Determining the physiological input stage – The retina

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In evolutionary terms, the retina is an externally displaced portion of the brain. 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. In addition, the curvature provides for the same proportion of the image scale regardless of the angle of incidence of the light.

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 approach involves inserting a film with the photographically active side facing outward, which decreases 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 cells and amacrine cells, and finally ganglion cells. Each of these neuron types occurs in different varieties and fulfills other tasks in addition to the following basic functions. For example, there are over a dozen different types of amacrine cells, as well as two major genera of ganglion cells: small magno cells and giant parvo cells. 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

Next Determining the physiological input stage – The photoreceptors in general

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