When their attention is otherwise engaged, people sometimes fail to notice a salient and fully visible, but unexpected object or event, a phenomenon known as inattentional blindness. Inattentional blindness can be viewed as a byproduct of attentional selection: Our ability to focus attention enables us to ignore irrelevant or distracting information, but it occasionally leads us to miss items that we might have wanted to experience.
The award-winning study on this topic by Simons and Chabris (1999) led to a best-selling book called The Invisible Gorilla: How Our Intuitions Deceive Us (Chabris and Simons, 2009). The “invisible gorilla” refers to the 1999 result as a case of “inattentional blindness” in which some participants didn’t recall seeing someone in a gorilla suit right in front of them while they focused on the passing of basketballs.