Researchers have rapidly taken advantage of adaptive optics (AO) technology for the study of the human visual system. Vision, the primary human sense, begins with light entering the eye and the formation of an image on the retina (Fig 1), where light is transformed into electro-chemical impulses that travel towards the brain. The eye provides the only direct view of the central nervous system and is, therefore, subject of intense interest as a means to the early detection of retinal and systemic diseases. However, ocular aberrations limit the optical quality of the human eye, thus reducing image contrast and resolution. With the use of AO, it is now routinely possible to compensate for these ocular aberrations and image cellular level structures such as cone and rod photoreceptors, smaller foveal cones, retinal pigment epithelium (RPE) cells, leukocyte blood cells and the smallest retinal blood vessels in vivo and without the aid of contrast enhancing agents.

The Human Eye and Visual System

The human eye behaves as a complex optical structure sensitive to wavelengths between 380 and 760 nm.  Light entering the eye is refracted as it passes from air through the tear film-cornea interface.  It then travels through the aqueous humor and the pupil (a diaphragm controlled by the iris) and is further refracted by the crystalline lens before passing through the vitreous humor and impinging on the retina (Fig 1).  The tear film-cornea interface and the crystalline lens are the major refractive components in the eye and act together as a compound lens to project an inverted image onto the light sensitive retina.  From the retina, the electrical signals are transmitted to the visual cortex via the optic nerve (Fig 1).

Figure 1. Gross anatomy of the human eye and detail of the retina. The major refractive elements are shown. Primary sources of aberration are the tear film – cornea interface and the crystalline lens.  The incident light on the retina is absorbed by the cone and rod photoreceptors after traversing several retinal layers.  Image modified from the National Eye Institute, National Institutes of Health.


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