“We think we should see anything in front of us, but in fact we are aware of only a small portion of our visual world at any moment. The idea that we can look but not see is flatly incompatible with how we understand our own minds, and this mistaken understanding can lead to incautious or overconfident decisions...This error of perception results from a lack of attention to an unexpected object, so it goes by the scientific name 'inattentional blindness.' This name distinguishes it from forms of blindness resulting from a damaged visual system; here, people don't see the gorilla, but not because of a problem with their eyes. When people devote their attention to a particular area or aspect of their visual world, they tend not to notice unexpected objects, even when those unexpected objects are salient, potentially important and appear right where they are looking. In other words, the subjects were concentrating so hard on counting the passes that they were 'blind' to the gorilla right in front of their eyes.”
- "What You See Is What You Get," by Manohla Dargis
Simons, D. J., & Chabris, C. F. (January 01, 1999). Gorillas in our midst: sustained inattentional blindness for dynamic events. Perception, 28, 9, 1059-74.