A matter of perception: why lifeguards sometimes fail to see victims on the bottom of the pool.(Risk Management)

It's a question that has haunted the industry for years: How can diligent, vigilant and well-trained lifeguards miss so many unconscious bodies on the bottoms of clear-water swimming pools and waterparks?

Actually, the answer may be quite simple: It's called, "perceptual blindness," and to save more lives, it's imperative that the industry understand it.

Perceptual blindness--including related phenomena known as inattentional blindness and change blindness--occurs commonly in humans. When people are engaged in an engrossing task, such as monitoring swimmers in a pool, they often fail to notice otherwise obvious events because they happen outside the immediate focus of attention. In this way, perceptual blindness can help explain why lifeguards fail to detect victims on the bottom of swimming facilities.

Real-life case studies of this blindness include drivers running over bicyclists, train engineers plowing into cars, submarine pilots surfacing under ships and airline pilots landing on other planes. In each case, the object or obstruction should have been easily noticed but was not.

That's because even though the observers were "looking" right at the missed events, their attention was focused on other visual stimuli, or they were otherwise cognitively engaged (e.g., talking on a cell phone). Strikingly, those involved in these crashes usually have no idea there was an object there, and cannot explain their failure to have seen it.

A demonstration of inattentional blindness goes something like this. Viewers are asked to monitor three basketball players in white T-shirts and count the number of times they pass the ball during a video clip. Thirty-four seconds into this experiment, a person wearing a gorilla suit walks through the game and even pauses to pound his chest before moving on. Despite their vigilance, approximately half the viewers never see the gorilla. Even after they are told about the gorilla and shown the video, they refuse to believe it. "Foul!" they cry, "that must be a different tape!"

Demonstrations of change blindness, which is the failure to notice large changes across different views of a scene, produce similar results. Research on these forms of blindness is being conducted at Harvard, Yale, Cornell, Illinois, Penn State, University of Paris and other institutions around the world.