The accommodation mechanism—that is, the mechanism that focuses the lens system of the eye—is essential for a high degree of visual acuity. Accommodation results from contraction or relaxation of the eye ciliary muscle. Contraction causes increased refractive power of the lens, as explained in Chapter 50, and relaxation causes decreased power. How does a person adjust accommodation to keep the eyes in focus all the time?
Accommodation of the lens is regulated by a negative feedback mechanism that automatically adjusts the refractive power of the lens to achieve the highest degree of visual acuity. When the eyes have been focused on some far object and must then suddenly focus on a near object, the lens usually accommodates for best acuity of vision within less than 1 second. Although the precise control mechanism that causes this rapid and accurate focusing of the eye is unclear, the following features are known.
First, when the eyes suddenly change distance of the fixation point, the lens changes its strength in the proper direction to achieve a new state of focus within a fraction of a second. Second, different types of clues help to change the lens strength in the proper direction:
1. Chromatic aberration appears to be important. That is, red light rays focus slightly posteriorly to blue light rays because the lens bends blue rays more than red rays. The eyes appear to be able to detect which of these two types of rays is in better focus, and this clue relays information to the accommodation mechanism with regard to whether to make the lens stronger or weaker.
2. When the eyes fixate on a near object, the eyes must converge. The neural mechanisms for convergence cause a simultaneous signal to strengthen the lens of the eye.
3. Because the fovea lies in a hollowed-out depression that is slightly deeper than the remainder of the retina, the clarity of focus in the depth of the fovea is different from the clarity of focus on the edges. This difference may also give clues about which way the strength of the lens needs to be changed.
4. The degree of accommodation of the lens oscillates slightly all the time at a frequency up to twice per second. The visual image becomes clearer when the oscillation of the lens strength is changing in the appropriate direction and becomes poorer when the lens strength is changing in the wrong direction. This could give a rapid clue as to which way the strength of the lens needs to change to provide appropriate focus.
The brain cortical areas that control accommodation closely parallel those that control fixation movements of the eyes. Analysis of the visual signals in Brodmann’s cortical areas 18 and 19 and transmission of motor signals to the ciliary muscle occur through the pretectal area in the brain stem, then through the Edinger-Westphal nucleus, and finally by way of parasympathetic nerve fibers to the eyes
