Difference between revisions of Eye strain

Some participants report feeling strain after long sessions of distance active focus. There are several theories as to why this may be the case:

Helmholtz theory of accommodation (1855) – This theory is also called the capsular theory of accommodation . Helmholtz theorized that when the eyes are viewing a distant object, the ciliary muscle relaxes and the zonular fibers between the ciliary body 1 and the equator of the lens stay flattened but when the object of focus is close, the ciliary muscles contract and the zonular fibers loosen. In the Helmholtz theory, the lens equator slides away from the sclera during accommodation and closer to the sclera when accommodation ends. In this theory, the zonular fibers are relaxed during accommodation and the zonular fibers are under tension when there is no accommodation reflex.

Schacher theory of accommodation (2006) - This theory states that when the lens is in focus, there is increased tension on the lens through the equatorial zonular fibers and when there is contraction of the ciliary muscle, the zonular fibers located equatorially increase their tensile strength. This results in the steepness of the central surface of the lens, an increase in thickness of the lens and a flattening of the lens edges. As the tension on the equatorial zonular fibers increase during accommodation, the anterior and posterior zonular fibers relax. The anterior and posterior zonular fibers serve as passive support structures for the lens, but the equatorial zonular fibers determines the refractive power of the lens.

Catenary theory of accommodation (1970) – This theory is also called the Coleman theory of accommodation. It states that the lens and the zonula fibers form a diaphragm, which is held in a catenary (a curve formed by a wire, rope, or chain hanging freely from two points that are not in the same vertical line) shape due to the difference in pressure between the aqueous and vitreous bodies of the lens. A change in diameter of the ciliary body results in a change of the catenary shape. It means there is a continuous pressure difference on the lens. The strength of this pressure difference is approximately 2.3 cm of water column, with major changes occurring during the initial seconds of the accommodation phase. The anterior capsule and the zonular fibers form hammock shaped surface that is duplicable but depends on the diameter of the ciliary body. The ciliary body however makes a shape like the pillars of a suspension bridge, but does not need to support the force around the equator to flatten the lens.

One suggestion for handling distance strain is to balance both near and far accommodation, which is referred sometimes as "dynamic vision" activity.