Imagine yourself in a dark classroom reading powerpoint slides. if an audience member were to check the internet using her cell phone and causing her screen to light up, chances are that many people would notice the change in illumination in the classroom. however, if the same thing happened in a brightly lit classroom during a discussion, very few people would notice. the cell phone brightness does not change, but its ability to be detected as a change in illumination varies dramatically between the two contexts. this is an example of