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Y. Wang, P. B. Kruger, J. S. Li, P. L. Lin, L. R. Stark; Individual Differences in Accommodation to Wavefront Vergence and Chromatic Aberration: Color Normal and Deutan Observers. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4026. doi: https://doi.org/.
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Longitudinal chromatic aberration (LCA) provides a cue to accommodation with small pupils. However, large pupils increase monochromatic aberrations, which may obscure chromatic blur (McLellan, Marcos, Prieto & Burns, Nature, 417, 174, 2002). We examined the effect of pupil size and LCA on accommodation in color normal and deutan observers.
Participants were nine normal trichromats, three deuteranomalous trichromats, and two deuteranopic dichromats (anomaloscope and D-15). Accommodation was recorded by infrared optometer (100 Hz) and pupil by video-camera (30 frames/s) while observers viewed a Maltese cross target in a Badal stimulus system. There were two illumination conditions: white (3000 K; 20 cd/m2) and monochromatic (550 nm with 10 nm bandwidth; 20 cd/m2) and two artificial pupil conditions (3 mm and 5.7 mm). The target moved sinusoidally (1-3 D at .2 Hz) during trials lasting 40 seconds. Six trials were run for each condition randomized in blocks. Gain and temporal phase-lag were calculated by FFT for each trial. Mean pupil diameter was calculated for each trial. Separately, static measurements of wavefront aberration were made with the eye accommodating to targets between zero and 4 D (COAS, Wavefront Sciences).
Dynamic gain to vergence modulation increased significantly with pupil size in monochromatic (p=.005) but not white light (p=.12), and significantly with addition of LCA at both pupil sizes (5.7 mm, p=.004; 3 mm, p=.02). There were no significant differences in dynamic accommodation between color normal and deutan individuals for any condition (.68≤p≤.96). Normals and deutan observers showed large individual differences in dynamic gain to vergence and LCA. Mean responses also varied among individuals, but deuteranomalous observers appeared to over-accommodate compared to color normal observers (.06≤p≤.12).
Large individual differences in accommodation to vergence and LCA are a hallmark of accommodation in normal and deutan observers. LCA continues to provide a signal at large pupil sizes despite higher levels of monochromatic aberrations. Monochromatic aberrations may defend against chromatic blur at high spatial frequencies (McLellan et al., 2002) but accommodation responds best at 3 c/deg where blur from higher order aberrations is less.
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