The Visual Pathway

Key points

• • Light signals leave each eye along the optic nerve, and the two optic nerves meet at the optic chiasma in front of the pituitary gland.
• • At the chiasma the fibres from the nasal (inner) half of each retina cross to the other side, while the temporal (outer) fibres stay on the same side.
• • Because of this partial crossing, each optic tract carries the opposite (contralateral) half of the visual field seen by both eyes.
• • The optic tract reaches the lateral geniculate body of the thalamus, from which the optic radiation fans out to the primary visual cortex in the occipital lobe.
• • Damage to one optic nerve blinds that whole eye, whereas damage at the chiasma, as from a pituitary tumour, knocks out both outer fields, called bitemporal hemianopia.
• • Damage behind the chiasma, in the tract or radiation, causes loss of the same half-field in both eyes, called a homonymous hemianopia.