Vision: Cells in the retina, at the back wall of the human eye, react to light changes with a change in (the voltage of) the electrical signal. The signal is then transmitted to the brain via the optical nerve. The size and number of brain regions which take part in image analysis is an indicator of the special importance visual perception has for humans. Apart from the primary visual cortex, which takes up about 15% of the total cerebral cortex, more than 30 different visual areas have been identified
All in all, about 60% of the cerebral cortex is involved in perception, interpretation, and reaction to visual stimuli. The visual system (in the human brain) processes visual stimuli in around 100–150 milliseconds – the signals are therefore dispersed at a speed of up to 350 km/h (217 mph). The maximal difference in brightness, which is distinguishable by the retina, is 1 to 10. For greater differences in light, the eye needs to adjust by changing the diameter of the pupil. In combination, these two processes allow for an adjustment to light intensities from 1 Lux (moonlight) to bright sunlight. UV-exposure of the skin leads to tanning and an increase in vitamin D production. Vitamin D is essential for the absorption of calcium. A modest amount of sunlight can assist in the prevention of diseases, such as osteoporosis.