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.
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.