The retina is a special type of light-sensitive tissue located in the interior of the eye that’s so analogous to brain tissue, it’s considered a part of the central nervous system. If you consider our eyes to be cameras, then the retina is the film. However, the retina does much more than just send a ‘picture’ to the brain – it actually has to compress the image so that it can be conveyed via the optic nerve, because the photoreceptors in the retina can take in more information than the optic nerve can convey.
Although it looks like a single layer, the retina is actually very complex and comprises ten layers of nerve cells, all of which are connected by synapses. Within each of these layers are several different types of cells: the photoreceptors called rods and cones, photosensitive ganglion cells, bipolar cells and other cells that assist with regulating light input as well as processing and transmitting images. Rods and cones each have their own function. Rods are more sensitive to light and are responsible for nightvision and peripheral vision; each one can respond to a single photon, or particle, of light. Cones, on the other hand, work in bright light and are responsible for seeing colour, fine detail and rapid movements.
Until the Nineties, it was thought that only rods and cones were involved in sight. Then researchers discovered a much rarer type of cell called photosensitive ganglion cells. These cells help regulate pupil size and the light/dark cycle, or circadian rhythms, that we function by.
Light reaches the rods and cones by first passing through transparent layers of nerve cells. When it reaches these photoreceptors, the light causes chemical changes in the rods and cones. The raw data is sent back through the layers of nerve cells, which process and encode the image before sending it via the optic nerve to the brain.
We all have a blind spot, or scotoma – a place on the retina where there are no photoreceptive cells to perceive light. Known as the optic disc, this area is where the optic nerve passes through the retina on the way to the brain. Although the blind spot is sizeable, we don’t notice it. That’s because the blind spot in each eye is in a different place, so the other eye “fills in’ the blanks. To the right we have included a ‘blind spot test’. When you close your left eye and focus on the circle, then slowly get closer to the page, the plus sign will vanish!
Iris scanning is the more common form of biometrics when it comes to eyes, but did you know that our retinas also have special identifying characteristics? It all comes down to the complex network of blood vessels in the retina; even twins’ retinas are different. When you look into a retinal scanner, a low-energy beam of infrared light is reflected by the capillaries. The resulting unique pattern is then stored as code within a database. Retinal scanners are fast and reliable, but the downside is that the scanners are expensive and the pattern can change over time with certain diseases or other eye problems.