Decision processes in perception.

About 5 years ago, the theory of statistical decision was translated into a theory of signal detection. Although the translation was motivated by problems in radar, the detection theory that resulted is a general theory for, like the decision theory, it specifies an ideal process. The generality of the theory suggested to us that it might also be relevant to the detection of signals by human observers. Beyond this, we were struck by several analogies between this description of ideal behavior and various aspects of the perceptual process. The detection theory seemed to provide a framework for a realistic description of the behavior of the human observer in a variety of perceptual tasks. 1 This paper is based upon Technical Report No. 40, issued by the Electronic Defense Group of the University of Michigan in 1955. The research was conducted in the Vision Research Laboratory of the University of Michigan with support from the United States Army Signal Corps and the Naval Bureau of Ships. Our thanks are due H. R. Blackwell and W. M. Kincaid for their assistance in the research, and D. H. Howes for suggestions concerning the presentation of this material. This paper was prepared in the Research Laboratory of Electronics, Massachusetts Institute of Technology, with support from the Signal Corps, Air Force (Operational Applications Laboratory and Office of Scientific Research), and Office of Naval Research. This is Technical Report No. ESD-TR-61-20. 2 For a formal treatment of statistical decision theory, see Wald (1950) ; for a brief and highly readable survey of the essentials, see Bross (1953). Parallel accounts of the detection theory may be found in Peterson, Birdsall, and Fox (1954) and in Van Meter and Middleton (1954). The particular feature of the theory that was of greatest interest to us was the promise that it held of solving an old problem in the field of psychophysics. This is the problem of controlling or specifying the criterion that the observer uses in making a perceptual judgment. The classical methods of psychophysics make effective provision for only a single free parameter, one that is associated with the sensitivity of the observer. They contain no analytical procedure for specifying independently the observer's criterion. These two aspects of performance are confounded, for example, in an experiment in which the dependent variable is the intensity of the stimulus that is required for a threshold response. The present theory provides a quantitative measure of the criterion. There is left, as a result, a relatively pure measure of sensitivity. The theory, therefore, promised to be of value to the student of personal and social processes in perception as well as to the student of sensory functions. A second feature of the theory that attracted us is that it is a normative theory. We believed that having a standard with which to compare the behavior of the human observer would aid in the description and in the interpretation of experimental results, and would be fruitful in suggesting new experiments. This paper begins with a brief review of the theory of statistical decision and then presents a description of the elements of the theory of signal detection appropriate to human observers.