Li~ht and Dark Adaptation of Tectal Neurones in the Crucian Carp: The Effect of Stimulus Parameters upon both Neuronal Threshold and Response Ma~nitude By

One of the salient features of vertebrate visual systems is in their ability to adapt and function over an extremely wide range of light intensities. Psychophysi-cal measurements show that the human eye can discriminate light intensities over a range of about 9 log units. Since Hecht (1937) proposed his photochemical theory that changes in the sensitivity of the visual system are directly ascribed to the bleaching and regeneration of the visual pigments, many experiments have been performed concerning the rivalry between photochemical and non-photoche-mical factors on visual adaptation (Lythgoe, 1940; Craik and Vernon, 1941). The experiment on rivalry has ended by the outcome of the studies on bleaching of photo-pigments; there may be large changes in visual sensitivity without significant changes in the concentration of visual pigment, and the photo-chemical factors play certain limited roles in adaptation (cf. Rushton, 1961). A new question, however, arises whether the main part of visual adaptation can be attributable to the neuronal level of the retina or elswhere in the visual pathway. The responses of neurones throughout the entire retinal structure of the mudpupp