Study topic · about 9 minutes

The visual pathway: from the eye to the brain

Vision starts in the retina, where rods and cones turn light into nerve signals. At the optic chiasm, about half the fibers of each optic nerve cross, so each hemisphere sees the opposite half of the world. The signals relay in the lateral geniculate nucleus of the thalamus, then the optic radiation carries them to the primary visual cortex. The fibers are sorted so neatly that damage at each step leaves its own pattern of lost vision.

Follow the visual pathway in 3D

It starts in the retina

The retina is a thin sheet of nerve tissue that lines the back of the eye. The cornea and lens focus the scene onto it upside down and reversed left to right, and the brain sorts this out later. Light-sensing cells called photoreceptors turn the light into electrical signals. They come in two kinds: rods and cones.

The fovea is a small pit at the center of the retina, packed with cones and free of rods. Whatever you look at directly lands there, which is why the center of your vision is so sharp. Away from the center, rods take over, which is why a faint star is easier to see out of the corner of your eye.

Rods and cones pass their signals, through other cells in the retina, to retinal ganglion cells. The axons of these cells, about a million from each eye, gather at the optic disc and leave the eye as the optic nerve. The optic disc has no rods or cones, so it makes a blind spot. You never notice it, because the brain fills in the gap and the other eye covers it.

The pathway, step by step

1. Optic nerve. Each optic nerve carries the signals from one eye through a bony canal into the skull. Strictly speaking, it is a tract of the brain, not an ordinary nerve.
2. Optic chiasm. The two optic nerves meet in an X just above the pituitary gland. Fibers from the nasal half of each retina, the half nearer the nose, cross to the other side. Fibers from the temporal half, nearer the temple, stay on their own side.
3. Optic tract. Behind the chiasm, each optic tract carries the opposite half of the visual field, from both eyes. Most of its fibers end in the thalamus.
4. Lateral geniculate nucleus (LGN). This relay station in the thalamus has six layers, and each layer receives input from one eye only. It passes the signals on toward the cortex.
5. Optic radiation. From the LGN, the optic radiation fans back through the white matter. Its lower fibers, which carry the upper part of the visual field, first loop forward into the temporal lobe. This detour is called Meyer's loop. The upper fibers, carrying the lower part of the field, run back through the parietal lobe.
6. Primary visual cortex (V1). The fibers end in V1, which lines the calcarine sulcus, a deep groove on the inner surface of the occipital lobe. It is also called the striate cortex, or Brodmann area 17.

Why does each side of the brain see the opposite half of the world? Light from your left visual field lands on the right half of each retina. That is the nasal half of the left eye and the temporal half of the right eye. The nasal fibers cross at the chiasm and the temporal fibers do not, so both reach the right hemisphere.

Each eye still sees both halves of the world. It is the visual field, not the eye, that is split between the hemispheres. Split-brain studies used this to show a picture to just one hemisphere: see left brain and right brain.

Follow the pathway in the Seeing lesson

The visual cortex and its upside-down map

The primary visual cortex holds a point-by-point map of the visual field: neighboring points in the world land on neighboring points of cortex. The fovea, a tiny spot in the eye, claims a large share of the map, which is why central vision is so detailed. Many cells here respond best to edges tilted at one angle, the first step toward seeing shapes.

The map is flipped. The calcarine sulcus has an upper bank, on the cuneus, and a lower bank, on the lingual gyrus. The upper bank sees the lower half of the opposite visual field, and the lower bank sees the upper half. The same rule runs through the optic radiation.

Visual field defects: finding the damage

Because the fibers are sorted so neatly, the pattern of lost vision points to where the damage is. Doctors map it with a visual field test: you look at a central point and report lights that appear around the edges.

The names sound harder than they are. Hemianopia means blindness in half of the visual field. Bitemporal means both outer halves, on the temple side. Homonymous means the same side in both eyes, and a quadrantanopia is the loss of a quarter.

A pituitary tumor grows up from below and squeezes the middle of the chiasm, where the crossing fibers run. Those fibers carry the outer half of each eye's view, so side vision fades on both sides. It often fades so slowly that people notice only when they start bumping into things. Read more about the gland in hormones and the brain.

A stroke of the posterior cerebral artery often leaves the center of vision intact, which is called macular sparing. The usual explanation is that the tip of the occipital lobe, where central vision is mapped, often also gets blood from the middle cerebral artery. Read more in blood supply of the brain.

Beyond V1: the "what" and "where" streams

From V1, signals spread to nearby visual areas that add color, motion and depth. Then the work splits into two broad streams: a ventral "what" stream and a dorsal "where" stream, also called the "how" stream.

The two streams work as a team. When you pick up a mug, the ventral stream knows it is your mug, and the dorsal stream steers your hand to the handle.

Prosopagnosia, or face blindness, is trouble recognizing familiar faces, sometimes even your own, while eyesight is normal. It can follow damage to the fusiform gyrus, for example from a stroke, or be present from birth. People with it often recognize others by their voice, hair or walk.

Reflexes: the pupil and the superior colliculus

Not every fiber in the optic tract goes to the thalamus. A small share goes to the midbrain, which runs fast, automatic responses to light. This is the path of the pupillary light reflex:

1. Light hits the retina, and the signal travels along the optic nerve and optic tract.
2. Some fibers leave the tract before the thalamus and end in the pretectal area of the midbrain.
3. Each pretectal area signals the Edinger-Westphal nucleus on both sides. This is where the pupil-shrinking fibers of the oculomotor nerve (cranial nerve III) begin.
4. These fibers relay in the ciliary ganglion, a small cluster of nerve cells behind the eye. From there, nerves tighten the ring-shaped muscle of the iris, and the pupil shrinks.

Because each side of the midbrain signals both eyes, light in one eye shrinks both pupils. The lit eye shows the direct response and the other eye the consensual response. So if one optic nerve is cut, light in that eye moves neither pupil, while light in the healthy eye still shrinks both. If one oculomotor nerve is damaged instead, that pupil stays wide whichever eye is lit.

The superior colliculus, a small bump on the back of the midbrain, also gets fibers straight from the eye. It turns your eyes and head toward a sudden flash or movement, often before you know what caught your eye. It helps launch the quick eye jumps called saccades, through eye-movement centers in the brainstem.

A few fibers also reach the body clock in the hypothalamus, so daylight can set your daily rhythm. Read more in sleep and the brain.

Memory tricks

In these study guides

• MCATSensing the environment
• AP Psychology1.6 Sensation
• Intro psychologySensation (sections 5.3 to 5.5)
• Anatomy and physiologyThe somatic nervous system (chapter 14)
• Medical schoolSensory systems and the thalamus
• GCSE BiologyThe eye and seeing (4.5.2.3, Biology only)

Take it further in 3D

• SeeingFrom the eye to the back of the brain. A short guided lesson in the 3D app.

Sources

1. Neuroanatomy, Visual Pathway, StatPearls Publishing (NCBI Bookshelf)
2. Neuroanatomy, Pupillary Light Reflexes and Pathway, StatPearls Publishing (NCBI Bookshelf)
3. Anatomy and Physiology 2e, 14.2 Central Processing, OpenStax

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