By Deborah Zelinsky, O.D.
“Vestibular information is integrated with proprioceptive and other sensory inputs to generate our sense of motion,” say authors of a recent study published in a 2021 issue of Current Opinion in Physiology (https://doi.org/10.1016/j.cophys.2020.12.001). This explains why patients with vestibular sensory loss or other vestibular impairments find “everyday activities like walking” to be difficult. “Even small head movements can produce postural and perceptual instability,” they state. In fact, among scientists and health professionals, a new term for this type of problem is “Triple P” – Persistent Postural-Perceptual Dizziness.
The scientists’ description of how the proprioceptive and vestibular systems “work together” to help us navigate and move through our environment is not surprising. In fact, the author of a 2022 online article (https://www.kenhub.com/en/library/anatomy/the-vestibular-system) defines the vestibular system as a “somatosensory portion of the nervous system,” providing us “with the awareness of the spatial position of our head and body (proprioception) and self-motion (kinesthesia).”
So, why is an optometrist talking about the vestibular system, a structural labyrinth located in the inner ear? Because eye movements link at a reflex level to head movements!
When head injury occurs or neurological disorders like Parkinson’s and Alzheimer’s disease develop, communication between neuron signals often gets disrupted. This disruption may then disturb the integration of these neurons with other body proprioceptors involving touch, sensation, pressure, and movement.
Proprioception: an “Internal Sense”
Experts often call proprioception an “internal sense.” Mechanosensory neurons found within muscles, ligaments, tendons, and joints, proprioceptors transmit information to the central nervous system, and in turn activate feedback loops. These feedback loops allow the body to move without conscious attention. At the same time, they communicate with proprioceptor-like neurons in other sensory systems, including the ears and eyes. The integrated function and coordination of these neurons stabilize posture and prompt body movement. The proprioceptors help with balance, stair climbing, and core posture control, thereby lessening clumsiness.
The Mind-Eye Institute studies the retina’s critical role in integrating various sensory maps, including eye-ear coordination, to process visual space and achieve proper spatial awareness. This synchronization of perceived auditory and visual space with proprioceptors optimizes performance. For instance, people can be scared of walking on stairs or through doorways, or picking up objects. These actions require subconscious planning of the amount of energy to exert. If the eyes send signals indicating something is going to be heavy, a person readies his or her arms to lift in a manner much different than when anticipating picking up a small box of feathers.
Visual processing involves a complex network of communication signals between the central nervous system (which includes the retina, brain, and spinal cord) and other circuitry, such as the emotional, motor, and sensory systems and thinking processes.
How Retinal Stimulation Affects the Brain
The Mind-Eye Institute team uses therapeutic lenses, filters, and other optometric interventions to stimulate the retina by changing the way light passes through it and affecting how the brain reacts to information about the environment. Our sensory systems are like musicians in an orchestra. Each musician may be highly skilled in a specific instrument, but without a conductor synchronizing what they play, the result is simply noise – not music.
Brain injury and neurological disorders like Alzheimer’s disease disrupt sensory circuitry and mapping of space. When either central or peripheral eyesight fail to interact appropriately and/or inputs from eyes and ears fall out of synchronization, patients often become confused about their environment. This confusion can create a narrowed perception and awareness of surroundings, inappropriate reactions and responses, and difficulties with decision-making skills, as well as learning and memory.
The Mind-Eye Institute developed and designed the internationally recognized and patented Z-Bell Test™ to evaluate a patient’s overall integration of retinal processing with awareness of auditory space – basically, the stability of the eye-ear connection. During the auditory portion of the test, a patient reaches out, with eyes closed, and tries touching a ringing bell. If the patient cannot do so, a Mind-Eye optometrist determines the optimal combination of lenses to place in front of the patient’s closed eyelids allowing the patient to find the bell immediately without conscious effort. Light still passes through the eyelids and activates parts of the brain not used for eyesight. With eyes closed, patients must visualize surrounding space in order to locate the bell. The optometrist repeats this with the visual portion of the testing.
Auditory localization and visual localization must match in order to lessen overall effort and sensory confusion and achieve proprioception. Brain-injured stroke patients, for example, often struggle with gaze stability. Hampered vestibulo-ocular reflex (VOR) function reduces dynamic visual acuity and causes balance issues. Imagine having to stare at your feet in an effort to maintain balance; you would not be able to process surrounding space easily or navigate and plan movements.
The Vestibulo-Ocular Reflex
Although not part of proprioception, the vestibulo-ocular reflex (VOR) plays a significant role in balance, providing an important feedback pathway between eyes and inner ears. The VOR helps further prevent falls by providing head and postural stability because of the ongoing interaction among tactile, auditory, and proprioceptive sensory inputs.
Scientists writing in a recently published book Neuroanatomy, Vestibulo-ocular Reflex (www.ncbi.nlm.nih.gov/books/NBK545297/) state “this reflex keeps us steady and balanced even though our eyes and head are continuously moving when we perform most actions.” When moving the head, “eye muscles are triggered instantly to create an eye movement opposite to that of our head movement at the exact same speed to readjust the visual world.” That adjustment, “in turn, stabilizes our retinal image by keeping the eye still in space and focused on an object, despite the head motion.”
Head injury can destabilize posture and balance by affecting this interactive VOR pathway. In the Encyclopedia of Behavioral Neuroscience (2010), authors write that the “VOR uses information from the vestibular labyrinth of the inner ear to generate eye movements that stabilize gaze during head movements. Without the VOR, when walking down the street, it [would be] impossible [for a person] to read signs or even recognize faces.” VOR disturbances resulting from disease or brain injury can cause vertigo, spatial disorientation, and balance issues. Persons also may experience temporary impairment of proprioceptive neurons because of exhaustion, mental disorders like depression, vitamin and hormonal deficiencies, and cytotoxic factors, such as chemotherapy.
Why the Interest in the Vestibular System
Back again to the question of why an optometrist might be so interested in the vestibular system. As Mind-Eye patients frequently indicate, stimulation of the eye’s retina using light can affect their listening, posture, and balance.
And, of course, proprioception conducts those parts of the “orchestra” and keeps them in sync.
Deborah Zelinsky, O.D., is a Chicago optometrist who founded the Mind-Eye Connection, now known as the Mind-Eye Institute. She is a clinician and brain researcher with a mission of building better brains by changing the concept of eye examinations into brain evaluations. For the past three decades, her research has been dedicated to interactions between the eyes and ears, bringing 21st-century research into optometry, thus bridging the gap between neuroscience and eye care.