The nervous system receives input through an array of sense organs (for example, the eye, ear, or nose) and transforms the information into neural processes through a procedure called sensation. (Using the computer analogy, sensation can be compared to computer input.) Each sensory system follows similar principles for the conversion of a physical stimulus into a psychological experience.
Receptors for each sensory system are limited by the amount of stimulation necessary to elicit a sensation and by the amount of stimulus change that can be detected. In the field of psychology called psychophysics, relationships between physical stimuli and psychological experience are studied. One technique to study such relationships is called the method of constant stimuli, in which stimuli of varying intensities are presented in random order to a subject. The results are used to determine the absolute threshold—the minimum intensity detected by a subject 50% of the time. (Your dog, for instance, has a much lower absolute threshold for sound than you do and hears a car in the driveway before you hear the knock on the door.)
The difference threshold—the minimum (physical) distinction between stimulus attributes that can be detected 50% of the time—is also of concern. The difference threshold is also called a just noticeable difference (JND).
Ernst Weber, a well‐known early investigator, observed that regardless of their magnitude, two stimuli must differ by a constant proportion for their difference to be detectable. His observations are formulated as Weber’s law, which states that the “just noticeable difference” is a constant fraction of the stimulus intensity already present. (If a room is quiet, you can hear a faint knock at the door. But if your CD player is blaring, it takes a loud bang on the door for you to hear it.) If you are exposed to a stimulus that doesn’t change over a period of time, sensory adaptation occurs, and you become less sensitive to the stimulus. If you have to study in a room with a constant noise outside, for example, you will usually eventually adapt to the noise, and it will become less offensive.
The first systematic studies of sensory thresholds were conducted by physiologist
Ernst Weber at the University of Leipsig in Leipsig, Germany, the same university where Wilhelm Wundt would later transform psychology into an experimental science. Weber’s experiments were designed to determine sensory thresholds, of which there are two types:
- Absolute threshold — the minumum intensity of a stimulus that one can detect
- Difference threshold — the minimum difference in intensity between two stimuli that one can detect.
The Absolute Threshold
If you’ve ever had a standard hearing test, you’ve experienced the testing for absolute thresholds. Typically this involves listening to various pitches of tone through earphones. You are given a button to hold and are told to press the button until you hear a tone, then release the button until the tone fades away, then press the button until you hear it again, and so on. The intensity at which you “lose” and regain the tone is your absolute threshold for that particular tone.
Weber defined the absolute threshold as the intensity at which the stimulus was detected on 50% of trials. A stimulus detected on, say, 20% of trials is by this definition below the absolute threshold for detection, although of course it is sometimes detected nevertheless. Such a stimulus is termed subliminal (below threshold; the German word for threshold is limen).
The Difference Threshold
As with the absolute threshold, Weber defined the difference threshold statistically. Starting with a standard stimulus intensity, one can increase or decrease the intensity until one can just barely tell that changed (comparison) stimulus is either more intense or less intense than the standard. The lower difference threshold is reached when the comparison stimulus is judged to be more intense than the standard on 25% of trials. The upper difference threshold is reached when the comparison stimulus is judged to be more intense than the standard on 75% of trials. The difference threshold is the average of the two differences between the comparison stimuli and the standard.
Signal detection. Factors other than the magnitude of the stimulus also affect sensory discriminations. When a discrimination—that is, the detection of a stimulus (a signal)—must be made against a background of noise, the procedure is called signal detection. Signal detection theory takes into account the fact that people are making decisions as they make sensory discriminations. When they attempt to separate a signal change from its background, they may guess, have biases in their judgments, or become less vigilant during the judging process. Knowledge of signal detection theory is useful in many situations—for instance, if one were teaching people to detect accurately small blips on radar screens in an air control tower. Another aspect of sensory perception, subliminal perception (perception without awareness), has been of interest in recent years. However, the data concerning the existence of the phenomenon are still controversial.