On the mechanisms of visual adaptation

Abstract

Due to adaptation, the visual system can “see well” (e.g. have a high contrast sensitivity) in widely changing conditions of illumination. These two requirements which seem to be contradictory, may be combined if the “working range” (i.e. the steepest part of R-logI-function) shifts along the I-scale in accordance with the illumination. In human subjects contrast sensitivity is highest near to the adapting level and decreases on both sides this level. The same can be shown in summed spike responses in the optic nerve of the frog: the R-logI-function is most steep near the adapting level in any state of adaptation. The R-logI-function were also measured in the horizontal cells of the turtle retina. The function measured by transients (on- and off-peaks) is very steep near the adapting level and shifts along the I-scale if the adapting light is changed. On the other hand the R-logI-function measured by plateau steady potentials is much less steep and does not shift during adaptation to a new background. It is supposed that transients as well as the shift of the R-logI-function are due to the feedback (perhaps a positive one) between horizontal cells and receptors. Polarizing of a horizontal cell can vastly change the local ERG, as well as give rise to “current ERG”. This shows that horizontal cells take part in the processing of signals in bipolars and can partly be responsible for adaptation shifts of R-logI-function of the visual system. The role of the pupil reflex as an adaptation mechanism is considered. The effectiveness of this reflex is increased due to the adaptation shift of its R-logI-function.