Dynamics of accommodative fatigue in rhesus monkeys and humans.

College of Optometry, University of Houston, 505 J Davis Armistead Bldg, 4901 Calhoun Rd, Houston, TX 77004, USA.
Vision Research (Impact Factor: 2.14). 02/2005; 45(2):181-91. DOI: 10.1016/j.visres.2004.07.036
Source: PubMed

ABSTRACT Changes in accommodative dynamics with repeated accommodation were studied in three anesthetized rhesus monkeys and two conscious humans. Maximum accommodation was centrally stimulated via the Edinger-Westphal nucleus in monkeys with a 4 s on, 4 s off paradigm (4 x 4) for 17 min, 4 x 1.5 for 27 min and 2 x 1 for 16 min. Humans accommodated repeatedly to visual targets (5 x 5; 5D and 2 x 2; 6D) for 30 min. In all cases, accommodation was sustained throughout. The anesthetized monkeys showed inter-individual variability in the extent of changes in accommodative dynamics over time while no systematic changes were detected in the human accommodative responses. Little accommodative fatigue was found compared to previous studies which have reported a complete loss of accommodation after 5 min of repeated stimulation in monkeys.

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    ABSTRACT: PURPOSE: To evaluate accommodative dynamics immediately before and after visual fatigue-inducing tasks with congruent (C) and noncongruent (NC) visual stimuli. METHODS: Accommodative dynamics for a 2-diopter (D) step (2.5 ↔ 4.5 D) stimulus were assessed using the WAM-5500 open-field autorefractor in 10 visually normal, asymptomatic, young-adult subjects before and after C and NC tasks performed on different days in a counterbalanced manner. For the C task, subjects altered binocular fixation (50 cycles) between reduced Snellen charts at 50 and 20 cm every 3 seconds to the beat of a metronome. For the NC task, subjects performed 50 cycles of accommodative flipper (±1.5 D) as rapidly as possible while binocularly fixating on a reduced Snellen chart at 40 cm. RESULTS: Mean steady-state (SS) response level reflecting its accuracy was significantly (p ≤ 0.05) reduced post-NC task for both increasing (4.5-D level) and decreasing (2.5-D) accommodation. Although the SS response variability was significantly (p ≤ 0.05) increased post-NC task at the 4.5-D stimulus level, it did not change significantly (p > 0.05) for the C task. There were no significant differences between the pretask and posttask group mean initial amplitude, time constant, and peak velocity for either the C or the NC task for both increasing and decreasing accommodation. CONCLUSIONS: The accommodative system exhibited consistent fatigue effects with respect to SS variability and response accuracy. There was no objective evidence of oculomotor learning for these specific tasks.
    Optometry and vision science: official publication of the American Academy of Optometry 12/2012; · 1.53 Impact Factor
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    ABSTRACT: Purpose: To estimate changes in surface shape and Gradient Refractive INdex (GRIN) profile in primate lenses as a function of accommodation. To quantify the contribution of surface shape and GRIN to spherical aberration changes with accommodation. Methods: Crystalline lenses from 15 cynomolgus monkeys were studied in vitro under different levels of accommodation produced by a stretching system. Lens shape was obtained from Optical Coherence Tomography (OCT) cross-sectional images. The GRIN was reconstructed with a search algorithm using the optical path measured from OCT images and the measured back focal length. The spherical aberration of the lens was estimated as a function of accommodation using the reconstructed GRIN and a homogeneous refractive index. Results: The lens anterior and posterior radii of curvature decreased with increasing lens power. Both surfaces exhibited negative asphericities in the unaccommodated state. The anterior surface conic constant shifted towards less negative values with accommodation, while the value of the posterior remained constant. The GRIN parameters remained constant with accommodation. The lens spherical aberration with GRIN distribution was negative and higher in magnitude than that with a homogeneous equivalent refractive index (by 29% and 53% in the un-accommodated and fully accommodated states, respectively). Spherical aberration with the equivalent refractive index shifted with accommodation toward negative values (-0.070 μm/D), but the reconstructed GRIN shifted it further (-0.124 μm/D). Conclusions: When compared with the lens with the homogeneous equivalent refractive index, the reconstructed GRIN lens has more negative spherical aberration and a larger shift toward more negative values with accommodation.
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