On Facts and Theories in Psychophysics: Does Ekman’s Law Exist?

Abstract

A classical treatment of JND has been to make some assumption about its subjective size in order to construct a scale relating sensation to stimulus intensity; Fechner’s law is one result of this tactic. Ekman’s law asserts that sensations one JND apart define a constant ratio, and leads to the conclusion that sensation is a power function of stimulus intensity. However, reconsideration of the cited evidence suggests that Ekman’s law is tautological with Weber’s law.

An alternative approach regards matching experiments as involving two physical continua. Given Weber’s law for both continua and cross-modal equivalence of JND, the matching continuum is a power function of the target continuum. Further, the exponent is inversely related to the Weber fraction on the target continuum. This second approach makes measures of resolving power compatible with cross-modal-matching data but avoids assumptions about the subjective size of the JND.

This paper was written while the author was a visitor in the Psychological Laboratories of the University of Stockholm, and was prepared with support from Grant GB 38292 from the National Science Foundation. I wish to thank Hannes Eisler, Gunnar Goude and Martha Teghtsoonian for their valuable comments on an earlier version.

I take satisfaction in noting that this paper grew out of a talk presented on 22 October 1972, to a group in which S. S. Stevens was a vigorous participant. On that occasion Stevens, happily wearing his Fechner Day button, debated psychophysical issues with even more enthusiasm than usual, so I remember vividly the opportunity to present to him in person the beginning of what I now write here in his memory.