Cross-coupling between accommodation and convergence is optimized for a broad range of directions and distances of gaze.
Accommodation and convergence systems are cross-coupled so that stimulation of one system produces responses by both systems. Ideally, the cross-coupled responses of accommodation and convergence match their respective stimuli. When expressed in diopters and meter angles, respectively, stimuli for accommodation and convergence are equal in the mid-sagittal plane when viewed with symmetrical convergence, where historically, the gains of the cross coupling (AC/A and CA/C ratios) have been quantified. However, targets at non-zero azimuth angles, when viewed with asymmetric convergence, present unequal stimuli for accommodation and convergence. Are the cross-links between the two systems calibrated to compensate for stimulus mismatches that increase with gaze-azimuth? We measured the response AC/A and stimulus CA/C ratios at zero azimuth, 17.5 and 30 deg of rightward gaze eccentricities with a Badal Optometer and Wheatstone-mirror haploscope. AC/A ratios were measured under open-loop convergence conditions along the iso-accommodation circle (locus of points that stimulate approximately equal amounts of accommodation to the two eyes at all azimuth angles). CA/C ratios were measured under open-loop accommodation conditions along the iso-vergence circle (locus of points that stimulate constant convergence at all azimuth angles). Our results show that the gain of accommodative-convergence (AC/A ratio) decreased and the bias of convergence-accommodation increased at the 30 deg gaze eccentricity. These changes are in directions that compensate for stimulus mismatches caused by spatial-viewing geometry during asymmetric convergence.
Nguyen
Dorothy
D
University of California, School of Optometry, 512 Minor Hall, Berkeley, CA 94720-2020, USA.
Vedamurthy
Indu
I
Schor
Clifton
C
D000065:Accommodation, Ocular / Q000502:physiology*; D000328:Adult; D003290:Convergence, Ocular / Q000502:physiology*; D004215:Distance Perception / Q000502:physiology*; D005246:Feedback / Q000502:physiology; D006801:Humans; D010775:Photic Stimulation / Q000379:methods; D015357:Vision Disparity / Q000502:physiology