A famous study by Daniel Simons and Christopher Chabris (1999) provides a powerful demonstration of inattentional blindness. In the experiment, participants were instructed to count the number of basketball passes made by a team in white shirts. While they were focused on this task, a person in a gorilla costume walked through the scene for nine seconds. Remarkably, nearly half of the participants failed to notice the gorilla, illustrating how intense focus on one activity can make people completely miss other, otherwise obvious, visual stimuli.