Our eye consists of several parts, as shown in this schematic image:
Below, we will give a brief description of the main parts of the eye that contribute to the visual process.
The pupil is the part of our eye that modulates the amount of light entering our eye. It is a hole in the middle of the iris. The light entering our eye does not reflect, which is why the pupil appears black.
The retina is the part at the back of the eye that receives the entering rays of light. The retina is made up of several different cells, including photoreceptors.
The retina contains about 126 million photoreceptors. These photoreceptors contain photopigments, which undergo decay when light falls on them. This creates an electrically charged current and allows signal transmission between neurons.
There are two types of photoreceptors:
Rods (+/-120 million)
Rods are sensitive to little stimulation and thus become active when there is little light. These receptors are therefore mainly used at night.
Cones (+/- 6 million)
The cones are sensitive to strong stimulation and are therefore mainly active during the day. This is because cones have photopigments that can be recharged very quickly. This allows them to recover quickly from light and to start decaying again. Cones are also essential for seeing colors.
In addition to photoreceptors, the retina contains another type of cell called ganglion cells. These lie behind the photoreceptors and ensure that signals are projected to the brain. We distinguish two different types of ganglion cells:
All cells of the visual system have a receptive field. This is a concept that is difficult for some people to understand because it is not part of the cell. We describe the receptive field of a cell as the place in the visual space where light must be to make a particular cell active. You can also think of it in another way, as a place on the retina. In that case, the receptive field is the part of the retina where light has to fall to activate the cell further on in the brain.
In the center of the retina is the fovea, also called the yellow spot. This is where the light coming from the center of your vision falls. There are only cones in the fovea, which means that the center of the field of vision is also the place where you can see the most details.
The location in the retina where the ganglion cells leave the eye and merge into the optic nerve has no photoreceptors. This means that light falling on this part of the retina cannot be processed. This area is also called the blind spot because you are "blind" to this area in your receptive field. This blind spot is "solved" in two ways. First of all, this spot is located in the peripheral part of your retina, the part where you cannot see very well in the first place. Secondly, the brain solves the phenomenon of the blind spot by copying the area in the receptive field that is just next to the blind spot. For example, when you look at a red sheet of paper, you do not suddenly see a black hole somewhere. The brain copies the image of the red area just next to your blind spot over the blind spot so that you will just see a whole red sheet of paper.
Author: Caroline Benjamins (adapted by Pauline van Gils)
Illustration: Pauline van Gils