Physiology of Vision in a Virtual World

Looking to learn more about general vision and the use of virtual reality in vision therapy? Below are the answers to some of your questions.

How does Human Vision Work?

Anatomy and Physiology of Human Vision

Entire textbooks are written just on human vision, so we've provided a brief review of the anatomy and physiology of the eye and visual system

Anatomy

The visible portion of the eye is made up of clear window - the cornea, the colored, muscular iris, the clear, thin, vascular conjunctiva, and tough, white sclera. The iris is able to constrict and relax, making the hole in the iris, the pupil, larger or smaller. Both the cornea and sclera are tough - the help protect the eye from damage and give it its nice rounded shape.

Six muscles called the extraocular muscles are tightly bound to the sclera. These muscles are controlled by the third cranial nerve, the fourth cranial nerve, and the sixth cranial nerve, and they help the eye move and rotate in all directions.

Behind the iris the crystalline lens (or just "lens). The lens is an important structure, as it helps focus light on the light-sensitive structure, the retina. Behind the lens is the thick, jelly-like fluid that makes up the majority of the volume of the eye. This structure is the vitreous humor, and it's an important shock absorber for the eye. It also helps with transferring some nutrients. If you've ever seen a floater in your eye, it's likely it's in the vitreous.

The light-sensitive structure of the eye is the retina. Light-sensitive cells called photoreceptors are stimulated by light and function to convert light into electrical signals to transmit to the brain. Photoreceptors are either cones (high-detail, color-sensitive cells) or rods (low-detail, motion-sensitive cells). Electrical signals travel from the photoreceptors, through specialized cells, and finally to the bundle of nerve fibers that make up the optic nerve.

Physiology of Vision

Field of View

Unlike some other animals, humans’ eyes are both set on the front of the face, permitting binocular vision. This forward-facing orientation of the eyes means each eye has a rather similar view of an image, with the majority of the field of vision overlapping. This overlapping portion of the visual field provides key detail information to the brain to help us localize objects in space, and is complemented by the slight horizontal offset of each eye.

Together, each eye supplies the brain with a VERY similar, yet slightly different view of the world. When visual information is then combined, the slight difference in viewpoints of the right and left eye allow humans to see depth (or stereoscopic vision). In total, the human field of view is around 200 degrees horizontally, with around 120 degrees dedicated to binocular vision. Contrast this to an animal such as a rabbit, which has more laterally displaced eyes. The trade-off for the rabbit is the sacrifice of stereo vision for a wider field of view by the two eyes a rabbit only has 30 degrees where it has an overlapped field of vision with depth perception but has nearly 350 degrees of a field of view (only 10 degrees of a "blind spot" where the back of the head is!)

Visual information in the periphery or center of the visual field can, of course, be detected by one eye, but the combined visual information is what is required for binocular processing. This information is transmitted throughout the majority of the brain.

Simulating human vision | Brian Barsky | TEDxPrague Watch video here

How does Virtual Reality (VR) affect human vision?

Virtual Reality is a completely computer-generated environment that uses a display (often head-mounted display) to immerse the user in the artificial environment. The sense of immersion is highly dependent on engaging the user's senses - vision, sound, and touch are most often stimulated, while taste and smell are often not addressed. Developers of virtual reality games and experiences design environments that a user can interact with to improve the immersion experience, and rely heavily on 3D images to trick the user's visual system that the environment is "real".

How is VR vision therapy different than traditional vision therapy methods?

VR technology allows immediate visual adaptation in an artificial environment. In this artificial environment, Vivid Vision settings can apply prism assistance to break suppression as seen in strabismus, improve 3D depth perception, and increase stamina for the eyes to team together. Unlike the real world, a VR environment can be easily manipulated to improve compromised visual skills compared to traditional vision therapy tools. Many people with binocular disorders lack normal visual function, by giving visual assistance in an artificial VR environment and making VR adaptations throughout a vision therapy program will improve their visual outcome.

How does virtual reality vision therapy work?

Vision therapy focuses on the concept of neuroplasticity by using perceptual learning - this is essentially targeted, practiced tasks that help the brain form new pathways (or improve on weak existing pathways) to improve a visual skill such as visual acuity, fusion, or stereopsis. Binocular training may use red/green or red/blue glasses, polarized glasses, or virtual reality. This is sometimes referred to as "dichoptic" or "anti-suppression" training if a patient's visual system ignores visual input from one eye (called cortical suppression or just suppression). Later phases of treatment focus on stereopsis (3D) skill training. Levi and colleagues suggest that stereopsis training (which is done with 3D glasses or virtual reality) may even be more beneficial than 2D training of one or both eyes, especially for patients with an eye turn (strabismus).

Who determines if a VR vision therapy treatment is right for someone?

A specialist in visual function, also known as a developmental optometrist will evaluate the following areas and make the suggestion for vision therapy that may include VR as part of a therapy protocol. A few items covered in the assessment may include:

Binocularity, or how the eyes interact with each other and how they transmit information to the brain. The doctor measures the eyes’ ability to aim together accurately in order to maintain single vision, and they check to make certain the eyes don’t slide out of alignment, such as with crossed or wandering eyes.
Oculomotility, or tracking. Developmental optometrists will also check their patients’ ability to control where they aim their eyes, such as the skill required for reading so we don't lose our place. They also make sure patients can follow a moving target smoothly and are able to make accurate eye jumps from one point to another.
Accommodation, or focusing. Developmental optometrists evaluate their patients’ ability to change their focus rapidly and smoothly when looking from distance to near and back again, such as from board to desk. In addition, developmental optometrists check to see if patients can maintain clear focus at near ranges for extended periods of time without blur or fatigue, such as required for reading small print.
Visual Perception. Developmental optometrists also run tests to determine if patients have developed the perceptual skills they need to understand and analyze what they see, checking skills such as visual memory, visual discrimination, visual closure, and visual figure-ground.
Visual-Motor Integration, or eye-hand-body coordination. Finally, developmental optometrists run tests to see if patients’ visual systems are efficiently transmitting information to the body’s motor centers for good balance and coordination.

Once the binocular assessment is complete and a vision therapy program is suggested, VR vision therapy may be an integral part of vision therapy.

How is success determined in vision therapy? What does a successful recovery look like?

Success is determined by the patient's perception of the final desired outcome. People may ultimately feel success by physical outcome (example: cosmetically aligned posture seen with strabismus), a positive academic outcome (example: following along with ease while copying from the whiteboard to a school page or paper), or an emotional outcome (example: feeling positive about oneself overall). A successful recovery is accomplished and acquired when the skill or goal that was planned is now acquired.

Why is there suppression? What is the purpose of it?

Suppression is a subconscious adaptation by a person's brain to eliminate the symptoms of disorders of binocular vision such as strabismus, convergence insufficiency, and aniseikonia. The brain can eliminate double vision by ignoring all or part of the image of one of the eyes. The area of a person's visual field that is suppressed is called the suppression scotoma (with a scotoma meaning, more generally, an area of partial alteration in the visual field). Suppression can lead to amblyopia.

In a more general term, think of your eyes as twins playing together in a visual game, one of the twins is more dominant and refuses the other twin to play along in the visual game. The non-dominant twin eventually gives up (result is suppression) and does not play along with the dominant twin now in charge. This leaves just the one twin playing alone in the visual game. Both twins are healthy but the adaptation result is called suppression which leads to poor visual acuity in the nondominant eye.

What are the root causes of amblyopia?

Amblyopia is the lack of development of clear vision (acuity) in one or both eyes for reasons other than an eye health problem that cannot be improved with glasses alone. It is a problem with how the brain perceives and interprets the information coming from the amblyopic eye. It often leads to a suppression of the information coming from the amblyopic eye. People incorrectly apply the term "lazy eye" to both strabismus and amblyopia, which is why it is a bad phrase to use. Patients are often told that amblyopia can only be treated until a certain age. This is outdated information. While early intervention is still ideal, it is never too late to treat amblyopia. Another misconception is that the amblyopic eye is the "bad eye." While it doesn't have the same level of eyesight as the non-amblyopic eye, there may be other visual skills, such as localization, at which it is good.

There is no root cause of amblyopia and amblyopia typically begins during infancy and early childhood. The most common causes of amblyopia are a constant strabismus (constant turn of one eye), anisometropia (different vision/prescriptions in each eye), and/or blockage of an eye due to trauma, lid droop, etc.

What is Accommodation?

Accommodation is the ability to adjust the focus of the eyes as the distance between the individual and the object changes. This process is achieved by the lens changing its shape. Accommodation is the adjustment of the optics of the eye to keep an object in focus on the retina as its distance from the eye varies. At this time, accommodation adaptations or focusing adaptations are not available in a VR environment.

What is Vergence?

Vergence is a broad term that relates to eye movement. For example, we can CONverg (look and fuse at close) and DIverge (look and fuse at distance). In VR, objects can be placed in a virtual world at various distances to work on skills to teach both eyes to work together as a team.

Why does accommodation change as you grow older?

Accommodation, or also known as focusing on near objects, occurs by a concerted action of the ciliary muscle on the zonule fibers which hold the lens in place. The ciliary muscle is a ring of smooth muscle that, upon contraction, relaxes the tension on the zonular fibers and allows the lens to become more spherical. This increase in axial thickness results in an increase in the dioptric power, facilitating accommodation for improved near vision. With ciliary muscle relaxation, the tension on the zonules increases, resulting in lens flattening and a reduction in dioptric power. All of these structures are modified by the aging process, but it is the reduction in lens flexibility that’s most associated with loss of accommodation. Decreased lens flexibility limits the lens rounding and thickening needed for near focus. Models of the process suggest that as the lens becomes less flexible with age, ciliary muscles apply greater tension to zonules, causing ligament fatigue. Muscular atrophy is also a contributing factor. These paired changes in the physical properties of the focal apparatus collectively result in the presbyopic condition. The end effect is the loss of near focus, accompanied by blurred vision, eye strain, and headaches in many individuals. For some, these secondary effects are exacerbated after reading or computer use.

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