Ewald Hering in the context of Karl-Ferdinands-Universität

In the classical science of vision, directional vision describes that when a point of the retina is stimulated by light, this not only leads to a light sensation (image), but also a directional sensation that is recorded by the brain in one combined image of both eyes, as a direction in which the observer is central (egocentric direction).

Alhazen (Ibn al-Haytham), an Arab scholar from the 11th century, was the first to propose that vision is possible because light reflects off objects and then enters the eye after which perceptions arise in the brain that are partly the result of activities of the observer, such as directing the eyes. In the 19th century, this idea was elaborated by Ewald Hering. He assumed that each eye saw direction (visual direction) and introduced the idea of a "cyclopean eye" for egocentric direction, as if we saw the world from a single central point between both eyes. Combined with the results of research on the horopter this can explain single and double images. The explanation of sensory fusion, in which two double images merge into one new image with a new direction, was only possible when nerve cells were found in the brain that become active when a specific direction is stimulated simultaneously in the left and right eyes. The perception of depth based on differences in the directions between both eyes (disparity) is discussed in a separate article, Stereopis. The following describes the main line of this, mainly classical, development and discusses concepts that help in reading the source documents.

The Natural Colour System (NCS) is a proprietary perceptual color model. It is based on the color opponency hypothesis of color vision, first proposed by German physiologist Ewald Hering. The current version of the NCS was developed by the Swedish Colour Centre Foundation, from 1964 onwards. The research team consisted of Anders Hård, Lars Sivik and Gunnar Tonnquist, who in 1997 received the AIC Judd award for their work. The system is based entirely on the phenomenology of human perception as opposed to color mixing. It is illustrated by a color atlas, marketed by NCS Colour AB in Stockholm.

The opponent process is a hypothesis of color vision that states that the human visual system interprets information about color by processing signals from the three types of photoreceptor cells in an antagonistic manner. The three types of cones are called L, M, and S. The names stand for "Long wavelength sensitive,” "middle wavelength sensitive," and "short wavelength sensitive." The opponent-process theory implicates three opponent channels: L versus M, S versus (L+M), and a luminance channel (+ versus -). These cone-opponent mechanisms were at one time thought to be the neural substrate for a psychological theory called Hering's Opponent Colors Theory, which calls for three psychologically important opponent color processes: red versus green, blue versus yellow, and black versus white (luminance). The Opponent Colors Theory is named for the German physiologist Ewald Hering who proposed the idea in the late 19th century. However, it has been argued that Hering’s Opponent Colors Theory lacks adequate phenomenological and empirical support, and may not be a necessary feature of normal human color experience. Correspondingly, considerable physiological and behavioral evidence proves that the physiological cone opponent mechanisms do not constitute the neurobiological basis for Hering's Opponent Colors Theory.