VISUAL-ADAPTATION - RETINAL TRANSDUCTION, BRIGHTNESS AND SENSITIVITY

Two types of experiments tested and confirmed the hypothesis that light-adaptation at scotopic and mesopic levels is produced by nonlinearities in the mechanisms of retinal transduction. In the brightness experiments [human] observers judged the terminal portion of a flash of white light composed of a test flash superimposed upon a preceding adapting field of the same size. For flashes on the order of several seconds and retinal illuminances about 2 log uits above absolute threshold, brightness varied as a power function of total intensity with an exponent of 1/3 independent of the level of the adapting field. The increment in brightness produced by a given test intensity at different adapting levels varied in accord with equations describing brightness in a steady-state. In the threshold experiments, increment thresholds were obtained at several levels of adaptation covering the same range of intensities used in the brightness experiments. Except near absolute threshold, increment threshold increased as a power function of adapting level with an exponent of 2/3 in accord with a prediction based upon a fixed threshold criterion of brightness. A theoretical basis for the Weber-Fechner relation was suggested.