Concentrating closely on a conversation can make people deaf to other sounds - even someone saying the words "I'm a gorilla", scientists have found.
Volunteers failed to notice the clearly audible "gorilla man" while their attention was focused elsewhere.
The "silent gorilla" experiment showed that sound as well as visual information can be ignored if the brain is sufficiently distracted.
In a famous demonstration of the effect, a previous study showed how someone in a gorilla suit could walk through the middle of a busy basketball game without being spotted.
For the new experiment, volunteers were asked to listen to separate conversations between two men and two women.
Half way through a "gorilla man" appeared who repeated the phrase "I'm a gorilla" for 19 seconds.
People concentrating on the men's conversations were better at detecting the gorilla man. However, most of those focused on the women's discussion failed to notice him.
The research is reported in the journal Cognition.
Psychologist Polly Dalton, from Royal Holloway, University of London, said: "The 'invisible gorilla' effect, where people fail to see a person in a gorilla suit walking through a basketball game, is now quite well-known. Our study provides the first demonstration of a similar 'silent gorilla' effect in hearing.
"We were surprised to find such extreme effects with a listening task, because people often think of hearing as an 'early warning system' that can alert us to unexpected events that occur out of sight. The fact that a lack of attention can cause people to miss even distinctive and long-lasting sounds questions this view. This has real-world implications in suggesting, for example, that talking on your mobile phone is likely to reduce your awareness of traffic noises."