Jason and Mark were talking in class, but so was everyone else. As they continued to discuss their day's adventures, it suddenly became clear to them that the teacher was staring at them. They didn't realize that the class had been called to order and what was once only one conversation among many was now disruptive. Jason apologized quickly and the teacher resumed her normal activities. This is a good example of how a consumer's ability to detect a difference between two stimuli is ________.