Sensory differential thresholds are studied by two models: a discrete model, the neural quantum theory, and a continuous model, signal detectability theory. A comprehensive series of auditory experiments is reported that was designed to obtain measures of both the detection and decision processes of the observers when the signal consists of a brief increment in the amplitude of a pure 1000-cps tone. The analyses performed on these experiments support the existence of a low differential threshold in hearing. These analyses include the location of the isosensitivity functions and the characteristics of the psychometric functions. The experiments do not, however, provide sufficient evidence to permit a definite conclusion about the validity of the neutral quantum theory, except that it is clear that the decision model applied to the quantum theory is incorrect for “high”-amplitude signals.