The Eyes Have It
By Ann Constantino,
Published in the Humboldt Independent on May 18, 2021
“I want all my senses engaged. Let me absorb the world’s variety and uniqueness.” — Maya Angelou
This is the first in a series of articles on the human senses.
Of the five senses we humans possess, it’s likely that the most important one to many of us is sight. We rely on it to bring crucial information to the brain, and we enjoy using it to see beauty in nature and art, as well as to see our loved ones, whether in person or on Zoom. It’s hard to imagine being without our precious vision and fortunately, we can delay many diseases or age-related deterioration with glasses, surgeries, and other treatments. Nevertheless, many of our modern habits, including poor diet and too much screen time, threaten to shorten the lifespan of good vision.
The human eye
The pupil enlarges or contracts depending on how much light is passing through it. A lot of light produces a small pupil, dim light enlarges it.
Humans have what is called a “camera-type eye”, meaning that light is focused through a lens onto a light-sensitive membrane, just as in a camera. The cornea is the first structure light engages at the very front of the eye and does some of the focusing.
Next, the light passes through the pupil and onto a second lens for more focusing. The pupil is the black center of the iris or colored part of the eye. The pupil enlarges or contracts depending on how much light is passing through it. A lot of light produces a small pupil, dim light enlarges it. The size of the pupil is controlled by the pupillary sphincter muscle.
The crystalline lens, through which the light next passes, must also change its shape depending on the distance of what is being viewed. Ciliary muscles change the shape of the lens depending on how far away you are focusing, flattening it for distant objects, and thickening it for close-up vision.
The light then passes through a jelly-like substance called the vitreous humor, which helps maintain the spherical shape of the eye and ensures the light falls onto the retina correctly. The retina interprets the light and prepares a message to be sent to the brain via the optic nerve.
Embedded in the retina are millions of light-sensitive cells, known as rods and cones. Rods interpret light and are useful in poor light and are excellent at motion detection. Cones interpret color and create a clear, sharp focus in bright light.
There are different types of cones and generally, the variety of cones determines the variety of colors an animal can see. Humans possess more cones than many animals, but not as many as birds who have as many as five types, enabling them to see colors humans cannot. Most animals with camera-type eyes have a ratio of cones to rods that suits their niche in the natural world.
Switching to seeing with your rods, which are most useful in dim light, is the reason it takes a moment after turning off the lights for your eyes to adjust. The rods provide enhanced peripheral vision and motion detection, suiting nocturnal creatures, or humans who happen to be PG&E customers.
After processing by the rods and cones, an electrical signal is picked up by the optic nerve of each eye, which delivers it to a part of the brain known as the visual cortex, but not before coming to a crossroads with the optic nerve from the other eye.
At this crossroads, known as the optic chiasm, some of the nerve fibers from each eye cross and some do not. Evolutionary scientists believe this to be a major advancement in vision, accounting for binocular vision and hand-eye coordination.
Information from one side of your field of vision is split at the optic chiasm, sending the information from the left side of the field of view to the right side of the visual cortex and vice versa. This arrangement particularly suits animals with eyes in the front of their head. Birds and fish, who mostly have eyes on the side of the head, are able to use their eyes somewhat independently because the optic nerve does not blend the information from each eye.
The modern habit of spending a lot of time in front of electronic screens has caused an uptick in near-sightedness.
The following are the most common vision conditions. Treatments for several of these have advanced in the past few decades, helping many people retain good eyesight further into advanced age.
The modern habit of spending a lot of time in front of electronic screens has caused an uptick in near-sightedness. Also known as myopia, near-sightedness is caused by the eyeball’s excessive length or the cornea’s excessive curve.
Not surprisingly, far-sightedness is caused by an under-curved cornea and an eyeball that is too short. Both conditions are usually correctable with glasses.
Another common cause of poor vision is astigmatism which is caused by an irregularly curved cornea or misshapen crystalline lens. It can be corrected with glasses, surgery, or a specialized contact lens. About 30% of people have astigmatism and up to 70% of glasses prescriptions include astigmatism correction.
Cataracts are the result of injury or natural aging, causing blurry or clouded vision. More than half of all people age 80 or older have cataracts or have been treated surgically for them. Early warning signs include poor night vision, halos around lights, colors appearing less bright, and sometimes double vision. Corrective lenses can help at first, but surgery may be required. Surgery removes the damaged lens and replaces it with an artificial one.
Age-related macular degeneration is the leading cause of permanent blindness in elders over 60. It is caused by gradual wearing down of the “macula”, the light-sensitive center of the retina. Cells that are needed to interpret light eventually die and those with the condition may experience blindness in the center of the field of vision. The direct cause is unknown, but heredity may be a factor, as are smoking, high cholesterol, obesity, and high blood pressure. There is no cure for the condition, but treatments can slow down or stop its progress. A recently released study shows that sufferers of macular degeneration are at greater risk for hospitalization and death from Covid-19.
Glaucoma is a serious group of eye diseases caused by high intra-ocular pressure (IOP) in the eye. Doctors are unsure of how the increased pressure causes irreversible damage to the optic nerve, often resulting in blindness. It tends to run in families. If caught early, blindness can be prevented. Treatments include eye drops, oral medications, surgery, laser treatment, or a combination. Cannabis has been shown to be an effective way to temporarily lower IOP, but further studies are needed to find compounds that sustain the pressure-lowering properties of the drug.
As with most health-related issues, consistent vigilance will enable you to retain healthy vision longer into the autumn years. A comprehensive eye exam is recommended every two years for those under 60. Older adults and those considered to be at risk by heredity or other health issues should be examined annually.
Ann Constantino, submitted on behalf of the SoHum Health’s Outreach department.