DISTRIBUTION OF PHOTON-ABSORPTION RATES ACROSS THE RAT RETINA

1. An investigation into the distribution of light intensity across th e rat retina was carried out on excised, intact rat eyes exposed to Ga nzfeld illumination from a helium-neon laser (543 nm). 2. Some of the light entering the eyes exits through the sclera where its intensity c an be monitored with an optical 'pick-up' that samples the intensity c oming from a small region of external sclera and underlying retina. Th e spatial resolution of the pick-up is such that it samples light that has passed through ca 2 % of the rods in the rat eye. 3. Some of the laser light is absorbed by the rod pigment, rhodopsin, which gradually bleaches. Bleaching in the retina, in turn, causes an exponential inc rease in intensity emanating from the sclera. By monitoring this inten sity increase, we are able to measure two important parameters in a si ngle bleaching run: the local rhodopsin concentration and the local in tensity falling on the rods. 4. With an ocular transmission photometer , we have measured both the local intensity and the local rhodopsin co ncentration across wide regions of rat retina. Both pigmented and albi no rats were studied. 5. The distributions of rhodopsin and intensity were both nearly uniform; consequently the product, (rhodopsin concent ration) x (intensity), was similarly nearly equal across the retina. T his means that the initial rate of photon absorption is about the same at all retinal locations. 6. Interpreted in terms of photostasis (the regulation of daily photon catch), this means that the rate of photon absorption is about the same in each rod, viz. 14 400 photons absorbe d per rod per second. Since this rate of absorption is sufficient to s aturate the rod, one possible purpose of photostasis is to maintain th e rod system in a saturated state during daylight hours.