Accommodation to wavefront vergence and chromatic aberration: color normal and deutan observers

Abstract

Purpose: 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, 2002). We examined the effect of pupil size and LCA on accommodation in color normal and deutan observers. Methods: 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 sinusoidally moving Maltese cross target (1-3 D at .2 Hz) 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). Separately, static measurements of wavefront aberration were made with the eye accommodating to targets between zero and 4 D (COAS, Wavefront Sciences). Results: Dynamic gain to vergence modulation increased significantly with pupil size in monochromatic (p=.005) but not white light (p=.12), and gain increased significantly with addition of LCA at both pupil sizes (5.7 mm, p=.004; 3 mm, p=.02). Mean RMS higher order aberration increased from .23 m with small pupils (3 mm) to .48 m with large pupils (5.7 mm). 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 both vergence and LCA. Mean responses also varied among individuals, but deuteranomalous observers over-accommodated compared to color normal observers (.06≤p≤.12). Conclusions: Large individual differences in accommodation to wavefront vergence and to 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, but accommodation responds best at 3 c/deg where blur from higher order aberrations is less (Mathews, 1998; Mathews & Kruger, 1994).