Slow optical changes in human photoreceptors induced by light

PURPOSE. The basic assumption of fundus reflection densitometry is that cha nges in reflectance are solely determined by photolysis and regenerating vi sual pigments. This study was undertaken to investigate small but systemati c deviations from this rule. METHODS. Spectral reflectance changes (450 - 740 nm) of the fovea were meas ured during light and dark adaptation over a period of 66 minutes in five h ealthy subjects. The directional properties of the fundus reflection were e xamined with a custom-built scanning laser ophthalmoscope (SLO) at 514, 633 , and 790 nm. The same instrument was also used to find the spatial distrib ution of the reflectance changes. RESULTS. In addition to fast changes consistent with visual pigment, slower reflectance changes (lasting 10-20 minutes) were observed at all wavelengt hs including 740 nm, Because visual pigment does not absorb at 740 nm, a se cond mechanism must be involved. Factor analysis generated two factors (i.e ., spectral curves) that explained more than 97% of the variations in the t ime course of the spectral reflectance. Total reflectance was modeled by me ans of an existing model for fundus reflection, and it was found that the f irst factor strongly resembled the rapid changes in absorption of the cone pigments. The second factor seems linked to slow changes in cone reflectanc e. Measurements: with the SLO showed a clear increase in directionally depe ndent reflectance from 6 Lo 30 minutes in the dark. This was observed only in the central 6 degrees of the retina. CONCLUSIONS. The characteristics of the slow reflectance changes all point to the cone photoreceptors as the origin. Most likely, alterations in the i ndex of refraction between the interphotoreceptor matrix and photoreceptors lie at the base of this hitherto unknown phenomenon.