First step: Generation of nerve impulses – The retina

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In evolutionary terms, the retina is an externally displaced portion of the brain surface. It is only 1/10 millimeter thick and has around 200 million closely packed, highly specialized nerve cells. The upside-down image of our environment falls on it. The retina is a curved plane that corresponds to the curvature of the eyeball, giving it the advantage of being the same distance from the lens at all points and providing a sharp image everywhere. Furthermore, regardless of the light’s angle of incidence, the curvature corresponds to the same fraction of the image scale.

The structure of the retina is notable for stacking its functional layers so that light only reaches photosensitive cone and rod cells after passing through the neuronal cells above them. This method is equivalent to inserting the film with the photographically active side facing outwards, which reduces contrast-reducing stray light. It is achievable without hazard because the neural plexus on top does not move, and our perception filters out such silent impulses from our conscious view. 

From back to front, the photoreceptors are followed by horizontal cells, then bipolar and amacrine cells, and finally ganglion cells. Each of these types of neurons exists in several forms and performs duties other than the ones listed below. For example, there are over a dozen different types of amacrine cells, as well as two major genera of ganglion cells: small magnocells and giant parvocells. Both of these are critical components of the „Categorization of Information“ part. Bipolar cells receive input signals directly from photoreceptors, and many of them have connections to ganglion cells. Horizontal cells transport data between individual receptors, while amacrine cells do the same for individual bipolar cells. This type of linkage allows for feedback (lateral inhibition) as well as the grouping of specific receptors or bipolar cells.

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