On the possible relations between discriminability and apparent magnitude.

Abstract

Some psychophysicists seek a unified theory in which the scaling of apparent sensory magnitude and the discrimination of differences in intensity can be encompassed. When contextual factors are held constant, there is a positive correlation over many sensory continua between the exponent of the power function for sensory scaling and the inverse of the Weber fraction. Variations in neural efficiency also show a similar correlation with apparent magnitude and discrimination. The role of other factors (such as contrast, adaptation and size scaling) remains unclear. Both Weber and Fechner considered aspects of these issues. Weber held that, for the same physical stimulus intensity, apparent magnitude and discriminatory ability increased with neural efficiency. Fechner held that changes in apparent magnitude had no effect on discrimination: a stable differential threshold was predicted by the Parallel Law (Weber's Law applied to internal sensations). Evidence is considered for the tactile sense and weight perception, and for visual size. The relation between apparent heaviness and weight discrimination is complex, varying with the state of adaptation and with neural efficiency. There is some evidence that both tactile and visual apparent size increase with a finer underlying neural structure; but it is unclear whether other tyoes of increase in apparent size lead to increased spatial acuity, or to differences in the Weber fraction for line length. The variety of values of apparent magnitiude and discrimination, and the lack of a monotonic relation between them, makes it unlikely that all known relationships could be encompassed in a unified psychophysical theory.