Visual Pathways

The human eye has many of the features of a camera, starting with a lens to focus light. The cornea of the eye bends light rays and is primarily responsible for forming the image on the retina, the receptive surface inside the back of the eye. The visual image focused on the retina is inverted top to bottom and reversed right to left.

The part of the world that you can see without moving your head or eyes is called your visual field. Each eye sees only a portion of this visual field. The visual field can be divided into right and left visual hemifields. The right visual hemifield is seen by the temporal left retina and nasal right retina, while the left visual hemifield is seen by the nasal left retina and the temporal right retina.

The visual field can also be divided vertically into superior and inferior divisions. Note that the visual fields of both eyes overlap extensively in the central portion of each visual hemifield. This region defines the binocular field of view. Vision in the periphery of the field of view is strictly monocular, mediated by the most medial portion of the nasal retina.

The center of the visual field projects its image onto the foveal region of the retina. On the retina, the highly-specialized fovea has a dense concentration of small-diameter cones and a one-to-one relationship between the cones and the bipolar and ganglion cells. Visual acuity is therefore especially high in this region.

Now let's examine the pathways from the retina into the brain. This diagram shows a view of the base of the brain, as seen from below. Note that in this view, the position of the two eyes is reversed.

The axons of retinal ganglion cells exit the retina via the optic nerve. The optic nerve exits the eye in a region called the optic disc. Because there are no receptors in this region, nothing can be seen in the corresponding part of the visual field. This "blind spot" does not appear as a dark spot; rather, it is simply a region from which we cannot obtain visual information.

The optic nerves cross at the optic chiasm, which is located just anterior to the stalk of the pituitary gland. In humans, axons from the nasal retina cross over to the opposite side of the brain. Axons from the temporal retina project to their own side of the brain.

After they pass the optic chiasm, the axons of the retinal ganglion cells are known collectively as the optic tract. Note that information from the left visual field is carried in the right optic tract, and information from the right visual field is carried in the left optic tract.

The vast majority of axons of the optic tract terminate in the lateral geniculate nucleus (LGN), which is the visual part of the thalamus. However, the axons of retinal ganglion cells also project to several other brain regions.

Some axons of retinal ganglion cells extend to the superior colliculi, a paired structure on the roof of the midbrain. The superior colliculi help coordinate rapid movements of the eyes toward a target.

Small bundles of optic-tract axons also project to the suprachiasmatic nucleus in the hypothalamus. Cells in the suprachiasmatic nucleus are involved in the control of daily (circadian) behavioral rhythms related to the light–dark cycle.

Finally, optic-tract axons from still other ganglion cells project to a collection of neurons that lies between the thalamus and the midbrain in a region known as the pretectum. This region is important as the coordinating center for the pupillary light reflex (that is, the reduction in the diameter of the pupil that occurs when sufficient light falls on the retina).

The lateral geniculate nucleus serves as the primary relay nucleus for visual processing by the cerebral cortex. The right lateral geniculate receives information from the left visual field (the nasal left retina and temporal right retina), while the left lateral geniculate receives information from the right visual field (the nasal right retina and the temporal left retina).

The spatial relationships among the ganglion cells in the retina are maintained in their targets as orderly representations or "maps" of visual space. The primate lateral geniculate nucleus has six layers, and inputs from the two eyes are maintained in separate layers within the LGN.

From the LGN, visual information is relayed to the visual cortex. Most of the axons from LGN neurons form the optic radiations, which terminate in the visual areas in the occipital cortex at the back of the brain.

In this lateral view of the brain, we see that axons carrying information about the superior portion of the visual field sweep around the lateral horn of the ventricle in the temporal lobe (a branch called Meyer's loop) before reaching the occipital lobe. Axons carrying information about the inferior portion of the visual field travel in the parietal lobe.

The topographic order of visual information is maintained in the visual cortex. The fovea is represented in the posterior part of the visual cortex, whereas more peripheral regions of the retina are represented in progressively more anterior regions. Note that the area of central vision—the fovea—is represented over an especially large part of the visual cortex.

Inputs from the two eyes converge at the cortical level, making binocular effects possible. The primary visual cortex (V1 or striate cortex) projects to other areas of the cerebral cortex (referred to as "extrastriate") that are involved in complex visual perception.

The dorsal stream, which includes the middle temporal area, leads from the striate cortex into the parietal lobe. This system is thought to be responsible for spatial aspects of vision, such as the analysis of motion, and positional relationships between objects in the visual scene.

Another system, the ventral stream, leads from the striate cortex into the inferior part of the temporal lobe. This system is thought to be responsible for high-resolution form vision and object recognition.