Models of Accommodation

The ability to see clearly at different distances is one of the most important functions of the human visual system. This is performed routinely and effortlessly in daily life by the process called accommodation. During this process, the accommodation system must sense that a new target is defocused beyond a blur threshold, develop the appropriate neurological control signal based on blur magnitude, and then adjust relatively rapidly the optics of the eye via the ciliary muscle until the target is once again in focus. Thus, it involves feedback regulation of visual optics via the sensing of retinal image blur. In addition, since blur per se does not provide the light vergence direction (Stark, 1968), the accommodation system must use perceptual cues and other sources of information to determine the appropriate direction of focus (Ciuffreda, 1991, 1998). It does this remarkably well, so that rarely does accommodation occur in the wrong direction under natural viewing conditions. Moreover, accommodation takes place repeated in daily life, so that the system must be continually available to provide clear vision in the performance of a variety of tasks at a range of different distances.