R Lang & Associates

The eyes appear so closely connected to the brain that they might as well be considered one and the same with the brain.

Many recognize eyesight as the primary sense and that our visual system is known to be very good with remembering images. It is said that if you hear a piece of information, after three days you will remember 10% of it, but if you add a picture you will remember 65%. Of all the senses, vision provides the most extensive information and detail about the environment. In primates, and the higher animals including birds, the visual areas of the central nervous system have developed far beyond other sensory systems.

Even though accumulated knowledge of the brain is quite detailed, much of the brain still remains a mystery.

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Since the retina is derived from the neural tube, it is considered part of the central nervous system.

The sense of vision begins when a light ray enters the Cornea and Lens of the eye and strikes the Retina. Just like a camera, the image is reversed on the retina. From here electrical signals are produced and sent through the Optic Nerve to other parts of the brain which process the image and allow us to see.

After light rays enter the cornea they pass through a small chamber of fluid called the Aqueous Humor, pass through the pupil and the lens, then through a much larger chamber of fluid called the Vitreous Humor, and finally striking the light sensitive retina. In a most basic concept, the retina converts light into electricity similar to a photovoltaic, or solar cell. But the retina contains much more complex features.

Since the retina is derived from the neural tube, it is considered part of the central nervous system. The retina contains five types of neurons, one of which is the visual receptor cells, or rods and cones. The rods are found mostly in the peripheral region of the retina and are related to night vision. The cones are found in the central area of the retina, called the Fovea, and are responsible for sharpness and discrimination of color. This is also referred to as Scotoptic and Photopic vision, whereas humans have both, some animals may only have scotoptic (night vision) or photopic (day vision).

Visual processing begins in the retina. Light energy produces chemical changes in turn producing electrical energy. The optic nerve of each eye meets the other at the optic chiasm where medial nerves of each cross to the other brain hemisphere but lateral nerves stay on the same side. The overlap allows for depth perception. The separate-but-interconnected right and left hemispheres of the brain imply independent activity for the right and left side of a human but also allow coordinated activity between the right and left. This is how monovision is accomplished.

Electrical impulses are then communicated to the visual cortex of the brain which makes sense of the electrical signals, and either store the information for future reference or send a message to a motor area for action.

There are actually three layers of cells in the retina. The first layer of rods and cones can be subcategorized into three types depending on how they respond to red, green, and blue light; and in combination allow us to see all the various colors. The second layer of the retina is a network of interneurons and the third layer are the ganglion cells. The neurons in these two layers exhibit complex receptive fields which detect contrast changes which might indicate edges or shadows.

The optic nerve primarily routes information via the thalamus to the visual cortex, an area of the more major cerebral cortex. Although this is where visual perception occurs, nerves also carry information regarding the mechanics of vision to two sites in the brainstem. The first of these is called the pretectum which controls pupil size in response to light intensity. The other site in the brainstem is the superior colliculus which concerns moving targets and information governing scanning of the eyes. It is responsible for moving the eyes in short jumps, or saccades, which allow the brain to perceive a smooth scan by stitching together a series of relative still images. This explains why there isn’t blurring as one pans across a landscape or as one scans objects across a room.

Most projections from the retina travel from the optic nerve to a part of the thalamus called the lateral geniculate nucleus (LGN), deep in the center of the brain. Here the LGN separates inputs into two parallel streams, one containing color and fine structure, and the other contrast and motion. The LGN is also a layered structure, the top four of six composed of cells that process color and fine structure called the parvocellular layer (small cells); and the bottom two layers, the magnocellular layers (large cells) which process contrast and motion. These layers project all the way back to the brain to the primary visual cortex.

The cells in the primary visual cortex (or V1) are ordered within a point-to-point mapping corresponding to the retina, which means neighboring areas in the retina correspond to neighboring areas in V1. This allows the brain to position objects in two dimensions, often referred to by a horizontal and vertical axis. The third dimension, depth, is mapped by comparing signals from both eyes. These signals are processed in a checkerboard pattern of connections alternating between right and left eye called ocular dominance columns, and allow the position of objects relative to each eye to be calculated by triangulation. Lastly, there are orientation columns in V1, stacks of cells activated by lines of a given orientation, which allow detection of edges on objects and begin the complex task of visual recognition.^†

The secondary visual cortex (or V2) is mainly responsible for the phenomenon of color constancy, which explains how a red apple still looks red even under different colors of illumination. Color constancy is thought to occur because V2 can compare an object to the ambient illumination and subtract out the estimated illumination color. This process, however, can be strongly influenced by what color the viewer expects the object to be.

Although adaptation to light and dark is partly controlled by the pupil, an important part of this adaptation is in the retina. The rods and cones of the retina have pigments which consist of protein and Vitamin A. Vitamin A helps give the pigments their color and the color enables the pigments to absorb light. When light bleaches out the color in the pigments, it generates an electrical signal that the optic nerve sends to the brain. Vitamin A then moves into a part of the retina called the retinal pigmented epithelium where the vitamin regains its original chemical structure and returns to the rods and cones. It rejoins with protein molecules to form fresh pigments. The renewal of the pigment that enables the eye to see in dim light – rhodopsin - occurs largely in the dark. After being exposed to bright light (a bright day, for instance) the eyes cannot see in dim light (e.g., a darkened movie theater) because of the bleached rhodopsin. It takes 10 to 30 minutes for the rhodopsin to be renewed. This is the time it takes the eyes to become accustomed to the dark.

Six muscles outside the eye govern its movements - the inferior, medial, lateral, and superior recti - and the superior and inferior oblique muscles. These are attached to the sclera, the opaque or white portion of the eye. However, the superior oblique first passes through a fibrous ring, the trochlea, attached to the frontal bone which acts as a type of pulley. The arrangement of these lateral and oblique muscles – known as the extraocular muscles – offer just about every type of movement needed for human vision.

As we review the sense of vision, it appears quite amazing the capabilities of the brain. Even though accumulated knowledge of the brain is quite detailed, much of the brain still remains a mystery. Some like to match the brain to our most sophisticated machinery, comparing it for example, to a large supercomputer. It is reminded that although the brain is unable to keep up with the lightning speed of a supercomputer, it is man’s brain that created his machines.

^† The columnar organization of the visual cortex was first described by David Hubel and Torsten Wiesel resulting in the 1981 Nobel Prize (also shared with Roger W. Sperry for his independent research on the cerebral hemispheres).

1. Ability Path, How the Eye and the Brain Work Together (Blind Babies Foundation, Oakland, CA).
2. Brain Rules, John Medina, Pear Press, 2014.
3. BrainHQ (Posit Science), How Vision Works, 2014.
4. Neuroscience (Chap 14 and 15), Valentin Dragoi, University of Texas Medical School, Houston.
5. Vision, Encyclopedia of Science and Technology, McGraw-Hill, 11^th Ed.
6. Eye, World Book Encyclopedia, Scott Fetzer-Chicago, 2013.
7. Sensory Reception, The New Encyclopedia Britannica, 15^th Ed.