumption that the measurements obtained in psychophysical experiments were estimates of a neural threshold in the observer which could not be measured directly. It was thought that the threshold was a sharp transition point between sensation and no sensation, and that a specific, critical amount of neural activity must result from stimulation for the threshold to be exceeded. The