Visual threshold estimation and its relation to the question: Fechner-law or Stevens-power function.

This investigation was initiated by the apparent rivalry between Fechner-law and Stevens-power function. We used the pupil reaction as on objective measure for the excitation of the visual system within both the threshold and the suprathreshold region. The course of the threshold as a function of adaptation light shows important differences at different field areas. The Weber-law is valid only in the case of large fields (30 degrees) as had been used by Weber himself. At smaller fields the threshold may be described as a power function with exponents of 0.5 at the smallest (0.5 degrees) and about 0.75 at medium sized (5 degrees) fields. In all these experiments the Fechner-hypothesis is suitable automatically because of the constant increment of pupil constriction at the threshold. The validity of the Fechner-hypothesis together with the approximation of the threshold course by power functions lead to Stevens-power functions for the suprathreshold region. The Fechner-law - a logarithmic function - is valid at large fields only. This consideration has been confirmed by our measurements in the suprathreshold sphere.