and Birdsall’s work on the same campus as it began. Tanner became acquainted through his interest in mathematical and electronic concepts as models for psychological and neural processes and Swets was attracted because of his interest in quantifying the ‘instruction stimulus’ in psychophysical tasks, in order to control the observer’s response criterion. As they pursued studies in sensory psychology, these psychologists had become dissatisﬁed with psychophysical and psychological theories based on overlapping bell-shaped functions that assumed ﬁxed decision cutpoints and incorporated measures only of discrimination capacity, which were possibly confounded by attitudinal or decision processes. Prominent among such theories were those of Gustav Theodor Fechner (1860), Louis Leon Thurstone (1927), and H. Richard Blackwell (1963); see Swets (1996, Chap. 1).
Fechner and Thurstone conceived of a symmetrical cutpoint, where the two distributions cross, because they presented two stimuli on each trial with no particular valence; that is, both stimuli were ‘signals’ (lifted weights, samples of handwriting). Fechner also studied detection of single signals, but did not compare them to a noise-alone alternative and his theory was essentially one of variable representations of a signal compared to an invariant cutpoint. This cutpoint was viewed as a physiologically determined sensory threshold (akin to all-or-nothing nerve ﬁring) and hence the values of the sensory variable beneath it were thought to be indistinguishable from one another.
Blackwell’s task and model were explicitly of signals in noise, but the observer was thought to have a ﬁxed cutpoint near the top of the noise function, for which the false-alarm probability was negligible, and, again, the values below that cutpoint were deemed indistinguishable. For both threshold theorists, the single measure of performance was the signal strength required to yield 50 percent correct positive responses, like the eﬀective SNR of electronic detection theory, and it was taken as a statistical estimate of a sensory threshold. In psychology, the curve relating percentage of signals detected to signal strength is called the psychometric function. There had been some earlier concern in psychology for the eﬀect of the observer’s attitude on the measured threshold, e.g., Graham’s (1951) call for quantiﬁcation of instruction stimuli in the psychophysical equation, but no good way to deal with non-sensory factors had emerged.
In 1952, Tanner and Swets joined Peterson and Birdsall as staﬀ in a laboratory of the electrical engineering department called the Electronic Defense Group. They were aware of then-new conceptions of neural functioning in which stimulus inputs found an already active nervous system and neurons were not lying quiescent till ﬁred at full force. In short, neural noise as well as environmental noise was likely to be a factor in detection and then the observer’s task is readily conceived as a choice between statistical hypotheses. Though a minority idea in the history of psychophysics (see Corso 1963), the possibility that the observer deliberately sets a cutpoint on a continuous variable (weight of evidence) seemed likely to Tanner and Swets. A budding cognitive psychology (e.g., ‘new look’ in perception) supported the notion of extrasensory determinants of perceptual phenomena, as represented in SDT by expectancies (prior probabilities) and motivation (beneﬁts and costs).
6. Psychophysical Experiments
The new SDT was ﬁrst tested in Swets’s doctoral thesis in Blackwell’s vision laboratory (Tanner and Swets
1954; Swets et al. 1961–see Swets 1964, Chap. 1). It was then tested with greater control of the physical signal and noise parameters in new facilities for auditory research in the electrical engineering laboratory. Suﬃciently neat empirical ROCs were obtained in the form of the curved arc speciﬁed by SDT. ROCs found to be predicted by Blackwell’s ‘high-threshold’ theory—straight lines extending from some point along the left axis, that depended on signal strength, to th