The figure below illustrates how signal detection theory conceives of what is going on inside of the nervous system during the detection of a faint or confusing stimulus or signal. When the signal is not present, the activity in the nervous system is not always the same. There are random variations. This aspect of our functioning is indicated by the blue line. It is our old friend the normal or bell-shaped curve. It has a mean strength and a standard deviation given arbitrary values below. This situation is the mess or noise that confuses the detection. When the signal is present, the curve moves to the right on the graph because the signals adds a constant value on to the noise. This situation is indicated by the purplish curve, the Signal + Noise curve, below. It is also a normal curve with the same standard deviation, just moved to the right. Now remember, the higher the curve the more likely that value of sensory strength is to occur. However, any part of the sensory strength axis under a curve can occur.