Vision begins when light waves enter the eye and pass through transparent structures that bend, or refract, the light to focus it precisely onto the retina. The outermost layer is the cornea, a clear dome responsible for the majority of light refraction. Directly behind the cornea lies the aqueous humor, a watery fluid filling the anterior chamber. Light then passes through the pupil, an opening regulated by the iris, and enters the crystalline lens. While the cornea does the heavy lifting of bending light, the lens provides fine-tuning through a process called accommodation. Finally, the light travels through the clear vitreous body that fills the large posterior cavity of the eyeball before striking the light-sensitive neural layer at the back.
The eye continuously adjusts to environmental lighting and the distance of viewed objects to maintain a clear image.
Phototransduction is the conversion of physical light energy into electrical signals within the retina. This process takes place in the specialized outer segments of photoreceptor cells:
In the dark, photoreceptors continuously release glutamate because sodium channels remain open. When light strikes rhodopsin or iodopsin, it bleaches the pigment, closes sodium channels, and hyperpolarizes the cell, which halts the release of inhibitory neurotransmitters and signals the downstream neurons that light has been detected.
Raw signals from rods and cones do not go straight to the brain; they undergo significant initial processing in the retina via bipolar cells and amacrine cells before converging on ganglion cells. The axons of these ganglion cells bundle together to form the optic nerve (cranial nerve II).
At the optic chiasm, nerve fibers from the nasal half of each retina cross over to the opposite side of the brain, while temporal fibers remain uncrossed. This arrangement ensures that visual information from the left visual field of both eyes is processed in the right hemisphere of the brain, and vice versa. The pathway continues through the optic tracts to the lateral geniculate nucleus of the thalamus, and finally via optic radiations to the primary visual cortex in the occipital lobe.
The visual system adjusts dynamically to drastic changes in illumination and distinguishes a vast spectrum of hues through specialized physiological mechanisms.