Videotapes, each 75 s in duration showed two teams of three players, one team wearing white shirts and the other wearing black shirts, who moved around in a relatively random fashion in an open area. The members of each team passed an orange basketball to one another. After 44 - 48 s of this action a woman wearing a gorilla costume walked from one side of the action to the other. The unexpected event lasted 5 s, and the players continued their actions during and after the event.
Before viewing the videotape, observers were told that they would be watching two teams of three players passing basketballs and that they should pay attention to either the team in white or the team in black. They were told that they should keep a silent mental count of the total number of passes made by the attended team. After viewing the videotape and performing the monitoring task, observers were immediately asked to write down their count(s) on paper. They were then asked to provide answers to a series of additional questions. (i) While you were doing the counting, did you notice anything unusual on the video? (ii) Did you notice anything other than the six players? (iii) Did you see anyone else (besides the six players) appear on the video? (iv) Did you see a gorilla walk across the screen? After any "yes" response, observers were asked to provide details of what they noticed. If at any point an observer mentioned the unexpected event, the remaining questions were skipped.
(Simons, D. J. & Chabris, C. F. (1999). Gorillas in our midst: sustained inattentional blindness for dynamic events. Perception, 28, 1059 - 1074.)