Slow optical changes in human photoreceptors induced by light

The basic assumption of fundus reflection densitometry is that changes in reflectance are solely determined by photolysis and regenerating visual pigments. This study was undertaken to investigate small but systematic deviations from this rule.

Spectral reflectance changes (450-740 nm) of the fovea were measured during light and dark adaptation over a period of 66 minutes in five healthy subjects. The directional properties of the fundus reflection were examined with a custom-built scanning laser ophthalmoscope (SLO) at 514, 633, and 790 nm. The same instrument was also used to find the spatial distribution of the reflectance changes.

In addition to fast changes consistent with visual pigment, slower reflectance changes (lasting 10-20 minutes) were observed at all wavelengths including 740 nm. Because visual pigment does not absorb at 740 nm, a second mechanism must be involved. Factor analysis generated two factors (i.e., spectral curves) that explained more than 97% of the variations in the time course of the spectral reflectance. Total reflectance was modeled by means of an existing model for fundus reflection, and it was found that the first factor strongly resembled the rapid changes in absorption of the cone pigments. The second factor seems linked to slow changes in cone reflectance. Measurements with the SLO showed a clear increase in directionally dependent reflectance from 6 to 30 minutes in the dark. This was observed only in the central 6 degrees of the retina.

The characteristics of the slow reflectance changes all point to the cone photoreceptors as the origin. Most likely, alterations in the index of refraction between the interphotoreceptor matrix and photoreceptors lie at the base of this hitherto unknown phenomenon.