Below the hypothalamus is the optic chiasm. This is where the optic nerves intersect. This ensures that the visual information from the left visual field is projected to the right hemisphere and the visual information from the right visual field is projected to the left hemisphere.

How exactly does this work? The visual field (field of vision) can be divided into a left visual field and a right visual field. Suppose you look straight ahead, then everything to the left of your nose is in your left visual field. The visual information from this left field then enters both eyes and is projected onto the right part of the retina of both eyes. The visual information from the right field is projected onto the left part of the retina of both eyes. From the retina, the information is transmitted through the optic nerves to the visual cortex. There are four optic nerves, one for the left and right portions of the retina of each eye. Before the optic nerves reach the cortex, two of the four intersect in the optic chiasm. This happens as follows:

The optic nerve of the left part of the retina of the left eye does not cross and projects to the left hemisphere.

The optic nerve of the right part of the retina of the left eye does cross and projects to the right hemisphere.

The optic nerve of the left part of the retina of the right eye does cross and projects to the left hemisphere.

The optic nerve of the right part of the retina of the right eye does not cross and projects to the right hemisphere.

In short, the information from the left visual field is projected onto the right part of the retina and ultimately processed by the visual cortex in the right hemisphere. So it is not the case, as is sometimes thought, that the things you see with the left eye are processed entirely by the right hemisphere.

People with a brain infarct or hemorrhage in the right visual cortex are therefore, as it were, (partially) blind to all the information in the left visual field. This is called hemianopsia. People who are blind in one eye can still perceive information from the left and right visual field, only this field has become smaller.

Author: Bart Aben (translated by Melanie Smekal)