ON THE POSSIBLE RELATIONS BETWEEN DISCRIMINABILITY AND APPARENT MAGNITUDE

Some psychophysicists seek a unified theory in which the scaling of ap parent sensory magnitude and the discrimination of differences in inte nsity can be encompassed. When contextual factors are held constant, t here 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 simila r 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. We ber held that, for the same physical stimulus intensity, apparent magn itude and discriminatory ability increased with neural efficiency. Fec hner held that changes in apparent magnitude had no effect on discrimi nation: a stable differential threshold was predicted by the Parallel Law (Weber's Law applied to internal sensations). Evidence is consider ed for the tactile sense and weight perception, and for visual size; T he relation between apparent heaviness and weight discrimination is co mplex, varying with the state of adaptation and with neural efficiency . There is some evidence that both tactile and visual apparent size in crease with a finer underlying neural structure; but it is unclear whe ther other types of increase in apparent size lead to increased spatia l acuity, or to differences in the Weber fraction for line length. The variety of values of apparent magnitude and discrimination, and the l ack of a monotonic relation between them, makes it unlikely that all k nown relationships could be encompassed in a unified psychophysical th eory.