On the detection of stepwise changes in a constant signal level

A stepwise change in the frequency of a continuous pure tone is taken as an element signal. The detectability of a single step is compared with the detectability of a pattern composed of two such steps in order to determine the ways in which the steps interact and how these interactions depend upon the time interval between the steps. It is shown that two positive steps separated by D msec interact by summation (d’ for two steps being greater than d’ for a single step) and that this summation is 100% even when D=100 msec. The extent of summation decreases as D is increased beyond 100 msec, leveling off at about 40% when D is near 1,000 msec. A positive step followed by a negative step (an increment) presents a more complex picture. When D=0 msec, the pattern is the null signal, and, of course, the two steps subtract completely. As D is increased, the extent of subtraction decreases, becoming zero when D is about 400 msec. Increasing D beyond 400 msec results in summation, reaching about 40% when D=2,000 msec. For D less than about 1,000 msec, the sign of the second step in a two-step signal is important: two steps of like sign summate, and two steps of unlike sign subtract. However, when the two steps are separated by about 2,000 msec, the sign of the second step is irrelevant: summation occurs, and the extent, about 40%, is the same for two steps of like sign and two steps of unlike sign. A brief theoretical discussion stresses the extant need for an information-processing theory of signal detection.