Contains research objectives and reports on one research project.U.S. Air Force Contract AF19(604)-1728, monitored by the Operational Applications Laboratory, Air Force Cambridge Research Cente

Green, D. M.

Swets, J. A.

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XIX. SIGNAL DETECTION BY HUMAN OBSERVERSProf. J. A. Swets P. D. Donahue Louise IarussiProf. 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 toilluminating 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. SwetsA. THE DETECTION OF A PULSED SINUSOID OF UNCERTAIN FREQUENCYDetection experiments were conducted under two conditions. Under one condition,the observer knew that a particular signal frequency would be used on all the trials of asequence. In the other condition, the observer knew only that the signal would alwaysbe some frequency in a given range. In the second instance the particular signal fre-quency used on any trial of the sequence was determined by a random process. Theobserver was therefore uncertain as to the signal frequency on any one trial, but he didknow the range of possible signal frequencies. Under both conditions the signals wereadjusted in amplitude so that their detectability would be the same at all frequencies.The effect of frequency uncertainty was very small. In the most extreme condition, inwhich the signal ranged from 500 to 4000 cps, the decrement in detectability was approx-imately 3 db, as compared with the condition in which the signal frequency was fixed.This work was supported in part by U.S. Air Force Contract AFl9(604)-1728,monitored by the Operational Applications Laboratory, Air Force Cambridge ResearchCenter, Bedford, Massachusetts.181(XIX. SIGNAL DETECTION BY HUMAN OBSERVERS)This result is inconsistent with several models that have been proposed to explainthe effects of signal uncertainty. It is also very puzzling when compared with thecritical-band hypothesis of Fletcher. In Fletcher's experiments, the masking noisewas progressively filtered to determine which frequency components of the noise wereactually effective in masking the sinusoidal signal. The result, which has since beenverified by several investigators, was that only the noise components in a small bandaround the signal frequency were important in masking the signal. This result has usu-ally been interpreted to imply that in a detection situation the first stage of the auditorymechanism must be likened to some type of narrow-band filter. It was supposed thatthere are many of these filters in parallel, and that the one having the center frequencyof the signal is the only one used to detect a sinusoidal signal in noise. Clearly, theresult of the experiment employing frequency uncertainty calls for a re-evaluation ofthe simple critical-band hypothesis.D. M. Green182

Signal Detection by Human Observers

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Signal Detection by Human Observers

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