If you don’t notice something within 1.5 seconds, you may never see it


If you were watching a basketball game and a person in a gorilla suit walked across the court, you would notice, right? A 20-year-old experiment showed that only about half of us would, and now it turns out that if you don’t notice new information in your visual field within about 1.5 seconds, you are unlikely to catch it at all.

“The reason we seem to have this inattentional blindness is that we can only take in so much visual information at once. One of the ways we deal with that is the deployment of selective attention,” says Katherine Wood at the University of Illinois at Urbana-Champaign.

Wood and her colleague Daniel Simons, who is also from the University of Illinois at Urbana-Champaign, used a series of experiments to see if increasing the length of time a new visual cue is in someone’s field of view can break through inattentional blindness. They asked people to watch black and white shapes moving in straight lines across a computer screen and bouncing off the screen edges at 45° angles.

The participants were told to count how many times the objects of one colour bounced off the edge. While they were doing this, a new, cross-shaped object passed across the screen. The cross showed for either 5 seconds or 2.67 seconds. “The noticing didn’t vary much, so we cut that short time in half again,” says Wood.

This time, two groups of participants saw the cross for either 5 seconds or 1.5 seconds. Slightly more than half of the people noticed the cross if it was displayed for 5 seconds, and slightly less than half noticed it when it was displayed for 1.5 seconds. In more detail, there was just a 12.7 per cent rise in the chances of noticing the object if it was displayed for 5 seconds rather than 1.5 seconds.

“The most natural thing to assume is that the longer it’s there, the more opportunity you have to notice it, so we were quite surprised when it turned out that it seems not to help you very much,” says Wood. The researchers found that 1.5 seconds is the limit at which most people will notice something, but they may need even less time.

Vanessa Beanland at the University of Otago in New Zealand says that in a real-world situation, unexpected objects could increase in relevance or threat over time, so we might notice them after a while. “We may not initially notice a driver behaving erratically in the distance, but as we get closer to them, their behaviour becomes more relevant to us and poses more of a hazard,” she says.

Inattentional blindness feels counterintuitive precisely because we don’t notice what we are missing, she says. “We remember vividly all the times we have noticed something unexpected or unusual, but have no idea how many times we have missed gorillas or unicycling clowns, unless a researcher explicitly brings it to our attention,” says Beanland.