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Thinking about Light from an Optometric Viewpoint

“The eye sees only what the mind is prepared to comprehend.”

When I started my journey driving home from an appointment one late winter afternoon, the sun was shining brightly. But the day quickly turned to dusk and dusk to darkness by the time I pulled the car into my driveway. As an optometrist who spends her days assessing patients’ brain function by evaluating the links between their eyesight, hearing, posture, and movement, I could not help but think about the important biological and psychological role that light plays in how we perceive, understand, and react to our environment.

The gradually dimming sunlight during my drive, coupled with the glaring headlights of oncoming vehicles, required me to keep “adapting,” changing my awareness of – and responses to – what was happening on the road around me. When the sun kept going behind clouds and then popping out again or when the sunbeams came through the trees as I was moving, the many changes in my environment were affecting my peripheral retinas. I kept having to put the visor down, then up, then down again. Driving west is not enjoyable when the sun is setting!

The regular shifting of light – from day to night – determines our circadian rhythm, “telling” us, through the release of hormones and production of internal biological chemicals, when to sleep, when to awaken, when to eat, when to exercise, when to work, and when to relax. If this normal shifting of light is disrupted, such as by flying across time zones, we can experience fatigue, functional and attention difficulties, and other symptoms commonly described as jet lag. Light is even associated with other basic functions like body temperature. For example, in the dark while we sleep, heart rate and body temperature drop, appetite is suppressed, and movement lessens.

Experts tell us to avoid the “blue” light from computer monitors, iPads, and mobile phones before going to bed. That is because blue light blocks receptors in the eyes that create melatonin, and without the melatonin, a person will not be as sleepy. Sleep also is less efficient and beneficial in persons who fall asleep with a television on, because the light from the screen flickering through their closed eyelids alters brain activity due to retinal signaling.

The retina is actually part of the brain. And, because it is so, certain receptors in it are able to feed information in the form of electrical impulses to more than the eyesight portions of the brain. Some information travels to the hypothalamus and other key brain structures, causing us to react and respond consciously, subconsciously, and unconsciously. Depending on its intensity and the angle at which it passes through our eyes, light stimulates different retinal receptors.

Bright light – such as the sunlight I encountered during the initial portion of my drive – stimulates the cones, the photoreceptors that predominate the central retina and allow people to see objects clearly. As light dims, the rod receptors, which are scattered more in the peripheral retina, become more dominant. The peripheral retina takes in the majority of the surrounding visual field – everything other than the point of conscious, fixed gaze. Eyeglasses can control which retinal sections have more stimulation that others.

Peripheral eyesight is what helps us remain safe in our environment. When we walk or run, peripheral eyesight ensures that we place our feet correctly, avoiding objects and barriers of which we are often unaware – like puddles and high sidewalk curbs. If we catch movement out of the corner of our eye, we are able quickly and subconsciously to judge the speed, size, and shape of an approaching object and determine if we should move out of the way, flee, put our arms up for protection, or, perhaps, stand our ground and address the approaching target. Meanwhile, our central eyesight is what governs how we use our hands and arms to reach out or grasp things as we move.

The interplay between these two eyesight systems — central and peripheral — provides us with depth perception and the awareness of where objects are located. There are other retinal connections that link with balance and posture centers and visual imagery skills, in addition to the sleep centers mentioned above.

In many patients who have sustained traumatic brain injury, bright light interferes with this depth perception and awareness of surrounding space. For patients with Alzheimer’s disease or dementia, the shift of eyesight from retinal cones in bright light to the peripheral rods in dim light may only add to their confusion and restlessness – a condition called Sundowner’s syndrome.

Each person’s reaction to light – and subsequently, the surrounding environment – is highly individualized. At the Mind-Eye Institute, we learn about a patient’s specific needs through comprehensive testing of his or her eyesight, visual skills, and ability to localize targets with eyes and ears. The synchronization of these two sensory systems (eyes and ears) link to the patient’s posture and movement. This information then allows our Mind-Eye team to tailor prescriptions for “brain” glasses.

“Brain” glasses can stabilize the balance between the activity in a person’s central and peripheral retinal receptors and improve synchronization of hearing and seeing targets in the same location, lessening overall confusion and sensory mismatches. The glasses achieve this by varying the intensity and angle of light through the retina, thereby creating new brain signaling pathways.

Loss of sensory synchronization due to brain injury, stroke, or other neurological disorders, can affect visual processing and lead to multiple symptoms – vertigo, headaches, light and sound sensitivities, anxiousness and stress, attention and comprehension problems, an inability to read and interact appropriately in social situations, memory problems, and a general feeling of not being oneself. The goal of the Mind-Eye Institute is to reduce those symptoms by changing light and guiding patients to development of larger comfort and tolerance ranges, thus enhancing quality of life.

For children – and adults – with learning disorders, variations in light using therapeutic eyeglasses help enhance under-developed visual processing skills – skills that regulate and control the ability to respond appropriately to what is happening.

Sometimes when driving, I get caught up in neuroscience podcasts that stimulate new ideas. Perhaps, the next time I am on the road and the sun is too bright, I will forego all scientific thoughts by just switching the car radio on to a relaxing music station.

Deborah Zelinsky, O.D.
Founder, Executive Research Director
Mind-Eye Institute