Dark-adaptation of the human rod system

Following substantial bleaching, dark-adaptation thresholds of a complete rod monochromat and of a subject with normal colour vision were measured using a Wright colorimeter. When precautions were taken to ensure that the fixation point fell on the same retinal area during the threshold measurements as during the bleaching period, the dark-adaptation threshold curves of the rod monochromat followed exactly the same course as those of the normal subject subsequent to the cone-rod break of the long-term, normal dark-adaptation curve; irrespective of the intensity and the duration of the bleaching and the wavelength of the test stimulation. In contrast to the normal subject, however, the dark-adaptation curves of the rod monochromat showed no evidence of any cone function at photopic intensities. Furthermore, as opposed to previous measurements which show a simple linear relationship between fraction of bleached rhodopsin and log threshold, the present results show that there is a close linearity between log fraction of bleached rhodopsin and log threshold. This linear relationship is obtained despite varying extents of bleaching and subsequent dark-adaptation periods.