Figure 4 shows two sets of probability of occurrence curves. When the signal is stronger there is more separation between the two probability of occurrence curves. When this happens the subject's choices are not so difficult as before. They can pick a criterion to get nearly a perfect hit rate with almost no false alarms. ROC curves for stronger signals bow out further than ROC curves for weaker signals.
Less noise reduces the spread of the curves.
For example, consider the two probability of occurrence curves in Figure 5. The separation between the peaks is the same but the second set of curves are much skinnier. So the subject would have an easier time setting their criterion in order to be right nearly all the time. In reality, we have no control over the amount of internal noise. But it is important to realize that decreasing the noise has the same effect as increasing the signal strength.