STEVEN’S POWER LAW

You have so far come to know that area of psychophysics concerns the relationship between stimulus intensity and sensory magnitude. Fechner, extending Weber’s law believed that a general equation, with the logarithmic relationship, does exist that holds good for all senses.

What Weber Says

Weber’s law tells us that each just noticeable stimulus increment is a constant fraction of the stimulus to which it is added. The sensations go up by an arithmetic series and the stimuli by a geometric series, therefore, we must mark off ever-larger intervals on the physical scale to correspond to equal intervals on the psychological dimensions. Woodworth (1938) stated as follows: “the sensation plods along with step by step while the stimulus leaps ahead by ratios.”

What Fechner Says

Fechner stated that the scale of subjective sensory intensity has almost the same properties which characterize physical dimensions, the unit of subjective sensory intensity being j.n.d.

Fechner’s formulation was very influential but a question was there. Is it that sensory magnitude can only be assessed indirectly as Fechner had claimed? To answer this question, a Harvard Psychologist S.S. Stevens (1906-1973) provided one straight forward method.

He asked subjects to estimate sensory magnitude directly. In his method, a series of stimuli were presented to subjects and they were asked to assign numbers that were proportional to the corresponding subjective impressions. Thus, if one tone sounds three times louder than another, the subject had to assign a number to the first that was three times larger than the number given to the second. (Stevens, 1961).

Steven’s Law

Two points were special to it. (i) The subject had little trouble in performing the task. They could judge their own subjective experience on a direct scale of subjective magnitude. (ii) The relation between this scale and physical intensity, as Stevens found out, was not logarithmic.

Hence Stevens conceived a power function, the formula being
S = kIN
Where – S stands for the subjective magnitude
I for stimulus intensity
k and N are constants.

This exponential function asserts that the intensity of a sensation is proportional to stimulus intensity raised to a certain power. When N is smaller than I, sensation grows more slowly than stimulus intensity and when N is larger than I, sensation
grows more rapidly than stimulus intensity. The nervous system plays an important role in its compression or expansion.

Let us take an example of the sensation of pain by electric shock. Here the organism is better served by expansion than by
compression. When the stimulus is quite intense, even a small increment in its intensity may spell the difference between survival and destruction.

Expansion of the subjective scale incites the victim to escape before serious harm is done. A look into the development of methods to measure sensation quantitatively from Weber to Stevens would reveal that these were great contributions to the field of Psychology. Until the middle of the 19th century, scientists had despaired of ever measuring psychological processes. But Fechner showed the way and now we are in a position to measure sensation more accurately in quantitative form.

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