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Cord R H Huchzermeyer, Jan Kremers; Psychophysical temporal contrast sensitivities for modulation of the four photoreceptor types using the silent substitution paradigm at different retinal illuminances. Invest. Ophthalmol. Vis. Sci. 2014;55(13):767.
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© ARVO (1962-2015); The Authors (2016-present)
The aim of this study was to measure temporal contrast sensitivites (TCS) for sine-wave modulation of each of the four photoreceptor types as a function of temporal frequency and retinal illuminance with a four primary LED stimulator using the silent substitution paradigm.
Two color normal subjects participated in this study. An 8-channel LED stimulator presented a 2°diameter center and a 13° outer diameter surround field, each with four independent primaries. Sine-wave modulation stimuli were presented in the white surround field while subjects fixated the darker steady central field. The highest mean retinal illuminance of the surround was 3 log phot Td, and this was decreased with neutral density filters (to -0.9 log phot td in 0.3 steps). Measurements were performed at 10 different frequencies between 1 and 28Hz. For each photoreceptor type (L-,M-,S-cones/rods) isolating stimuli were constructed using the triple silent substitution paradigm based on the 10° cone fundamentals and the 10° scotopic luminous efficiency. At each condition, threshold contrasts were determined using a randomly-interleaved double staircase algorithm.
The TCS-curves for the L- and M-cone isolating stimuli are consistent with the notion of two channels mediating thresholds: one low-pass (chromaticity-dependent) and one band-pass (luminance-dependent). In contrast, the TCS-curves were low-pass for the S-cone and rod isolating stimuli. There was a steady sensitivity decrease for the S-cones whereas the rod sensitivity remained constant up to about 10 Hz beyond which the sensitivities rapidly decreased. The rod TCS could also be measured at high illuminances. At 1Hz, sensitivies were lower than those of the three cone types. At frequencies above 8Hz, rod sensitivity was higher than for S-cone stimuli, lower than for L-cone stimuli, and about equal as for M-cone stimuli.
Measuring TCS for photoreceptor isolating stimuli in the parafovea may offer some advantages. Individual correction for prereceptoral filtering (especially macular pigment) may not be as important as for central stimuli. Furthermore, measurements may even be feasible in subjects with poor vision. Therefore, this protocol may be more suitable for patients with retinal disease to investigate pathologies in photoreceptors and retinal circuitry.
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