Accommodation (vertebrate eye)

Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies. In this, distances vary for individuals from the far point—the maximum distance from the eye for which a clear image of an object can be seen, to the near point—the minimum distance for a clear image.
Accommodation usually acts like a reflex, including part of the accommodation-convergence reflex, but it can also be consciously controlled.
The main ways animals may change focus are:

Changing the shape of the lens.
Changing the position of the lens relative to the retina.
Changing the axial length of the eyeball.
Changing the shape of the cornea.
Focusing mechanisms

Focusing the light scattered by objects in a three dimensional environment into a two dimensional collection of individual bright points of light requires the light to be bent. To get a good image of these points of light on a defined area requires a precise systematic bending of light called refraction. The real image formed from millions of these points of light is what animals see using their retinas. Very even systematic curvature of parts of the cornea and lens produces this systematic bending of light onto the retina. Due to the nature of optics the focused image on the retina is always inverted relative to the object.
Different animals live in different environments having different refractive indices involving water, air and often both. The eyes are therefore required to bend light by different amounts leading to different mechanisms of focus being used in different environments. The air/cornea interface involves a larger difference in refractive index than hydrated structures within the eye. As a result, animals living in air have most of the bending of light achieved at the air/cornea interface with the lens being involved in finer focus of the image. Generally mammals, birds and reptiles living in air vary their eyes' optical power by subtly and precisely changing the shape of the elastic lens using the ciliary body.
The small difference in refractive index between water and the hydrated cornea means fish and amphibians need to bend the light more using the internal structures of the eye. Therefore, eyes evolved in water have a mechanism involving changing the distance between a rigid rounder more refractive lens and the retina using less uniform muscles rather than subtly changing the shape of the lens itself using circularly arranged muscles.

Land based animals and the shape changing lens

Varying forms of direct experimental proof outlined in this article show that most non-aquatic vertebrates achieve focus, at least in part, by changing the shapes of their lenses.
What is less well understood is how the subtle, precise and very quick changes in lens shape are made. Direct experimental proof of any lens model is necessarily difficult as the vertebrate lens is transparent and only functions well in the living animal. When considering vertebrates, aspects of all models may play varying roles in lens focus. The models can be broadly divided into two camps: those models that stress the importance of external forces acting on a more passively elastic lens and other models that include forces that may be generated by the lens internally.

External forces

The model of a shape changing lens of humans was proposed by Thomas Young in a lecture on the 27th Nov 1800. Others such as Hermann von Helmholtz and Thomas Henry Huxley refined the model in the mid-1800s explaining how the ciliary muscle contracts, rounding the lens to focus near. The model may be summarized like this: normally the lens is held under tension by its suspending ligaments and capsule being pulled tight by the pressure of the eyeball. At short focal distance the ciliary muscle contracts, stretching the ciliary body and relieving some of the tension on the suspensory ligaments, allowing the elastic lens to become more spherical, increasing refractive power. Changing focus to an object at a greater distance requires a thinner less curved lens. This is achieved by relaxing some of the sphincter-like ciliary muscles allowing the ciliarly body to spring back, pulling harder on the lens making it less curved and thinner, so increasing the focal distance. There is a problem with the Helmholtz model in that despite mathematical models being tried, none has come close enough to working using only the Helmholtz mechanisms.
In 1992, Ronald Schachar proposed a model for land based vertebrates that was not well received. The theory allows mathematical modeling to more accurately reflect the way the lens focuses while also taking into account the complexities in the suspensory ligaments and the presence of radial as well as circular muscles in the ciliary body. In this model the ligaments may pull to varying degrees on the lens at the equator using the radial muscles, while the ligaments offset from the equator to the front and back are relaxed to varying degrees by contracting the circular muscles. These multiple actions operating on the elastic lens allows it to change lens shape at the front more subtly. Not only changing focus, but also correcting for lens aberrations that might otherwise result from the changing shape while better fitting mathematical modeling.
The "catenary" model of lens focus proposed by Coleman demands less tension on the ligaments suspending the lens. Rather than the lens as a whole being stretched thinner for distance vision and allowed to relax for near focus, contraction of the circular ciliary muscles results in the lens having less hydrostatic pressure against its front. The lens front can then reform its shape between the suspensory ligaments in a similar way to a slack chain hanging between two poles might change its curve when the poles are moved closer together. This model requires precise fluid movement of the lens front only rather than trying to change the shape of the lens as a whole. While this concept may be involved in the focusing it has been shown by Scheimpflug photography that the rear of the lens also changes shape in the living eye.

Internal forces

When Thomas Young proposed the changing of the human lens' shape as the mechanism for focal accommodation in 1801, he thought the lens may be a muscle capable of contraction. This type of model is termed intracapsular accommodation as it relies on activity within the lens. In a 1911 Nobel lecture Allvar Gullstrand spoke on "How I found the intracapsular mechanism of accommodation" and this aspect of lens focusing continues to be investigated. Young spent time searching for the nerves that could stimulate the lens to contract without success. Since that time it has become clear the lens is not a simple muscle stimulated by a nerve so the 1909 Helmholtz model took precedence. Pre-twentieth century investigators did not have the benefit of many later discoveries and techniques. Membrane proteins such as aquaporins which allow water to flow into and out of cells are the most abundant membrane protein in the lens. Connexins which allow electrical coupling of cells are also prevalent. Electron microscopy and immunofluorescent microscopy show fiber cells to be highly variable in structure and composition. Magnetic resonance imaging confirms a layering in the lens that may allow for different refractive plans within it. The refractive index of human lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens. This index gradient enhances the optical power of the lens. As more is learned about mammalian lens structure from in situ Scheimpflug photography, MRI and physiological investigations, it is becoming apparent the lens itself is not responding entirely passively to the surrounding ciliary muscle, but may be able to change its overall refractive index through mechanisms involving water dynamics in the lens still to be clarified. The accompanying micrograph shows wrinkled fibers from a relaxed sheep lens after it is removed from the animal indicating shortening of the lens fibers during near focus accommodation. The age related changes in the human lens may also be related to changes in the water dynamics in the lens.

Human eyes

The young human eye can change focus from distance to as near as 6.5 cm from the eye. This dramatic change in focal power of the eye of approximately 15 dioptres occurs as a consequence of a reduction in zonular tension induced by ciliary muscle contraction. This process can occur in as little as 224 ± 30 milliseconds in bright light. The amplitude of accommodation declines with age. By the fifth decade of life the accommodative amplitude can decline so that the near point of the eye is more remote than the reading distance. When this occurs the patient is presbyopic. Once presbyopia occurs, those who are emmetropic will need an optical aid for near vision; those who are myopic, will find that they see better at near without their distance correction; and those who are hyperopic will find that they may need a correction for both distance and near vision. Note that these effects are most noticeable when the pupil is large; i.e. in dim light. The age-related decline in accommodation occurs almost universally to less than 2 dioptres by the time a person reaches 45 to 50 years, by which time most of the population will have noticed a decrease in their ability to focus on close objects and hence require glasses for reading or bifocal lenses. Accommodation decreases to about 1 dioptre at the age of 70 years. The dependency of accommodation amplitude on age is graphically summarized by Duane's classical curves.

Amplitude of accommodation

The amplitude of accommodation is a clinical measurement that describes the maximum potential increase in optical power that an eye can achieve in adjusting its focus. It refers to a certain range of object distances for which the retinal image is as sharply focused as possible. Amplitude of accommodation is measured during routine eye-examination. The closest that a normal eye can focus is typically about 10 cm for a child or young adult. Accommodation then decreases gradually with age, effectively finishing just after age fifty. The average amplitude of accommodation, in diopters, for a patient of a given age was estimated by Hofstetter in 1950 to be 18.5 − with the minimum amplitude of accommodation as 15 −, and the maximum as 25 −. However, Hofstetter's work was based on data from two early surveys which, although widely cited, used methodology with considerable inherent error.