Chapter 3. Visual Pathways from Eye to Brain

Light from the left half of the visual field strikes the right half of each retina, while light from the right half of the visual field strikes the left half of each retina. Within each retina, this light is transduced into neural signals in the 1 million axons of the retinal ganglion cells (RGCs) that emerge from the back of each eye to form the optic nerve.

The left and right optic nerves travel only a few centimeters until they meet at the optic chiasm, where the optic nerve from each eye splits in half. The axons from the RGCs in the right half of the right retina and the right half of the left retina (i.e., from the right temporal retina and the left nasal retina) combine into the right optic tract, which continues into the right hemisphere of the brain. The axons from the RGCs in the left half of the right retina and the left half of the left retina (i.e., from the left temporal retina and the right nasal retina) combine into the left optic tract, which continues into the left hemisphere.

Thus, as a consequence of the splitting of the optic nerves and their recombining into the optic tracts, neural signals carrying information from the left visual field go to the right hemisphere of the brain, while signals carrying information from the right visual field go to the left hemisphere. This is referred to as the contralateral representation of visual space.

Within each hemisphere, about 90% of the axons in the optic tract go to the lateral geniculate nucleus, which sends signals to the primary visual cortex (area V1) via the optic radiations; most of the other axons in the optic tract branch off to the superior colliculus.