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

      Wang, Yinan (2009-05-18)
      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).
    • Accommodation to wavefront vergence: adapting ‘averse channels’

      Chen, Angela (2008-06-02)
      Purpose: Accommodation responds to wavefront vergence, but the mechanisms for vergence detection are unknown. One possibility is that accommodation responds to the angle of incidence of light at edges blurred by defocus. ‘Averse channels’ that sample light from opposite sides of the pupil were hypothesized by Makous (1968, 1977). Makous named the phenomenon a “transient” Stiles-Crawford effect because he found that there was a reduction in sensitivity to light entering one side of the pupil that lasts for a short period of time. Such ‘averse channels’ that sample modulation across the pupil are a possible mechanism for detecting the sign of defocus. The purpose of the present experiment is to determine whether such ‘averse channels’ can operate separately of each other to specify the sign of defocus for accommodation. Method: Accommodation was monitored continuously while subjects viewed a vertical monochromatic (548nm) luminance edge (1.0 contrast) that stepped either to a far or near direction in a Badal optometer. Various levels of adapting field were used to reduce the contrast of the edge (0.48, 0.36, 0.26, or 0.17 contrast) to determine the contrast threshold for accommodation. In a final experiment, an adapting field entered the eye through the nasal or temporal side of the pupil, to selectively adapt nasally or temporally tuned ‘channels’, while the target stepped randomly toward or away from the eye. The orientation of the vertical edge was either bright on the right side or bright on the left. Results: In a preliminary experiment, only five out of twenty-six subjects showed reliable and consistent responses, with gains >0.5 for both positive and negative step change in vergence. The contrast threshold for accommodation to step changes in target vergence was approximately 26%. Data from three subjects who accommodated reliably to both directions of step changes in vergence do not support the claim that ‘channels’ sample light separately from opposite sides of the pupil to determine the sign of defocus. Conclusion: Potential factors that account for the poor accommodative responses in the present study include: 1) the target was illuminated with monochromatic light, 2) the target was a simplified stimulus in which the blur was in only one direction (one spatial phase), 3) the target was viewed monocularly, 4) the 3mm artificial pupil increased depth-of-focus and removed the normal dynamic behavior of the pupil, 5) our subjects were untrained. The negative results suggest alternate hypotheses: 1)‘averse channels’ work in conjunction with each other, not separately, to detect vergence, 2) ‘averse channels’ can function separately to detect vergence, but the signal was undetectable using the current method, and 3) ‘averse channels’ do not mediate vergence detection.