Our eyes take everything in. But how is this visual information turned into images we can perceive? This amazing process is done with the help of special cells called rods and cones.
Before we get into what rods and cones do, let’s go over a quick refresher.
When light enters the eye, it will hit tissue in the eyeball called the retina. This super-sensitive tissue takes light energy patterns and converts them into electric discharges, called action potentials, that are they carried to the areas of the brain that help process visual information.
More precisely, the retina, which is linked to the central nervous system, is effectively the deepest layer of the eye. It’s composed of a thick layer of nerve cells that cover nearly the entire innermost wall of the eyeball. What this means is that when we train our eyes on an object, the visual image of it will hit the retina at a precise spot along what’s known as our visual axis. Along this axis, one area is most important. Called the fovea, it sits in the centre of the retina, and is what provides our sharpest vision abilities – the only part of the retina that gives us 20/20 vision. It also is responsible for precise detail and colour.
So the fovea is very important. In fact, half of the nerve fibres in the optic nerve are relaying information from the fovea. Meanwhile, the other 50 per cent of the nerve fibres carry information from the remainder of the retina.
You can get a sense of where these vital parts of the eye are, and how the eye takes in visual (light) information, in the accompanying graphic.
Okay, now back to the rods and cones.
These cells are found in the retina and act as photoreceptors. That means they convert light into signals. They do this by having a special response to light, which activates aphoto-sensitive pigment. This pigment is attached to the outer membrane of rods and cones, as visualized in this graphic.
And there are a lot of rods and cones. In the human eye, you can find an estimated 5 to 7 million cones, and about 110 to 130 million rods. These special cells are each found in different parts of the retina (a very complex part of the human body). Cones are in the fovea (central) area, while those millions of rods are found in what’s called the peripheral retina.
To get a sense of just how rods and cones exist in separate parts of the retina, check out this chart showing the density of the special cells both near and away from the central fovea region of the retina.
Okay, so far so good. But just what kind of light-processing abilities are rods and cones responsible for? Each is special, taking on separate and distinct duties that allow us to see the world around us.
Rods see visual information in black, white and the many shades of grey in between. They’re also the special cells that take in forms and shapes. Ever wonder how we can still see what’s around us when it’s dark out? Thank the rods for our night vision. They’re so sensitive they can react to visual information even when there is little light.
Cones add colour to our lives. They sense colour, but with a catch – they need much more (day) light than rods do to process what we’re looking at. In normal or bright light, cones are working at their best, bringing us the multitude of colours around us. But this breadth of colour is itself the product of a special process, as cones come in only three types, one for each colour: red, green and blue (RGB). However, working together in unison, they can make out the complex blend of light waves. As a result, we can perceive not just the RGB colour trio but literally millions of colours on the colour spectrum.
Pretty amazing what rods and cones do for us, isn’t it? If you have more questions, don’t hesitate to ask your optometrists during an eye appointment.
And remember, eye health is just as important as keeping other parts of your body healthy –a point that is a bit blurry for some people! Regular vision exams are crucial to keeping your vision sharp and healthy, and detecting any problems.
Dr.Christina John is a graduate of School of optometry, University of Waterloo in 1994. She offers her service with new technologies and is very dedicated in spreading awareness about vision problems and its remedies. She used to write many articles on eye in order to spread awareness among common people.