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Peter J. DeLint, Tos T. J. M. Berendschot, Jan van de Kraats, Dirk van Norren; Slow Optical Changes in Human Photoreceptors Induced by Light. Invest. Ophthalmol. Vis. Sci. 2000;41(1):282-289.
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purpose. 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.
methods. 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.
results. 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°
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
index of refraction between the interphotoreceptor matrix and
photoreceptors lie at the base of this hitherto unknown
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