120 Neural Aspects of Vision \1 : I 



Accordingly, the molecular reactions are deferred to Chapter 19. 



A fundamental test of any theory of the neural aspects of vision is the 

 explanation of color discrimination. The subjective sensations of color 

 are familiar to most humans. However, at the lowest intensities where 

 objects are barely visible to the dark-adapted eye, there is no sensation 

 of color. At light intensities which are just slightly greater than this, 

 colors begin to be sensed. 



The sensation of color is a complex function of the wavelengths of 

 light reaching the eye. Just how complex this function is has been 

 emphasized by a set of experiments referred to at the end of Section 4. 

 However, when a large patch of light of the same wavelength is presented 

 to the eye, it is identified as a single color. 



A light consisting of a very narrow wavelength band is called mono- 

 chromatic. The other extreme, equal intensities at all wavelengths, is 

 called white. The sensation of white can be evoked by many compositions 

 simpler than uniform intensity throughout the visible spectrum. 

 Certain pairs of colored lights, such as blue and yellow, appear white 

 when mixed in equal proportions. The pairs of colors producing white 

 are called complementary. Sets of three colors, such as red, green, and 

 blue, as well as sets of four or more colors, also give a sensation of white. 

 Likewise, varied groups of colored lights can produce any given color 

 sensation. 



Psychophysicists distinguish several different qualities of sensations 

 associated with color vision. These include luminosity (or luminous 

 intensity), hue, and saturation. Luminosity 1 is defined as a "measure 

 of the flux of luminous energy per unit solid angle emitted by a source." 

 The luminous energy is in turn "an evaluation of radiant energy in 

 terms of its ability to produce brightness." 



A colored light, as well as a given luminosity or brightness, will 

 always have a certain hue 2 which is defined as " the quality of a sensation 

 according to which an observer is aware of differences of wavelength of 

 radiant energy." A given colored light may not be a pure hue but may 

 be mixed with white light. This is measured by the saturation 3 which 

 is the "quality of sensation by which an observer is aware of different 

 purities of any one dominant wavelength." For example, pink repre- 

 sents a mixture of red and white ; it is said to be less saturated than a 

 pure red color. Hue and saturation taken together constitute chroma- 

 ticity. 



It has been known for many years that sets of three stimuli existed, 



1 Quoted from: Committee on Colorimetry, The Optical Society of America, 

 The Science of Color. (New York: Thomas Y. Crowell Company, 1953.) 



2 Ibid. 



3 Ibid. 



