Dr. Prashant Garg
Executive Chair, LV Prasad Eye Institute, Banjara Hills, Hyderabad
People often compare the eye with a camera. Why so?
Answer: Yes, the eye can be compared very well with a camera, because the primary goal of the camera and the eyes are the same. Both are involved in capturing high fidelity information of the external world for us to enjoy. Anatomically also, the camera and the eye share a lot of similar parts – both have lenses that refract the light entering the eye. The screen in the camera can be compared to the eye’s retina – that is where the image of the external world is formed. The USB cable that connects your digital camera to the
computer for image processing is akin to the optic nerve that sends information from the eye to the brain for processing.
As a follow up to the previous question, do we see with the eye or the brain?
Answer: We see with both the eyes and the brain. The retina in the eye can, in fact, be thought of as an extension of the central nervous system. That is why many disease conditions involving the brain and the central nervous system manifest first in the eye. Now, coming to the answer for what helps us see – the eye is an optical structure that helps form an image of the external world. The brain is the computer that processes this image and helps us parse important information like details of various objects, color, 3D perception and motion. It is also the brain’s processing of the information sent by the eye that helps us interact with the external world (like crossing a road, working on a mobile phone, etc.). Without the eyes, the brain does not receive any information for processing. Without the brain, the information formed in the eye cannot be processed. So, both share a very symbiotic relationship that helps us enjoy the visual world around us.
Now, we have two eyes, so there must be two images formed in the brain. Still how do we see only one image of the world?
Answer: The main reason why we have two eyes is to perceive the world around us in 3D. For example, when you go to watch a 3D movie in a theatre, it is our brain’s capability to use the information from the eyes as a single unit to perceive the 3D picture vividly. The brain merges the information from the two eyes into a single entity – this process is called binocular fusion. This is achieved through specialized regions in the occipital cortex of our brain. Many people, in fact, do not see things in 3D. For them only one eye is working at a time. For instance, children who have different refractive errors in the two eyes typically see with only their dominant eye – the image from the other eye is “suppressed” by the brain. Even worse, some individuals with eye deviations see the world double. Technically this is called “diplopia” and it can be devastating for patients. Such patients are treated using optical aids or surgery to restore single vision.
Does every organism in the animal kingdom see the world in 3D? If not, why is the evolutionary pressure for seeing the world in 3D?
Answer: Not at all, 3D vision is a special attribute of only a small set of organisms in the animal kingdom. Humans and, our cousins, monkeys have this special attribute. Other animals like rabbits, catsor horses do not see the world in 3D. Whether one sees, in 3D or not is largely defined by the activities of the organism. Organisms that are higher on the food chain – the predators (like humans and monkeys) do a lot of activities that involve calculation of 3D vision – jumping from one branch of the tree to another in monkey or participating in one motor activity like threading a needle in humans. This requires the two eyes to work together as a team, as noted earlier. To achieve this, we have evolved eyes that are placed in the front of our head and we have developed specialized areas in the brain for processing 3D vision. Other organisms like rabbits primarily forage for food and they are more of a prey for other organisms. For them, ensuring that they have a wide eld of view to escape from predators is more critical than having 3D vision. So, they have evolved with eyes on the sides of their head so that they can maximize their field of view. In general, 3D vision comes as a trade-off with a loss in field of view – humans cannot see what is happening in the back of their heads, while the rabbit nearly has 360-degree field of view.
We see the world in color. How is this achieved?
Answer: Color vision, once again, is a special attribute to only some organisms in the animal kingdom. Humans see color vision because our retina has three different types of cells – cone photoreceptors –that are sensitive to different wavelengths of light. Humans can perceive anywhere from 480nm to 760nm of light – violet to red light. The information from these specialized photoreceptors is then further processed by the brain to get ne estimates of color vision. Humans are not the only ones with color vision capability. In fact, many organisms including some fishes have access to many different wavelengths of light, including those in the infrared and ultraviolet spectrum. Snakes, for instance, have sensitivity to infrared light that they use as heat sensors to detect their prey in the night. Some species of bees have sensitivity to ultraviolet light – so, these animals see the world very differently from us humans. It is important to note that our ability to see color is restricted to only bright light. On a sunny day, our cone photoreceptors are maximally active, and they can produce a vivid sensation of color. In the evening or in darkness, another type of retinal cell – rod photoreceptors – are active and they can only sense luminance changes and not color information.
Why do we tear? Is that a good thing to happen?
Answer: Tears play an important role in maintaining our ocular health. Bulk of the tears is produced by a gland located just above the eyes called the lacrimal gland. The tears lubricate the front part of the eye, wash away dust and other irritants that fall on the eye, they have anti-microbial properties that kill the pathogens that fall on the eye, and they also create a smooth surface for producing good quality images on the retina. Every time we blink – adults blink about 14 – 15 times every minute – tears are evenly spread over the front part of the eye to achieve all the goals listed above. Tear film deficiency is a problem for many people, and this can lead to a very uncomfortable life and loss of vision. Such patients are treated using tear substitutes or perform surgeries to fix their problem. So, yes, tearing is a good thing to happen for maintaining eye health.
As a follow up question, if the lacrimal gland is constantly producing tears, why are we not tearing all the time?
Answer: This is because the tears that lubricate the eye are drained out of the eye through a tiny drainage pathway that is located right next to where the eyelids merge with the nose. This pathway, technically called the lacrimal drainage channel, opens into the nose and eventually into the throat as well. That is why, every time you have an eye drop instilled in your eye, you can feel that drop come through your nose and the bitter taste may also be felt in your throat. In fact, we have a clinical test that checks for the patency of this pathway.
What are the most common causes of eye ailments in a child, young adults and the elderly?
Answer: In children, the most common eye ailment these days is short-sightedness or Myopia. This is caused by the eyeball growing too long for its age, resulting in blurred vision. Excessive use of gadgets like smartphones and poor exposure to sunlight are risk factors for Myopia progression. In younger children and infants, we also worry about a condition called Retinoblastoma, a deadly cancer of the eye, and lazy eye, where only one of the two is effectively working. These conditions may be best identified by an eye care professional during an eye screening program or during the child’s regular visit to the eye doctor or pediatrician. In young adults, excessive use of computers may lead to eye fatigue, difficulty in the two eyes working as a team and burning sensation due to poor blinking. These problems are very common among IT professionals, which is why we ask them to take routine breaks from their computer work. In those in their 40’s and 50’s, an inability to focus on near objects – a condition called Presbyopia – is a major problem. This is when individuals become dependent on wearing near vision spectacles to aid their day-to-day activities.
In the elderly, Cataract is the most common cause of vision loss. This happens because of the clouding of the eye’s lens, and this may be fixed surgically by replacing the biological lens with an artificial lens. Glaucoma is another problem faced by the elderly. This happens due to an increase in the pressure within the eye. This problem is a silent thief of vision, as the disease may progress without even the patient realizing the problem. For all these reasons, an annual eye examination is recommended for every individual in society.