© 2009 The International Eye Trust.

The retina is composed of several layers of nerve cells.The Photoreceptor Layer is composed of Rods (for night vision) and Cones (for finely detailed imaging in the light). The light is absorbed by the rods and cones and begins a process of information signaling that sends signals through a web of nerve cells that are in contact with the rods and cones. The information is directed indirectly, to the Retinal Ganglion Cells (RGC), which ultimately form the Optic Nerve. Light absorbed by the rods sends information to the Bipolar Cells and then to the Amacrine Cells and then to the RGC; whereas, light absorbed by the cones sends information to the bipolar cells and then directly to the RGC. The amacrine cells are axonless cells that modulate the visual information between the rods and cones and the RGC. Another type of nerve cell in the retina is the Horizontal Cell, responsible for lateral inhibition, increasing contrast between the illuminated and the nonilluminated parts of the retina. Horizontal cells allow the rods and cones to "talk" with each other.

For many of us as we get older our vision starts to fade. When light from the world around us passes through the pupil of the eye, it is focussed by the lens onto a light sensitive layer at the back of the eye called the retina.

This thin membrane converts the different light intensities, shapes and colors into an array of electrical signals which then travel along the optic nerve to the visual cortex, the part of the brain which deals with sight. This area then decodes and interprets this complex electrical message into the vision we experience in our eyes. As we age the retina can be subject to degenerative defects with fewer and fewer of the light receptors in the retina firing correctly or at all, so much of the signal is lost. This can result in diminished vision and even blindness.

The human eye

On the retina, lies a highly concentrated section called the Macula, which has the highest visual acuity. At the center of the macula, lies a small indentation called the Fovea. The fovea is approximately 1.5 mm in diameter. It contains the highest density of cones and very few rods. The axons from the RGC in the fovea, lead to form the optic nerve at the Optic Disk. This is where the optic nerves lead into the visual system of the brain.
The RGC forms the optic nerve, which carries the signaling to the Visual System of the brain. The signals are processed by the Lateral Geniculate Nucleus (LGN), a sort of "processing" station in the brain. From the LGN the signals are sent on to the visual system where they are processed further and then sent to higher Cortical area.