At the outset, subjects learned to associate a label with each element in a set of perceptual magnitudes (visual extents), using traditional paired-associate learning methods, Subsequently, on some trials, subjects indicated which pair of two pairs of labels corresponded to the more similar perceptual referents, and, on other trials, they selected the more dissimilar pair. It is shown that these similarity comparisons satisfy the axioms (transitivity and intradimensional subtractivity) necessary to conclude that they are based on computation of the difference of the differences of analogue-based interval scale representations.The findings also permitted refutation of the idea that memory for elementary percepts arises from their reperception. Notably,the memory exponent was 0.697, but the perception exponent was 0,546, and the reperception idea requires that the memory exponent be the square of the perception exponent (0.546 2 = 0.298). Symbolic distance effects and enhanced response time-based semantic congruity effects, typicallyfound with binary comparisons, extend the range of commonalties found between perceptual and memory psychophysics.Memory psychophysics extends Fechner's (1860/1966) outer psychophysics to a determination of the quantitative relationship between the physical magnitude of a stimulus and its subjective magnitude-not as it is perceived but, rather, as it is remembered. In this article, we explore the properties of the representation oflong-term memory for elementary visual percepts and the representation of the percept itself, and we examine their interrelationships in detail. We begin with a brief review of the principal findings based on the application of classical psychophysical techniques to remembered stimuli, and we discuss some of the limitations of that work. We then address the fundamental question of whether a memorial This research was supported by a grant from the Natural Sciences and Engineering Research Council of Canada to W.M.P. Portions of this work were presented at the XXV International Congress of Psychology, Brussels, July, 1992 . The experiment was conducted by R.K. as part of her master's degree requirements at the Department of Psycho 1-ogy, Carleton University. Our thanks to Geoffrey Loftus, Gregory Lockhead, and two anonymous reviewers for their helpful comments. Correspondence should be addressed to W. M. Petrusic, Department of Psychology, Carleton University, Colonel By Drive, Ottawa, Ontario, K1S 5B6, Canada (e-mail: biILpetrusic@carleton.ca).-Accepted by previous editor, Geoffrey R. Loftus representation of a basic element of sensory experience can be maintained in memory at the level of an interval scale, by extending the axiomatic approach offundamental measurement (Krantz, Luce, Suppes, & Tversky, 1971) to the domain of memory psychophysics by requiring comparative judgments ofsimilarity/dissimilarity. Specifically, the subjects were presented with two pairs of pairs (quads) and were required, on some trials, to indicate which pair was more similar and, o...