Change blindness refers to the difficulty observers have in noticing large changes to visual scenes. For example, failing to notice that an object has been moved or disappeared altogether from the observer's sight. Change blindness occurs for a number of reasons. Where your attention is focused is one of them. When you're reading a book, you usually don't pay attention to the paintings that may be hanging from the wall in front of you. According to researcher Daniel Simons, attention is limited, so we have to pick and choose what we focus on. In reality, we can only focus on one thing a time, so the one thing we choose to focus our attention on, we do it in great detail. Mostly everything else passes by our awareness. Besides attention, age, how objects are presented to us, and drug use can also influence change blindness. In the 'Person Swap' experiment conducted by Derren Brown, a man dressed in all black posed as a lost pedestrian asking passerby's for directions. While the participant was giving directions, they were interrupted by men who were carrying a large painting. The man who was originally asking for directions was switched with another, also dressed in black. Most participants barely even recognized the switch, providing evidence that shifting a person's attention by causing a distraction increased change blindness.