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Darren Koenig, Ayeswarya Ravikumar, Heidi Hofer; Two-point detection, discrimination, and appearance without higher order aberrations in the dark-adapted fovea. Journal of Vision 2013;13(15):T31. doi: https://doi.org/10.1167/13.15.31.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: We investigated the impact of the cone mosaic on spatiochromatic reconstruction of foveal two-point stimuli displayed with adaptive optics. We compared results to theoretical predictions to determine whether cones contribute independently to detection and appearance and, if not, the extent of summation for these processes.
Methods: We measured detection thresholds, discrimination thresholds, and spatial and color appearance for 25ms, 550nm, aberration-corrected two-point stimuli, with retinal size of ~0.5', separated from 0.5' to 12' in the central fovea (3 subjects) and at 1.1 deg (4 subjects). We used a bleaching protocol at 1.1 deg to isolate cone responses.
Results: Detection thresholds were inconsistent with independent cones and reflected summation on a scale of 5–6' at both eccentricities, however spatial appearance reports indicated access to finer grained mechanisms. Discrimination thresholds averaged 1.9' and 4' in the central fovea and 1.1 deg, respectively. Color appearance was highly variable for two-point stimuli, as for single points, but strongly coupled for small separations, reflecting a color mechanism with a scale of ~30' at 1.1 deg.
Conclusions: Two-point detection and appearance were inconsistent with independent cone contributions, even with tiny adaptive optics stimuli, creating challenges in recovering ‘single cone’ sensations. Furthermore, spatial and appearance reports indicated access to even finer grained and coarser grained mechanisms, than those governing detection. Foveal discrimination thresholds were consistent with previous reports, with larger thresholds at 1.1 deg as expected from cone spacing. Overall, results suggest detection, spatial appearance, and color appearance are independent processes with different limiting factors.
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