The Visual Pathway to the Brain
The photoreceptive cells in the retina (rods and cones) convert the light energy to nerve impulses which are sent via nerve fibers in the retina to the optic nerve, and through several other nerve connections in the brain (lateral geniculate body to the optic radiations to the cortical center of the occipital lobe located at the posterior cerebral cortex of the brain). The nasal nerve fibers of the optic nerve of each eye cross at a point just behind the pituitary gland known as the optic chiasm. The temporal fibers of the optic nerve do not cross. The result is that each side of the brain gets part of the picture from both eyes. In the diagram to the right one can see how the visual fields perceived by each eye (the red and green ovals at the top) are passed by nerve impulses from the retina through the optic nerves, optic chiasm, optic tract, lateral geniculate body to the visual cortical center of the brain. The light from the temporal fields (In green) falls on the nasal retina and the light from nasal visual fields (in red) falls on the temporal retina of each eye. By following these color-coated pathways we can see where the final destination of the nerve impulses project the light image. When visual field testing is done in an optometrist's office, a defect picked up in this noninvasive test can indicate where in the visual pathway the pathology lies, although the cause (tumor, blood clot, hemorrhage) would not necessarily be known.