Background

Approximately 1 million retinal ganglion cells form the optic nerve of each eye. The optic nerve of the left eye and the optic nerve of the right eye meet just a couple of centimeters behind the eye in an area called the optic chiasm. It is so-called because it looks like a capital X which is also the shape of the ancient Greek letter chi. Here at the optic chiasm, your visual system does one of its most interesting tricks, which is also one of the most difficult aspects of visual anatomy for people new to the area to keep straight. So again, we warn you to pay close attention and review this information multiple times.

The optic nerve from each eye splits in half at the optic chiasm. The axons from the ganglion cells from the right half of the right retina and the ganglion cells from the right half of the left retina combine, forming the right optic tract, which then proceeds to the right hemisphere of the brain. Axons from the ganglion cells in the left half of the right retina and the left half of the left retina combine, forming the left optic tract, which then proceeds to the left hemisphere of the brain.

To keep track of the terminology used here, the half of the retina toward the nose is called the nasal retina. That is the left half of the right retina and the right half of the left retina. The half of the retina nearest your temples is called the temporal retina. That is the right half of the right retina and the left half of the left retina. The simple rule at the optic chasm is that nerves from the nasal retina cross and nerves from the temporal retina do not cross.

Use this activity to explore and highlight the pathways light follows from entering the eye all the way to the lateral geniculate nucleus (LGN).

Instructions

Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab

On the Illustration tab, you can stimulate a region of the retina and see the effect on the hypercolumns shown.

Settings

Below is a list of the ways that you can alter the illustration. The settings include the following:

Left World: trace how light coming from the left half of the world travels to the lateral geniculate nucleus (LGN).
Right World: trace how light coming from the right half of the world travels to the LGN.
Left Eye: trace how light entering the left eye travels to the LGN.
Right Eye: trace how light entering the right eye travels to the LGN.
Nasal Retina: trace how light falling on the nasal half, toward the nose, of each eye's retina travels to the LGN.
Temporal Retina: trace how light falling on the temporal half, toward the temples, of each eye's retina travels to the LGN.
All: Highlight all of the pathways that light follows to the LGN.
None: turn off all highlighting.
Left LGN: trace the pathways light follows in getting to the left LGN.
Right LGN: trace the pathways light follows in getting to the right LGN.

Reset

Pressing this button restores the settings to their default values.

Try these settings explore pathways from eye to LGN

You can highlight different parts of the pathways that carry light from the eyes to the lateral geniculate nuclei.

On the left of the screen, the person is looking at the plus (+) sign. It falls on the foveae of both eyes. Light is upside down and backward, so the light from the left half of the world falls on the right half of the retina of both eyes. Follow the arrows. Red is for what comes from the right half of the world and green for left. The two retinas are, for illustration purposes, divided at the fovea. The nasal retinas are toward the middle, where the nose is, and the temporal retinas are toward the outsides.