Development and Aging of Human Visual Focusing Mechanisms

摘要

The ability of the young adult human eye to focus an image onto the retina over a range from infinity to a few centimenters is a function of the shape, thickness, and placement of the crystalline lens relative to the cornea. The cornea provides a fixed refractive contribution to image formation, while the lens provides a variable contribution controlled by coupling ciliary muscle contraction to lens elastic recovery. The precise mechanism by which this coupling occurs remains the subject of study, although most proposed mechanisms are based on a Helmholtzian model. The gradual reduction in accommodative range with age leading to presbyopia is characterized by the retreat of the nearest comfortable focal point toward the far point; because lens shape becomes more sharply curved with age, this maintenance of far focus at the expense of near must be addressed in terms of other compensatory age-related changes within the lens. The development of the lens and the factors leading to image formation during accommodation are described. The potential factors contributing to age-related accommodative loss are assessed, and models of the process evaluated for both human and non-human primates. The lens-based geometric model of accommodation and presbyopia appears to fit the human visual system optimally.