How does seeing work?
The visual perception is one of the most complex functions of the human body. In the meantime, the stimulus absorption by the eye, the nerve transmission, and the structures of the visual cortex can be understood quite accurately. however, how the brain reassembles and adequately interprets this information in detail is far from being conclusively explored. But what do we actually perceive on a physical level?
Our perception is based primarily on light waves, On the retina are different types of receptors, which convert these light waves into electrical signals and transmit them via the optic nerve to the brain. The spectrum of light that our eye is able to perceive extends from approximately 380 nm to 780 nm. When looking at a rainbow, the colors violet, blue, green, yellow and red stand out in particular. In between there are still countless subtotals, as well as differences in the brightness of the colors. Estimates of the number of colors perceived by humans vary between 100,000 and 15 million colors.
Many species of animals also perceive (380-780nm) light outside this spectrum. For example, Ultraviolet radiation below 380 nm (insects) or infrared radiation above 780 nm (some fish species). At the same time, this means that "our" perceived world is only a fraction of what exists in reality. In the course of evolution, this spectrum proved beneficial for the ecological niche occupied by the human. Animals with other living environments, on the other hand, can benefit from a different perceived radiation area.
In the following the essential processes of the perception process are presented. The structure of the eye and the comparison of sticks and cones each have their own article.
Expiration of visual perception
1. The eye
Light waves fall through the cornea (cornea) of the eye, via the pupil, lens and glass body to the retina. While the cornea protects the eye from outside influences and ensures the refraction of light, the pupil regulates the incidence of light. The lens bundles the light, which then passes through the underlying glass body, a gelatinous substance, and hits the retina. By means of the retina, the light waves are converted into electrical excitations. For this purpose, the so-called bipolar nerve cells on the retina process the information from the photoreceptors (sticks and cones) and pass them on to the ganglion cells. The ganglion cells form the innermost layer of the eye and transmit the electrical excitations to the optic nerve (optic nerve).
2. The optic nerve
The left and right eye each have an optic nerve, which in turn consists of about 800,000 nerve fibers. At an optic chiasm both optic nerves cross and pass in a tract of an optic. Because of an optic nerve crossover the left brain's half receives information of the right eye, and the right brain half of the left eye.
3. The brain: thalamus, brainstem and visual cortex
The optic nerve optic nerves carry visual information in the form of electrical excitations to the lateral geniculate nucleus, a nucleus in the thalamus. Here about 85% of the nerve fibers, the other 15% run further to the hypothalamus and epithalamus. This information is not for visual processing, but reflexes related to vision, such as the pupillary reflex, the blinking reflex or the accommodation of the eyes.
In the corpus geniculatum, a substantial portion of the visual processing occurs. From there, the information is passed on to the visual cortex. The visual cortex has a particularly high density of nerve cells, as it maps the incoming information. For the interpretation, recognition of objects or the face recognition then further, connected with the visual cortex, brain structures involved.