††† In the intensity discrimination experiment discussed above, the intensities shown at any given time are I and I + DI. If the observer is assumed to spend some time before making a decision, the result is obtained when the observer is adapted to the intensity level I. When the intensity level the observer is adapted to is different from I, the observerís intensity resolution ability decreases. Suppose we run the same intensity discrimination experiment discussed in Section 2.3.1. but with I and I + DI surrounded by a much larger region with intensity Io, as shown in Figure 2.19. The result of plotting DI/I as a function of I and Io is shown in figure 2.19.
Figure 7.20. When Io, equals I. the result is the same as that in Figure 2.17. When Io is different from I, however, the j.n.d. DI increases relative to the case Io=I, indicating that the observerís sensitivity to intensity decreases. The result shows that sensitivity to intensity is highest near the level that the observer is adapted to. This is another way in which the visual system responds to a wide range of intensities at different times.