Scaling of Sensory Magnitude

Imagine strolling along a boardwalk at noon on a sunny day; how brightly the sunlight makes the sea sparkle! It's time for lunch and you enter a dimly lit restaurant. At first you can't see a thing—all seems black! But in a few minutes, you can see well enough to read the menu. You have a nice lunch and then emerge, blinking, back into the bright sunlight. Again, you can't see a thing—only an all-pervading white glare! But again, after a few minutes, your vision is restored and you can view the sailboats prancing among the waves. How is it that your eyes can make these remarkable adjustments and provide clear vision under a huge range of light conditions? More specifically, and among other such questions, how does brightness (of, e.g., lamps in the dim restaurant, light reflected from the menu pages) increase with increasing time in the dark? This is a psychophysical question and the answer is given by a psychophysical scaling of the brightness sensation under experimental conditions. The resulting relationships between brightness of lights and time in the dark compose a set of fundamental facts that vision science must explain. They also constrain and inform studies of the physiological mechanism responsible for such dark adaptation. Any mechanism that purports to explain the experience of dark adaptation must predict these relationships. Many similar relationships among relevant variables for vision, hearing, taste, smell, touch, and pain experiences have been measured by psychophysical scaling, describing and elucidating phenomena such as adaptation and recovery, temporal and spatial summation, binocular and binaural summation, spatial inhibition, and so forth.

More generally, researchers measure the magnitude of various aspects of sensory or perceptual experiences in order to induce empirical laws, such as those of dark adaptation, as foundations of a complete understanding of human perception. Psychophysical scaling provides a useful and much-validated set of techniques to do this. A scale is a mathematical rule by which numbers are assigned to objects or events in order to measure some quantity associated with them. Modern measurement theory provides formal rationalizations for psychophysical scaling, but the techniques can be used without them. The minimum necessity is to understand that certain scale types allow a broader range of uses than do others, with ratio (ratios, differences, and zero are meaningful) and interval (only differences are meaningful) scales being the most useful. All techniques discussed yield interval scales at the minimum.

Some sensory or perceptual experiences can be arranged along prothetic continua (e.g., brightness, loudness, apparent size), whereas others cannot (e.g., shape, color, pitch). On prothetic continua, changes in stimulus intensity result in a change in the magnitude, the “how-muchness,” of the perceptual experience. Prothetic continua are often represented in the brain by variations in the quantity of neural activity. Loudness, for [Page 874]example, seems to be represented by the total amount of neural activity in the auditory cortex. Prothetic continua can be meaningfully measured by all scaling techniques.

Other sensory or perceptual experiences can be ordered along a metathetic continuum that is qualitative rather than quantitative, involving changes in “kind.” Here, changes in a stimulus (e.g., light wavelength) result in changes in a sensory quality (e.g., color). Some sense impressions have both prothetic and metathetic aspects; for example, light has both brightness and color. The techniques of this entry are not generally suited to measure metathetic continua, although sometimes they can be adapted by framing a quantitative question, such as “How similar do these colors appear to be?” or “How similar to the standard is this shape?” The similarity judgments can be scaled using nonmetric multidimensional scaling, and measurements of useful underlying quantitative variables can often be recovered.

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