Understanding the Eye

The human eye works similarly to a camera, where the eye captures light and movement from the surrounding world and sends the information to the brain to be interpreted.

The front of the eye is covered with a clear and curved membrane, known as the cornea, which focuses light while protecting the eye. After the light passes through the cornea, it travels through a space called the anterior chamber, through the pupil, and then through a lens that helps you to focus. Finally, light passes through the vitreous, a gel-like substance in your eye, and strikes the back of the eye, the retina (as shown in the picture).

Like the film in a camera, the retina captures images by capturing light. It then converts these images into electrical signals, which the brain receives and decodes.

The retina and its communication with the brain is the primary focus of research at the Donald K. Johnson Eye Institute.

Eye Anatomy Diagram

How the Eye Works

In a normal eye, light is focused by the cornea and lens to converge on the photoreceptors, the primary light sensing neurons that are in the outer layer of the retina. There are two distinct types of photoreceptors: cone photoreceptors, which primarily capture information about colours and fine details, and rod photoreceptors which capture information about movement and are responsible for peripheral and night vision. The photoreceptors convert light into electrical impulses, which are then propelled through the network of nerve cells at the front of the retina.

The front layer of the retina is made up with bridging neurons, known as retina ganglion cells. The heads of these cells make up the front layer of the retina, while the long “tails" of the cells (called axons) come together to form the optic nerve, which passes back through the retina and out the rear of the eye. The optic nerve is comprised of the axons of over one million retinal ganglion cells.

In the brain, electrical signals from the ganglion cells are interpreted into shapes, colours and motions. The brain sends signals back through cranial nerves to direct the movements of the eye. Voluntary movements allow an individual to focus on specific details, but the eye also makes rapid involuntary motions. These movements, called saccades, are unconscious, but essential to the process of vision. These movements vary the stimuli received by the photoreceptors and the ganglion cells, allowing the cells to function properly.

Diagram of the process of vision

Common Eye Diseases

Eye-Disease Infographics
View a plain-text version of the above infographic.

More than 5.5 million Canadians live with eye diseases, such as glaucoma, macular degeneration, inherited retinal disease and cataracts. This number is expected to double over the next 20 years.

Blinding Eye Diseases

Vision loss can affect people of all ages. Today, an estimated 1.5 million Canadians identify themselves as having a sight loss, and an estimated 5.5 million more have an eye disease that could cause sight loss. Some of the leading causes of vision impairment are age-related macular degeneration, cataract, diabetic retinopathy, glaucoma and uncorrected refractive errors.

At the Donald K. Johnson Eye Institute, our clinicians and scientists are studying these diseases, conducting cutting-edge research and developing new treatments for blindness and vision loss.

Age-Related Macular Degeneration

Light is captured in the eye by specialized nerve cells called photoreceptors in the retina. A damaged or destroyed photoreceptor cannot naturally be replaced or repaired by the body. Research to replace damaged photoreceptors is a major focus of research at the Donald K. Johnson Eye Institute, led by Dr. Valerie Wallace Dr. Wallace's work focused on using stem cell transplants to replace damaged cones and rod photoreceptors.

Photo of Cone Cells  

Age-related macular degeneration (AMD) occurs when the cone photoreceptors are damaged. These cells are found in the centre of the retina (the macula) and are responsible for fine vision and for colour vision. There are two forms of the disease: wet AMD and dry AMD.

  • In Dry AMD, waste products slowly build up under the retinal pigmented epithelium, below the macula where cone photoreceptors are clustered. The photoreceptors rely on the retinal pigmented epithelium for nourishment and support, as the damage occurs, they are less able to function. Vision loss due to dry AMD is usually slow, and many people with dry AMD never progress to serious vision loss.
  • Wet AMD is due to abnormal blood vessel growth underneath the retinal pigmented epithelium, likely a response to the damage caused by dry AMD. At the moment there is no established way to predict which patients will progress from the dry to the wet form of AMD. Wet-AMD associated vision loss can be sudden and severe, however, drugs have recently been developed to help control abnormal blood vessel growth, preventing photoreceptor damage and sparing vision.

Despite advances in slowing down dry AMD, once a photoreceptor is destroyed, vision loss is permanent. These are the patients that might be helped by transplants of cone and/or rod photoreceptors. Research to replace photoreceptors may also benefit people with inherited genetic conditions that affect cone and rod cells.

Genetic conditions that affect rod photoreceptors typically affect peripheral and night vision first, progressing slowly over many years until the cone photoreceptors are also affected. Retinitis pigmentosa is one example of a blinding condition that works in this way. Dr. Brian Ballios, a clinical scientist at the DKJEI is using human stem cell models to investigate how gene mutations cause photoreceptor disease. Dr. Philippe Monnier, a scientist at the Donald K. Johnson Eye Institute, is leading research to block programmed cell death in vision cells, work intended to slow the progress of these conditions.

Photo of Mature Cone photoreceptor  

In an ongoing clinical trial with the low vision rehabilitation experts at TWH, Dr. Michael Reber's lab provides audiovisual stimulation in virtual reality to patients with macular degeneration. The goal is to maximize the use of residual vision for activities of daily living such as mobility and reading.

Learn more about Age-Related Macular Degeneration, Retinitis Pigmentosa and other retinal dystrophies that affect cone and/or rod photoreceptors from Fighting Blindness Canada.


Once light passes through the cornea, it is focused by the lens of the eye. As a person ages, this lens can become cloudy, obscuring fine vision. In Ontario, surgery to replace a cloudy lens with a new synthetic one is common and is an effective treatment for sight-obscuring cataracts.


Corneal Disease

The transparent round dome at the front of the eye is called the cornea. It protects the inner eye from damage. It has no blood vessels but is instead nourished and cleaned by tears and by the layer of clear fluid (the aqueous humour) behind it. The cornea self-repairs scratches and other damage using a pool of corneal stem cells located in the outer ring of the cornea, the limbus. However, severe injuries and infections can damage the limbus, preventing repair and causing blindness. Dr. Allan Slomovic, an ophthalmologist at the Donald K. Johnson Eye Institute, has begun performing limbal stem cell transplants that can restore a badly damaged cornea. By implanting healthy stems cell in the limbus, the cornea can regain the ability to renew itself.

Institute scientist, Dr. Jeremy Sivak, also studies the cornea, working to understand the essential mechanisms that keep the cornea clear and the interplay between blood vessels and inflammation that can lead to disease.

Diabetic Retinopathy

Insulin is a hormone that controls the amount of sugar in the blood. When a person's body is not able to produce enough insulin or able to respond properly to the insulin that is produced, they develop diabetes. Diabetes is one of the most common chronic diseases in Canada, an estimated 2.4 million Canadians are living with the disease. Diabetes can cause ill effects and complications throughout the body, including serious damage to the retina of the eye.

The vision cells of the retina rely on two important networks of tiny blood vessels in the retina to supply them with nutrients and oxygen. The retina of the eye is the most metabolically active part of the body processing nutrients and oxygen at a tremendous rate, thus the functioning of these tiny blood vessels is tremendously important. Increased levels of sugar in the blood can cause blockages in these tiny vessels and damage the walls of the blood vessels. It may cause small vessels to rupture and leak fluid. This causes swelling in the retina and can damage vision. Because diabetes is relatively common, diabetic retinopathy is the most common cause of vision loss in working-age Canadians.

Photo of Retinal Ganglion Cells

The macula is the centre of the retina, where most of a person's fine detailed vision happens. When high blood sugar damages the vessels in this area and causes swelling, the condition is called diabetic macular edema (DME). DME is considered a consequence of more generalized diabetic retinopathy, and may cause severe vision loss without rapid treatment.

Diabetic retinopathy is a leading cause of blindness in working-age Canadians, and it is often preventable with early treatment. However, approximately 40% of people living with diabetes in Ontario had not had an eye exam for more than two years. Many of those people are from low-income communities. In order to fulfill this gap, Dr. Michael Brent, Director of the Clinical Trials Program, co-leads a Diabetic Retinopathy Screening Program with mobile imaging equipment, known as Project OPEN, helping to improve eye care and increase screening rates for vulnerable populations across Ontario. Clinics are in place across the province, in marginalized communities, to perform outreach and arrange screenings for residents.

Visit the Canadian Diabetes Association to learn more about diabetic retinopathy.

Glaucoma / Optic Neuropathies

Glaucoma occurs when the retinal ganglion cells and their projections that form the optic nerve are damaged. Once these cells with their long slender axon fibers are injured, visual information can no longer travel from the eye to the brain and vision is lost. Unfortunately, permanent damage can occur before a person has symptoms of vision loss. Improved testing and screening for glaucoma to prevent permanent damage, is an ongoing focus for clinicians like Dr. Irfan Kherani, Dr. David Mathew and Dr. Matthew Schlenker.

Although increased eye pressure is an important risk factor for glaucoma, not all patients exhibit this indicator, lowering the eye pressure doesn't always slow down the disease, and the underlying causes of the disease are not well understood. Nerve damage is triggered by messages sent between cells, but a better understanding of the origins of these messages is needed.

Photo of Signals Glaucoma

Dr. Jeremy Sivak studies cell communications and works to identify ways to block destructive messages and protect vulnerable cells. In particular, his research has examined signals of metabolic stress, meaning that the cells of the retina are struggling to get enough oxygen and nutrients.

Both glaucoma and serious injuries (traumatic damage) can cause currently irreversible damage to the optic nerve. However, Dr. Philippe Monnier is studying the early development of retinal ganglion cells and how their long axons are generated. The ultimate goal of this work is to understand how retinal nerve cells could be regenerated and guided to make connections with other cells restoring the flow of visual information.

Visit the Glaucoma Research Foundation of Canada to learn more about glaucoma.

Inherited Retinal Disease

Inherited retinal diseases (IRDs) are a group of diseases that can lead to severe vision loss and/or blindness. IRD is caused by gene malfunctions and can affect individuals of all ages.

Clinician investigator, Dr. Brian Ballios, and his team of specialists are moving towards a model of "same-day care," with the goal of helping IRD patients to receive diagnostic testing, complete specialized imaging of the retina, and begin genetic testing all in one day. In addition, Dr. Ballios is looking to use stem-cell therapies to identify treatments for these genetic blinding eye diseases. Dr. Ballios's research is focused on establishing models of specific human inherited retinal diseases, and studying how healthy and diseased human cells interact to improve the potential of stem cell-derived therapies.

When the photoreceptor cells in your retina die, the ability to detect light decreases, and vision is lost. There are no treatments to restore vision after photoreceptors die, but Dr. Valerie Wallace from the Donald K. Johnson Eye Institute and her team are looking for ways to restore vision by transplanting healthy photoreceptors into the retina. Eventually, this team hopes to establish a photoreceptor transplant centre and begin human trials of photoreceptor transplants at the institute, in partnership with the Institute's clinician scientists.

Photo of Retinal Astrocytes Neurons

The damages caused by IRDs are irreversible. However, Dr. Philippe Monnier recently discovered that blocking a protein called "neogenin" may be a new approach to help photoreceptor cells survive. This is an exciting step towards developing a new experimental drug that promotes the survival of light-sensing cells in the retina and may be able to slow down vision loss for IRDs.

Low Vision

When light passes through the lens of the eye, it is processed across the retina, however the highest resolution images are processed in the fovea at the centre of the macula. Human eyes make rapid unconscious movements to allow different parts of our environment to be captured by the fovea. The unconscious movements of both eyes are coordinated and are critical to normal vision.

New technology is also being developed to make more detailed study of eye movements. The Donald K. Johnson Eye Institute's affiliated scientist Dr. Moshe Eizenman is working to improve these tools. This equipment has many potential applications to improve the diagnoses of eye diseases and also to diagnose and monitor other conditions that affect the brain such as Alzheimer's disease and eating disorders.

Photo of Visual Rehabilitation program

Dr. Michael Reber develops a visual rehabilitation program for individuals with low vision based on cognitive training and virtual reality. In collaboration with Dr. Samuel Markowitz and Dr. Monica Nido from the Low Vision Rehabilitation clinic, the team is working towards stimulating the healthy part of the retina and corresponding regions in the brain to compensate for vision loss. This will rejuvenate visual perception restoring some daily life activities.​

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