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Arthur G. Shapiro; Cone-Specific Mediation of Rod Sensitivity in Trichromatic Observers. Invest. Ophthalmol. Vis. Sci. 2002;43(3):898-905.
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purpose. The slope of the rod threshold versus the illuminance (TVI)
function changes with the wavelength of the background light. This
study was conducted to determine whether the changes in slope are due
to the stimulation of specific cone classes.
methods. An eight-channel optical system was used to generate lights that
differed in cone and rod photoreceptor illuminance. Rod flicker TVI
functions were measured in normal trichromatic observers at mesopic
light levels. The independent variables were (1) the relative
contribution of the short (S)- and long (L)- wavelength cones to the
background light (i.e., the background lights varied along S-only and
L-only lines), and (2) the temporal frequency of the flickering lights
(4, 7.5, and 15 Hz).
results. The 4-Hz rod flicker TVI function had a slope of 0.87 when measured
near W (MacLeod-Boynton chromaticity of 0.66, 1.0). At 4 and
7.5 Hz, an increase in the relative L-cone illuminance steepened the
slope of the rod-only TVI curve, but an increase in the relative S-cone
illuminance had no effect. The slope of the 7.5-Hz TVI function
decreased at higher illuminance levels. At 15 Hz, the thresholds could
be measured over only a limited range.
conclusions. The L-cone system contributes to the desensitization of the rod system
at mesopic light levels, whereas, in the range of lights used in these
experiments, the S-cone system apparently does not. The possibility
that S-cone stimulation desensitizes the response to rod signals at
higher levels of S-cone illumination cannot be
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