06/08/2016

How Your Sense of Sight Works

Your vision has a prominent and unique place among your senses. The human brain biases vision over every other sense, doing everything it can to preserve your sense of sight. This is likely because visual distortion fundamentally changes the way we experience and interact with the world around us.

While vision is perceived through the eyes, it is directly connected to our brains through an extension of the brain called the optic nerves. The optic nerves enter the eyes and fan out in the back to form an area called the retina. The retina is where the eye’s visual receptors are located. Our visual receptors are called rods and cones—so named for the fact that they are rod-shaped and cone-shaped.

(As a sidenote, this is one of the few instances where a neurological term actually matches what it is describing).

Cones perceive color and fine details, while rods are sensitive to light, allowing us to see in the dark to a limited extent. Genetic disorders that affect our ability to see color—or “colorblindness”—are a result of poorly functioning cones. Colorblindness comes in 3 types, corresponding to the 3 varieties of cones that perceive colors: red, green, and blue.

DISORDERS OF THE EYE & OPTIC NERVES

Disorders which degenerate cones of all types can lead to light sensitivity, as cones are receptors that process better in the presence of bright light. These disorders are rare, and are likely the result of genetics rather than a brain injury. Genetic disorders that affect both rods and cones cause actual blindness.

Optic nerves are most often affected by demyelinating disorders, like multiple sclerosis. Demyelinating disorders damage the protecting sheath around nerves, impairing their ability to conduct impulses. Another optic nerve disorder is optic neuritis (inflammation of the nerve), which impairs vision.

MOVING FURTHER INTO THE BRAIN

As we enter the midbrain through the optic nerve, we reach the Edinger-Westphal nucleus. This group of neurons is responsible for controlling pupil size—which determines how much light is received and processed by the retina.

It’s also at this point that the binocular vision of the eyes crosses over to the side of the brain where it is processed. For both eyes, the field of vision is processed on a different side from which it is perceived. Put simply, things you see on your left are processed on the right side of the brain and vice versa.

To clarify, this is for both eyes —where visual stimuli is represented is dictated by your field of vision, not by “which eye” saw it. This is why it’s called “binocular vision,” as your two eyes form a single picture.

Past the Edinger-Westphal nucleus, visual signals travel to the occipital lobe through bundles of nerve fibers connected to the back of your brain, or “optic radiations.” The visual information is “viewed” as inverted by the occipital lobe. From here, your brain has two “streams,” or processing pathways: the dorsal (“where”) stream and the ventral (“what”) stream. Visual signals travel through both streams from the occipital lobe, where meaning is assigned to what you’re seeing.

The dorsal stream, for instance, projects information to the parietal lobe. The brain interprets the stimuli to understand where it is located in 3D space—that’s why the dorsal stream is known as the “where” stream. The ventral stream projects information to the temporal lobe, where the brain processes memory, word recall, and identification. This allows you to identify what you’re seeing—ergo, the “what” stream.

HOW COLORS ARE PERCEIVED BY YOUR BRAIN

If you remember from our last post on the sense of hearing, different notes are represented in different parts of the brain. Similarly, different colors are represented in different parts of the brain. Red, orange, and yellow colors stimulate the left brain more than the right. Blue, indigo, and violet colors stimulate the right brain over the left. Green colors, however, are equally represented in both sides of the brain.

Why does this matter?

Because certain visual stimuli corresponds to certain parts of the brain, our ReceptorBased® rehabilitative therapists can employ colored light stimulation to focus on specific areas of dysfunction. The way the brain processes color allows us to address areas of brain performance with precision and accuracy.

THE “BLIND SPOT”

Where the optic nerve enters the eye, there is a small area where there are no rods and cones. We don’t perceive this part of our vision because areas of the integration centers of the brain will fill in the lack of vision automatically. The brain function employed to fill in the missing area of our vision is known by neurologists as the “blind spot.” The size of the blind spot can vary—our specialists, as well as ophthalmologists and optometrists, can measure the size of a person’s blind spot with non-invasive means.

Here’s how:

The size of your blind spot will depend on how much power your brain devotes to filling in the missing visual information. Research from Dr. Carrick and others have found that the size of a person’s blind spot corresponds to a decrease in activity on the matching side of the brain.1

Monitoring that decrease in brain function yields a way of diagnosing the size of a patient’s blind spot. In turn, this allows our specialists to provide visual therapies to shrink the blind spot and strengthen brain function. As you can see, vision depends as much on the brain’s health as it does on your eyes.

OVERLAP WITH OTHER SENSES

Like hearing, vision overlaps with the vestibular system to contribute to your sense of balance. If you’ve ever been on a Universal Studios ride involving a movie screen, you’ve experienced this phenomenon. Even when riders are stationary, the movement on the screen can stimulate a sense of vertigo, movement, or imbalance.

Using the same processes, we can use visual stimuli to affect vestibular processing without engaging the whole system directly. This is particularly beneficial for patients who are highly sensitive to vestibular stimulation. Using visual stimulation or virtual reality to engage the vestibular system is like taking a “back door,” allowing us to improve function without direct vestibular stimulus.

If you have balance issues, vision issues rooted in neurological disorders, or your patient’s neurological problems are not responding to traditional therapies, consult our ReceptorBased® rehabilitative specialists. Our therapies, taught by the Carrick Institute, are able to target specific regions of the brain to restore or improve function quickly and effectively.

1Carrick, FR. “Changes in brain function after manipulation of the cervical spine.” Journal of Manipulative and Physiological Therapeutics. 1997;20:529–545.