References

Yerkes, R. M., & Dodson, J. D. (1908). The relation of strength of stimulus to rapidity of habit formation. Journal of Comparative Neurology and Psychology, 18, 459-482. Brown, W. P. (1965). The Yerkes-Dodson law repealed. Psychological Reports, 67, 663-666.

(1989). Caffeine, impulsivity, and memory scanning: A comparison of two explanations for the Yerkes-Dodson effect. Motivation and Emotion, 13, 1-20. Anderson, K. (1994). Impulsivity, caffeine, and task difficulty: A within-sub-jects test of the Yerkes-Dodson law. Personality and Individual Differences, 16, 813-819. Teigen, K. (1994). Yerkes-Dodson: A law for all seasons. Theory & Psychology, 4, 525-547.

YO-HE-HO THEORY. See LANGUAGE ORIGINS, THEORIES OF.

YO-YO EFFECT. See HUNGER, THEORIES OF.

YOUNG-HELMHOLTZ COLOR VISION THEORY. = Helmholtz's color vision theory = Young's color vision theory = three-ele-ment/tri-receptor/triple receptor/trichromatic color vision theory. In 1801, the English physician/physicist Thomas Young (1773-1829)

proposed that color vision is due to three different kinds of visual fibers. Young's original theory was based on Sir Isaac Newton's (1642-1727) earlier demonstration in physics of the existence of three primary colors (red, green, and blue). Because Young found it difficult to conceive of each sensitive point in the retina as containing an infinite number of particles to be capable of vibrating in perfect unison with every possible undulation of light energy, he suggested that there are only three kinds of fibers corresponding to three primary colors (red, green, and blue). Young expanded his theory somewhat in 1807, but it remained unrecognized, essentially, as it sat in the Philosophical Transactions of the Royal Society of London until 1852 when the German physiologist/psychologist Hermann von Helmholtz (1821-1894) rediscovered and popularized it [the Scottish physicist James Clerk Maxwell (1831-1879) is reputed, also, to have "rediscovered" Young's early work at about the same time as did Helmholtz]. Helmholtz wrote his quantitative line-element treatment of color vision and color discrimination short-ly before his death [cf., Helmholtz's theory of accommodation - states that the shape of the lens of the eye becomes convex as the ciliary muscle relaxes, and flattens as the ciliary muscle contracts; even though the concept of the lens changing shape originated with the French philosopher Rene Descartes (1596-1650), it was Helmholtz who elaborated the physiological mechanism involved in the pro-cess]. As he developed his quantitative theory, Helmholtz studied whether or not hue could be discriminated on the basis of gradations in the intensity of three fundamental processes (red, green, blue) that are evoked whenever the retinal cones are stimulated by light energy. Today, this theory is known as the Young-Helmholtz theory, and it postulates three types of cones (red, green, blue), each containing a different chemical substance where each is sensitive maximally to a different region of the electromagnetic spectrum. Consistent with the law of specific nerve energies, the red cones (if stimulated in isolation) would give a "red" sensation, green cones would give a "green" sensation, and blue cones would give a "blue" sensation. Also, according to the theory, the rate of firing

("excitability") of each cone type depends on the wavelength of the stimulating light. Thus, the phenomenal or subjective experience of hue (i.e., "color") depends on the relative frequencies of impulses set up in the three types of fibers, brightness (i.e., "intensity") depends on the total frequency of impulses in all three fibers, and saturation (i.e., "purity") depends on the amount of white produced in any quantifiable fusion of the fibers. All the other hues (including yellow, purple, and white or gray) are due to various combinations of the three component activities. The Young-Helmholtz color vision theory contains widely accepted ideas by psychologists today and has the advantages of accounting for the laws of color mixing and of parsimony over other theories that advance the involvement of more than three receptor processes in the visual experience. However, the Young-Helmholtz theory does have a number of difficulties associated with it: accounting fully for the experiences of color-blind individuals [cf., Daltonism, which is red-green color blindness, and is named after the English chemist John Dalton (1766-1844) who had it and first described it]; accounting for the brightness functions of both normal and color-blind persons; and accounting for contrary evidence that shows that the blue component in color vision has different properties than either the red or green components. See also ACCOMMODATION, LAW/PRINCIPLE OF; COLOR MIXTURE, LAWS/THEORY OF; COLOR VISION, THEORIES/LAWS OF; SPECIFIC NERVE ENERGIES, LAW OF. REFERENCES

Young, T. (1801). On the mechanism of the eye. Philosophical Transactions of the Royal Society of London, 91, 2388.

Young, T. (1807). On the theory of light and colours. In W. Savage (Ed.), Lectures in natural philosophy. Vol. 2. London: Joseph Johnson, St. Paul's Church Yard. Helmholtz, H. von (1852). On the theory of compound colours. Philosophy Magazine, 4, 519-534. Helmholtz, H. von (1892). Versuch das psy-chophysische gesetz auf die farben unterschiede trichromatischer augen anzuwenden. Zeitschrift fur Psychologie und Physiologie Sinnesor-gange, 3, 1-20.

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