The famous Christopher Chabris and Daniel Simons 1999 experiment. In his study, the subjects watched a video of a group passing a basketball around. Half the group wore white shirts. The other half wore black. The viewers were asked to count how many passes each team made.
The total count was irrelevant. The experiment was actually about ‘selective attention’. Half way through the video, a student wearing a gorilla suits walks through the middle of the players and thumps its chest, before walking off the other side.
Apparently 50% of people who watch this video (unaware of its purpose, of course) do not notice the gorilla at all. How could they miss something so blindingly obvious?
According Chabris and Simons’ book, The Invisible Gorilla, the reason is this:
‘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.’
(Excerpt from http://www.dailyreckoning.com.au/invisible-gorilla-australian-real-estate/2014/10/08/)