The Process of Vision
Image: Light is captured by photoreceptors at the back of the retina, and electrical signals are sent forward to retinal ganglion cells that link to the brain.
In a normal eye, light is focused on the retina by the lens. It passes through the clear jelly of the eye and through the nerves at the front of the retina to be captured at the base of the retina in specialized nerve cells called photoreceptors. There are two distinct types of photoreceptors: cone photoreceptors that primarily capture information about colours and fine details, and rod photoreceptors which capture information about movement and are responsible for peripheral and night vision. The photoreceptors convert light into electrical impulses, which are then sent forward through the network of nerve cells at the front of the retina.
The foremost nerve cells, called retinal ganglion cells, are extremely long slender cells. The heads of these cells make up the front layer of the retina, while the long “tails” of the cells (called axons) come together to form the optic nerve which passes back through the retina and out the rear of the eye. The optic nerve is comprised of the axons of over one million retinal ganglion cells.
In the brain, electrical signals from the ganglion cells are interpreted into shapes, colours and motion. The brain sends signals back through cranial nerves to direct the movements of the eye. Voluntary movements allow an individual to focus on specific details, but the eye also makes rapid involuntary motions. These movements, called saccades, are unconscious, but essential to the process of vision. These movements vary the stimuli received by the photoreceptors and the ganglion cells, allowing the cells to function properly.