Think back on all the times someone has tossed something your way – a book, a pen, a sweatshirt — with the intention that you would catch it. Much of the time, you probably do catch whatever it is, or at least kind of fumble at it. Every so often, though, maybe you miss it entirely, with no idea it’s coming your way until it smacks you in the side of the face.
It’s okay. It doesn’t mean you’re a klutz. It could mean you’re experiencing motion blindness, an inability to see moving objects. The blind spot isn’t just for driving — plenty of people have one all the time. But a study published this week in Psychological Science made the case that it has nothing to do with blindness, and in fact isn’t a vision problem at all — that, instead, it happens when something gets lost in translation between the eyes and the brain.
Issues of this type fall under the umbrella of agnosia, in which the brain has a hard time interpreting the visual information it receives. One of the most well-known is face-blindness; in that case, people can still “look right at [a face] and tell you where the nose and the mouth are,” study co-author Bas Rokers, a psychology professor at the University of Wisconsin, explained in a statement. “They just can’t integrate that information to figure out identity, because identity depends on the relationship between those elements.”
Similarly, the researchers argued that motion blindness is the breakdown of one of two visual cues that we use to make sense of motion. The first is changing disparity, or how the brain combines information from both eyes to make sense of a moving object’s true location in space. Stare at something in front of you and close one eye, and it’ll shift slightly; similarly, as you watch something move, each eye will take in that change from a slightly different vantage point. Changing disparity cues bridge-build a gap between the two. And the second is inter-ocular velocity, which “works the same way, but compares the speed of the object as it moves across each retina,” Rokers said.
For the study, Rokers and his colleagues fitted their volunteers with special glasses that allowed them to focus each eye on a different visual task, like watching dots moving across a screen, and also allowed the researchers to test the two cues one at a time. The majority of the participants struggled with one or the other — implying (a) that a deficit in motion-detecting abilities is fairly common, and (b) that we typically over-rely on one cue to compensate for the weakness of the other.
In fact, the researchers noted, most people with this type of agnosia, which typically affects just one sliver of the whole field of vision, will go through life without knowing about it. You’ll just have a misstep every now and then, unaware that your brain’s tripping you up.