Why do I see what I see

See

Structure and function - as we can see

The eye has the task of converting the electromagnetic waves of light into a sequence of nerve impulses, which can then be passed on to the brain.

"Being able to see" means: light from the outside must reach the retina unhindered through the entire eye and stimulate nerve cells there. The actual picture of our environment then arises in the brain.


"Bulbus oculi" is the name of our eyeball in technical terms. As the name suggests - it is spherical, a hollow body filled with liquid. The dermis, choroid and retina surround the wall of the eyeball from the outside in.

The choroid lines the back of the eye and is permeated with numerous blood vessels. Nutrients and oxygen are transported to the light-sensitive cells of the retina via the blood.

The dermis does not surround the whole eye either, but merges into the transparent cornea in the front part. It is something like a window through which light falls, five layers, about half a millimeter thick and free of blood vessels. Because they would take our unobstructed view.

The same goes for the lens and the vitreous. The metabolism of the vitreous humor is correspondingly sluggish. It draws its nutrients from the aqueous humor, which fills the cavities of the eye and is constantly renewed. One of these cavities is the anterior chamber of the eye, just behind the cornea.

Behind it is the colored iris or iris in a ring. It can use two muscles to reduce or enlarge the pupil in the middle and thus determine the amount of incident light.

The lens focuses the incident light behind the pupil. It is transparent and clear in the healthy eye. In addition, it is elastic and can change its refractive power through deformation. This allows the eye to adjust to the different viewing distances.

Once the light has passed through the cornea, pupil, eye lens and vitreous humor, it finally falls on the retina. The eye works in a similar way to a camera.

The light rays arriving in parallel are bundled in such a way that they meet on the retina precisely in the fovea, the focal point. The lens, in conjunction with the jelly-like glass body, creates an upside-down image of the outside world. Only our brain reverses the flood of images in real time.

The light-sensitive photoreceptor cells are located in the retina. The photoreceptor cells are most densely packed in the middle of the retina, the macula. It is a yellow spot about 1.5 millimeters in size and the point of sharpest vision.

The sensory cells in the retina convert the electromagnetic waves of light into nerve impulses and pass them on to the optic nerve. From there, the stimuli reach the visual center of the cerebrum via the diencephalon. Only here do images emerge from the signals from both optic nerves in the eyes.

Our brain performs at its best. It compares all images with the information that is already stored, with objects and feelings that we have collected in the course of our lives. From all this data, our brain creates the impressions that we perceive in fractions of a second.

Colored, light and dark - cones and chopsticks

The human eye has two types of sensory cells: cones and rods - six million cones and 120 million rods. In normal lighting conditions, cones and rods work equally. Both together give us the complete color picture of the environment.

Cones and rods have different spectral sensitivities. Rods have their maximum sensitivity in blue-green light at around 500 nanometers in wavelength, while cones in green-yellow light at around 560 nanometers.

The cones give us the information about the colors. They only send impulses to the brain from a certain brightness. And only then does our sense of a colored world arise. They also enable sharp vision.

The greatest density of cones is found at the point of sharpest vision, i.e. at the yellow spot or macula. Here every visual cell is connected to a nerve fiber and the resolution increases dramatically.

If the brightness decreases, we only see shapes, outlines and shades of gray. Now only the chopsticks are active. These photoreceptor cells get by with a relatively low light intensity. You report us light or dark.

That is why for us "all cats are gray at night". In addition, the sharpness of the image suffers from the "rod vision". Because unlike the cones, not every rod is connected to a nerve fiber. When it's pitch black, the human eye goes on strike.

Ametropia

myopia

If you are nearsighted, you can see everything in focus close up, but out of focus in the distance. Reason: The light beam is already bundled in front of the retina. The parts of the eye that are important for the visual process are not exactly coordinated with one another.

The most common cause is an eyeball that is too long or too high a refractive power of the cornea or lens. These factors are partly inherited and develop particularly in the first three decades of life. External factors, such as intensive close work or reading in poor light, can promote the development of myopia.

Farsightedness

With farsightedness, far-sightedness is good, but near-sightedness is blurred. This is the focal point, i.e. the sharp image behind the retina. At least in theory. Of course, the rays also hit the retina here. However, they are not fully united at this point.

As a result, we see the picture blurred. Cause: Most of the eyeball is too short. But it can also be because the refractive power of the lens and cornea is too weak. Farsightedness is usually innate.

Presbyopia

For some, the arm is not long enough to hold the book far enough away from the eye that the letters are no longer blurred. It was all much better in the past. With age, the eyes subside. It is very natural. Presbyopia therefore does not belong to the ametropia in the narrower sense.

Presbyopia is caused by a loss of elasticity in the lens. It can no longer bulge so much. As a result, we can no longer focus properly on objects that are closer.

From the age of 40 to 50, this leads to difficulties in focusing the eye closely. It's a normal aging process that affects more or less everyone.

Astuteness

Better known as astigmatism or astigmatism. In people with astigmatism, the surface of the cornea is not smoothly rounded like the surface of a sphere, but rather deformed. It is arched to different degrees in different places.

This leads to a distorted image on the retina and thus to blurred vision. The curvature of the cornea is usually congenital, but it can also be caused by scars after corneal injuries or inflammation.