Dark adaptation, absolute threshold and Purkinje shift in single units of the cat's retina.

Granit (1943, 1944) observed that the threshold of retinal ganglion cells in the cat drops when they are allowed to dark-adapt after exposure to bright lights, and that there is a shift in spectral sensitivity analogous to the Purkinje shift in the human. The drop in threshold was rather small compared to that found in human dark adaptation, and the final threshold reached was not nearly as low as one would expect in an eye adapted for nocturnal vision. Pirenne (1954) has emphasized that there is a big discrepancy (a factor of 103-106) between the lowest threshold obtained by electro-physiological methods and the much lower thresholds found by psychophysical methods in humans (Hecht, Shlaer & Pirenne, 1942). In addition, the existence of a random maintained discharge from retinal ganglion cells (Kuffler, FitzHugh & Barlow, 1957) raises some doubts about the relative parts played by the retina and the central nervous system in determining thresholds. We therefore set out to measure the absolute threshold of the large type of ganglion cell (Rushton, 1949) isolated by metallic micro-electrodes of the type used by Granit & Svaetichin (1939). The opportunity was taken of following threshold through the period of dark adaptation, and some observations of the Purkinje shift were made.