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Light Perception - Introduction
»And if you see a long tunnel, stay away from the light!« (Donkey from ‘Shrek’)
A LIGHT PERCEPTION
1 INTRODUCTION
Orientation towards and with help of the light is phylogenetically old and thus widely spread
in the animal kingdom (Merkel 1980). The visual system is a very complex, but also very well
investigated sensory system. Large parts of the cortex and several subcortical systems are
involved in its numerous functions, including perception and localisation of objects,
controlling the eye movements, and visual control during spatial movements (Zeki 1994). The
eye itself represents the entry gate to the visual system; the information available to the visual
brain centers depends on the eye’s features (cf., Němec et al. 2007).
The physical precondition to vision, light, has been, in the history of science, described
either as particles or as waves (light waves are made up by photons, which are nothing else but
discrete packets of energy or quanta). These two descriptions, however, are not compatible,
and the twentieth century made it clear that “somehow light was both wave and particle, yet it
was precisely neither”. This paradoxon (the “wave-particle duality”) was finally explained by
quantum electrodynamics (cf., Pedrotti & Pedrotti 1993; Tipler 2004).
During the light perception process, a photon is absorbed by an atom; it subsequently
excites an electron and elevates it to a higher energy level. If the energy is great enough, the
electron may escape the positive pull of the nucleus and jump to an energy level high enough
to liberate it from the atom. On the other hand, when an electron descends to a lower energy
level in an atom, it emits a photon of light equal to the bridged energy difference (cf., Hecht
2001; Tipler 2004).
Though the physical principles of light perception are the same across the animal
kingdom, visual capabilities and underlying structures and mechanisms differ starkly. Light
sensitivity, i.e. the excitation possibility through light is in most cases bound to specific light
sensory organs, eyes, with at least one visual pigment. The respective vertebrate sensory organ
is the lens eye, a dioptrical apparatus that follows physical optics and creates a sharp and light
intensive picture on the eye’s background; this type of eye with the light perceiving structures
being turned away from the light source is called inverse (Czihak et al. 1990). The vertebrate eye
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