Living Below the JND (the "Just Noticeable Difference"): Psychophysics in Everyday Life

Fechner and Volume Knobs

Seldom does one appreciate the contributions that psychophysical research has had on everyday life. When people think of experiments in this field, they think of experimental subjects trying to detect whether two lights are equally bright or different from each other. But psychophysics is much more than that, as recent research in neuroscience reveals. Psychophysicists study the relation between the nature of stimuli out in the word and the associated internal, mental experiences (called "outward psychophysics" by Fechner, the father of the field), or they examine the liaison between those mental experiences and neural activity, called "inward psychophysics," which is today a very active field in neuroscience. The brightness of a light, for example, depends not only on the degree of neural activation at a certain region of the brain but also on the way in which the region is activated and interacts with other regions.

When presenting sensory stimuli, psychophysicists get great pleasure from making one feel as if nothing is changing at all from one moment to the next in the external world, while at the same time they're actually changing things (e.g., ambient luminance or the color of a square) so gradually that one doesn't notice a difference. That is, they like to introduce changes in sensory stimulation that fall below the JND, the "just noticeable difference" (that is, the minimal amount of change in the nature of a stimulus that leads to perceptual changes). This is how Swiss watch designers have the second hand move from 0 seconds to 60 without one ever noticing the hand move. (Some say that the sunsets below the JND, but my friend tells me that he can see some movement moments before the sun vanishes from the horizon.) Similarly, in a psychophysics experiment, the room could be going from brightly lit to pitch dark without one ever detecting a change in light conditions during the change. In this way, some psychophysicists earn their living by helping special-effects programmers create realistic computer-generated images of dinosaurs, helping magicians make things disappear, and by helping audio engineers calibrate volume knobs. Your iPod volume works according to, not physical laws, but to psychophysical laws, thanks to the work of the psychophysicist. (The decibel is based on Fechner's psychophysical laws.) A bohemian friend of mine said that life is best lived below the jnd, but I never understood what that meant.