Scientists distinguish between the sensation of black and the perception of black. Theories have been advanced to explain both aspects, one espoused by a group of physiologists whose best known representative was physicist and polymath Hermann von Helmholtz, the other developed by physiologist and brain expert Ewald Hering. For Helmholtz, the sensation of black derived from the absence of light, from the lack of light stimuli. By contrast, Hering claimed that all visual perception including seeing black involved light, explaining the perception of black as the result of contrast. Von Helmholtz discovered that the human retina has three different kinds of cone enabling us to see colours and that these do not register stimuli at night. For Hering, this did not delve deeply enough. Incorporating the brain into his theory, he proposed that both the sensation and the perception of black result from a physiological-chemical reaction taking place in a nerve fibre. The two theories were eventually reconciled. For the perception of black this meant that first, a surface appears black when it does not reflect the light striking it, that is, when it absorbs waves that the eye can register; and secondly, although we register phenomena with our sense organs, lack of information received by the rods and cones in our retina does not make it impossible to see black, because ultimately we perceive with our brain.
Nevertheless, it has not been established how exactly the brain enables us to perceive black. We possess nerve cells that react to red and green, to circles and triangles, and to moving bars, but none of our nerve cells register black, which our brain processes in the areas reserved for shapes and movement.