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Subjective constancy or perceptual constancy is the perception of an object or quality as constant under changing conditions.

In vision this means perceiving a color as "constant under changing conditions of illumination" (Erickson 1975, p.11-12) and "is the achievement of a very complicated 'calculation' by an unconsciously working apparatus within our central nervous system" (Lorenz 1961, p.171).

"The facts behind color-constancy phenomena...are that we require fine color discriminations less frequently than gross discriminations, and when gross discriminations enable us to maintain focus on objects of prime interest, we 'systematically overlook' differences beyond the necessary degree of fineness. The mechanism which accomplishes this 'systematic overlooking' is the information-processing system of the organism, and the principle according to which it is accomplished is that this system never expands more of its capacity on a given perceptual task than is necessary according to the current needs and interests of the agent." (Sayre 1968, p.151-152)

There are several types of perceptual constancies:-

• Shape constancy
• Size constancy
• Color constancy
• Light constancy
• Distance constancy
• Location constancy

In music, subjective constancy is the identification of a musical instrument as constant under changing timbre or "conditions of changing pitch and loudness, in different environments and with different players." (Erickson ibid)

In speech perception this means that vowels or consonants are perceived as constant categories even if acoustically, they vary greatly due to phonetic environment (coarticulation), speech tempo, speaker's age and sex, speaker's dialect, etc.

• Erickson, Robert (1975). Sound Structure in Music, University of California Press. ISBN 0-520-02376-5.
• Lorenz, K. (1961). "The Role of Gestalt Perception in Animal and Human Behaviour" in Aspects of Form, p.157-178, ed. Lancelot Law Whyte, Indiana University Press.
• Sayre, K.M. (1968). "Toward a Quantitative Model of Pattern Formation" in Philosophy and Cybernetics, p.149-152, ed. Frederick J. Crosson and Kenneth M. Sayre, Simon and Schuster.

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