Contains a report on a research project.This work was supported in part by United States Air Force (Contract AF19(604)-1728

Swets, J. A.

Contains research objectives and reports on one research project

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XXIII. SIGNAL DETECTION BY HUMAN OBSERVERS*Prof. J. A. Swets E. F. Shipley S. A. SewallProf. D. M. Green M. J. McKey A. G. WassermanRESEARCH OBJECTIVESRecent research has shown that the theory of statistical decision, or the theory ofsignal detectability derived from it, constitutes an accurate description of the behaviorof human observers in the detection and identification of simple auditory and visual sig-nals. A primary concern of this project is to extend the range of the psychophysicalapplication of the theory through further development of the theory and through experi-mental study of detection and identification situations that are less restricted than thesimple situations investigated previously; that is, the highly idealized situations thatwere appropriate to a preliminary examination of the value of the theory in psychophys-ics. Thus we are interested in problems involving sequential observations of a signal,complex signals (including speech), large ensembles of signals, large signal-to-noiseratios, and so forth.A second concern of the project stems from the fact that the application of decisiontheory has identified new independent and dependent variables in psychophysical testing,with the consequence that methods have been refined and conceptions of sensory proc-esses have been revised. For example, the theory specifies the mode of control exertedby an observer over his criterion for signal existence, and has guided the design ofmethods of data collection and analysis that permit experimental separation of theeffects of variation in the criterion from variation in sensitivity. This result, in addi-tion to illuminating the detection process and providing more reliable indices of detect-ability, makes possible the unification of data collected under a variety ofpsychophysical procedures. Given these new tools and conceptions, we are inter-ested in re-examining some of the classical problems in sensory psychology and inexamining other substantive problems in vision and audition that show promise as aresult of these developments.J. A. SwetsA. SEQUENTIAL OBSERVATIONS OF SIGNALS IN NOISETwo studies were undertaken to apply the theory of statistical decision to detectionproblems employing multiple observations. The signals were pulsed tones, presentedin a background of white noise. In the first study, the number of observations of eachsignal event was controlled by the experimenter. In the second study, the observersdetermined how many observations they made before making a terminal decision.In the first study, two types of noise were used: variable noise (noise that is sta-tistically independent from one observation to another) and constant noise (noise that isexactly the same in each of the observations of a given signal event). With variablenoise, and a signal consisting of a single tone, the detectability of the signal increasesas the square root of the number of observations, as predicted. The use of constantnoise permits an estimation of the proportion of the total noise that affects detectionThis work was supported in part by U.S. Air Force Contract AF19(604)-1728,monitored by the Operational Applications Laboratory, Air Force Cambridge ResearchCenter, Bolling Air Force Base, Washington 25, D. C.181(XXIII. SIGNAL DETECTION BY HUMAN OBSERVERS)that is of internal origin. The result, obtained with each of two very different levels ofexternal noise, is that internal noise is approximately equal to external noise.In other experiments in the first study, signals composed of several frequencies,and signals representing various degrees of uncertainty about signal frequency, wereused. Their results were compared with the predictions of two alternative models ofthe process of frequency analysis. Although lack of space precludes presenting thedetails and derivations to support this statement, the essential result is that theobserver is apparently capable of controlling the width and number of the frequencyband(s) to which he is sensitive. In some cases, apparently he listens to a singlenarrow band; in others, apparently he listens to several bands, centered at the fre-quencies contained in the signal. Which strategy is adopted is a function of both theobserver and the task.In the second study, the observers performed a sequential test of two hypotheses:noise alone exists and signal plus noise exists. That is to say, after each observationthey accepted one of these hypotheses or requested another observation. In this appli-cation of the theory of sequential decision, the observers are assumed to establishdecision criteria in their observation space in such a way as to achieve the optimumerror rates. The optimum error rates are determined by the a priori probabilities ofthe two hypotheses, the values associated with the various possible outcomes of a ter-minal decision, the cost of making an additional observation, and the signal strength.The optimum error rates, in turn, determine the mean number of observationsrequired to achieve these error rates. Our observers behaved in an optimal manner:Their error rates and their average number of observations preceding a terminaldecision varied appropriately with changes in the parameters that ideally determinethem. A preliminary comparison of the observers' efficiency in sequential tests andin tests in which the number of observations was fixed by the experimenter shows thatthe sequential tests yield the same error rates with a 30 per cent saving in the numberof observations required.(Papers describing the results discussed here have been accepted for publication inthe Journal of the Acoustical Society of America.)J. A. Swets182

Signal Detection by Human Observers

Contains research objectives.U. S. Air Force Contract AF19(604)-7459, monitored by Operations Analysis Office, Air Force Command and Control Development Division, Bedford, Massachusett

XXIII. SIGNAL DETECTION BY HUMAN OBSERVERSProf. J. A. Swets Linda E. Branneman P. D. DonahueProf. D. M. Green Susan T. SewallRESEARCH OBJECTIVESAbout five years ago, it was shown that the behavior of the human observer - indetecting and identifying simple auditory and visual signals - may be described in termsof a theory of signal detectability modeled after statistical-decision theory. A primaryconcern of this group is to extend the range of the psychophysical application of thetheory through further development of the theory, and through experimental study ofdetection and identification situations that are less restricted than the simple situationsinvestigated previously. Thus we are interested in problems involving sequential obser-vations of a signal, complex signals (including speech), large ensembles of signals,large signal-to-noise ratios, and so forth.Our second concern stems from the fact that the application of decision theory hasidentified new independent and dependent variables in psychophysical testing, with theconsequence that methods have been refined and conceptions of sensory processes havebeen revised. For example, the theory specifies the mode of control exerted by anobserver over his criterion for signal existence, and it has guided the design of methodsof data collection and analysis which permit experimental separation of the effects ofvariation in the criterion from variation in sensitivity. This result, in addition to illu-minating the detection process and providing more reliable indices of detectability,makes possible the unification of data collected under a variety of psychophysical pro-cedures. Given these new tools and conceptions, we are interested in re-examiningsome of the classical problems in sensory psychology, and in examining other sub-stantive problems in vision and audition that show promise as a result of these develop-ments.During the past year we have conducted experiments on intensity discrimination,on observer consistency, on effects of various phase relations in binaural listening, andon memory aids in the detection setting. We have investigated some theoretical aspectsof sensory thresholds, critical bands in audition, sequential-observation procedures,and reaction time.J. A. SwetsThis work was supported in part by U. S. Air Force Contract AF19(604)-7459,monitored by Operations Analysis Office, Air Force Command and Control DevelopmentDivision, Bedford, Massachusetts.205

Contains research objectives.U. S. Air Force Contract AF19(604)-7459, monitored by Operations Analysis Office, Air Force Command and Control Development DivisionOperations Analysis Office, Air Force Command and Control Development Divisio

XIX. SIGNAL DETECTION BY HUMAN OBSERVERSProf. J. A. Swets P. D. DonahueProf. D. M. Green Susan A. SewallRESEARCH OBJECTIVESAbout five years ago, it was shown that the behavior of the human observer - indetecting and identifying simple auditory and visual signals - may be described in termsof a theory of signal detectability modeled after statistical-decision theory. A primaryconcern of this group is to extend the range of the psychophysical application of thetheory through further development of the theory, and through experimental study ofdetection and identification situations that are less restricted than the simple situationsinvestigated previously. Thus we are interested in problems involving sequential obser-vations of a signal, complex signals (including speech), large ensembles of signals,large signal-to-noise ratios, and so forth.Our second concern stems from the fact that the application of decision theory hasidentified new independent and dependent variables in psychophysical testing, with theconsequence that methods have been refined and conceptions of sensory processes havebeen revised. For example, the theory specifies the mode of control exerted by anobserver over his criterion for signal existence, and it has guided the design of methodsof data collection and analysis which permit experimental separation of the effects ofvariation in the criterion from variation in sensitivity. This result, in addition to illu-minating the detection process and providing more reliable indices of detectability,makes possible the unification of data collected under a variety of psychophysical pro-cedures. Given these new tools and conceptions, we are interested in re-examiningsome of the classical problems in sensory psychology, and in examining other sub-stantive problems in vision and audition that show promise as a result of these develop-ments.During the past year we have conducted experiments on pitch perception, colorperception, signal uncertainty, intensity discrimination, and sequential-observationprocedures.J. A. SwetsThis work was supported in part by U. S. Air Force Contract AF19(604)-7459,monitored by Operations Analysis Office, Air Force Command and Control DevelopmentDivision, Bedford, Massachusetts.195

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Signal Detection by Human Observers

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