It may affect 1 in 50 people, but you’ve never heard of it.

Close your eyes and imagine a dog. Picture the color of its fur, the wag of its tail, the way its ears perk up when it sees you. I bet it’s a pretty cute dog, right?

Now close your eyes and imagine seeing nothing. No dog, no wagging tail, no ears to speak of — just blackness.

For the majority of people, imagination is something that just happens; when you are told to visualize a dog, your brain can easily conjure the images and project them like a movie, using your mind’s eye. If someone tells you to imagine a beach, you either picture a beach that you’ve previously visited, or you make up your own damn beach, complete with pizza umbrellas and robot sharks and whatever other weird stuff you’re into.

But for some people, visualizing a dog or a beach or anything at all is completely impossible. The inability to conjure images using the mind’s eye is a real condition, and it’s called aphantasia — the absence of fantasy. This neurological affliction is believed to affect up to one in every 50 people; the kicker is that most of them don’t realize there’s anything wrong. But how could they? They’ve never experienced the world—or themselves—any other way.

Understanding aphantasia

Professor Adam Zeman is a neurologist at the University of Exeter Medical School. In 2005, he received a visit from a 65-year-old man who had recently undergone a minor surgical procedure. The man was concerned because after his surgery, he lost the ability to see with his mind’s eye. Professor Zeman and his team of researchers studied the man’s brain and discovered that when the man was asked to call to mind faces using his mind’s eye, certain brain regions failed to activate. In 2015, Zeman and his team published their findings in a study and officially coined the term “aphantasia.”

Back in 2005, Zeman and his fellow researchers were able to help their patient because that man realized something was different: Before his surgery, he was able to experience mental imagery. When it was over, he no longer could. But before the study was published, many people who suffered from aphantasia didn’t even realize they had a problem.

Such was the case for Blake Ross, a Facebook engineer. Ross was 30 years old when he read the article about Dr. Zeman and his research. It completely transmogrified his thinking — before the article, he never knew that other people were able to visualize their thoughts. When people told him to count sheep, he thought they were speaking metaphorically.

Ross’s inability to use his mind’s eye also extends to other senses: He can’t hear music in his brain and he says he’s never had a song stuck in his head. He describes his dreams as a series of plot points rather than visual or sensory experiences. His memory is similarly compromised: He can easily recall a long series of random numbers, but has trouble answering questions about what he did earlier in the day.

Aphantasia: just another way of experiencing the world

If you’ve reached the end of this article without having a major aha moment, you probably don’t have the condition. In 2009, psychology professor Bill Faw surveyed 2,500 people and his results suggest the condition affects approximately 2% of the population. If you’re concerned you might be one of them, try taking this test or reach out to Adam Zeman, who is still conducting research on the subject. And don’t worry — Zeman doesn’t consider aphantasia a disorder. It’s just another way of experiencing the world.