CHROMATIC AND ACHROMATIC MECHANISMS FOR ACCOMMODATION

Title CHROMATIC AND ACHROMATIC MECHANISMS FOR ACCOMMODATION
Author, Co-Author Philip Kruger, Steven Mathews, Karan Aggarwala, Sujata Nowbotsing
Topic
Year
1992
Day
Monday
Program Number
2:45 pm
Room
Ireland B
Affiliation
Abstract The current view of accommodation is that the eye changes focus to reduce blur and to improve retinal image contrast. Negative feedback in the form of blur reduction is necessary for effective accommodation, because defocus blur does not provide a directional signal. But Fincham (1951) suggested that achromatic and chromatic odd-error cues exist, which would make negative feedback less important. We have confirmed the existence of a chromatic mechanism, but the idea of an achromatic directional cue has not been tested. A high speed infrared optometer monitored accommodation while subjects viewed a 3 c/d sine-wave grating in a Badal stimulus system. Blur feedback was eliminated by linking the target position to the accommodation of the subject - changes in accommodation were compensated by equivalent changes in target distance. Target vergence was then modulated sinusoidally at 0.2 Hz over a 2 D amplitude. There were 5 stimulus conditions: 1. Normal (usual amount of longitudinal chromatic aberration (LCA) was present); 2. Doubled (twice as much LCA); 3. Neutralized (no LCA was present); 4. Monochromatic (550 nm); and 5. Reversed (LCA was opposite to normal). Six trials of 40 sec duration were run for each condition, and mean gain and phase-lag were computed from a FFT. Subjects accommodate well under open-loop conditions as long as the normal effects of LCA are present. Doubling the aberration has no effect on accommodation, but reversing LCA is severely debilitating. When the effects of LCA are eliminated (achromatizing lens or monochromatic light) subjects continue to accommodate but with reduced gain. Assuming that blur feedback was effectively eliminated in the experiment, our results suggest that both achromatic and chromatic odd-error signals exist for driving accommodation. (Supported by NEI Grant EYO 5901)
Affiliation of Co-Authors
Outline