Would you notice a gorilla walking through a basketball game?

Probably not, if you were busy counting. In 1999, Daniel Simons and Christopher Chabris asked people to watch a short video of two groups of players, one in white T-shirts and one in black, passing a basketball around. The viewers had to count the passes made by one team. In some versions of the video, a person in a full gorilla suit walks right through the scene. When the video ended and viewers were asked whether they had noticed anything out of the ordinary, in most groups 50 percent of them had not seen the gorilla.

The effect is called inattentional blindness, a term chosen by Arien Mack and Irvin Rock in 1992. It is not a problem of eyesight. The gorilla was on the screen, in the middle of the picture. The researchers attribute the miss to a failure to attend to it while engaged in the difficult task of counting passes. The idea behind the research is that we consciously experience only those objects and events to which we directly attend, which means that the vast majority of information in our field of vision goes unnoticed.

The experiment became one of the most famous psychological demonstrations ever, and its authors were awarded an Ig Nobel Prize. It was also used in a British public safety advert, with the gorilla replaced by a moonwalking bear, to warn that motorists can fail to see cyclists for the same reason.

Experience changes the odds. In a study that used the gorilla video with basketball players, 62 percent of adult basketball experts noticed the gorilla, while only 38 percent of adult novices did.