Based on their surveys, Dr. Zeman and his colleagues estimate that 2.6 percent of people have hyperphantasia and that 0.7 percent have aphantasia.

Now Dr. Zeman and Dr. Pearson are studying an even larger swath of people who experience extremes of mental imagery. One of the original 21 people with aphantasia who were studied by Dr. Zeman, Thomas Ebeyer of Kitchener, Ontario, created a website called the Aphantasia Network that has grown into a hub for people with the condition and for researchers studying them. Visitors to the site can take an online psychological survey, read about the condition and join discussion forums on topics ranging from dreams to relationships. So far, more than 150,000 people have taken the surveys, and over 20,000 had scores suggesting aphantasia.

“This really is a global human phenomenon,” Mr. Ebeyer said. “I’ve heard from people from Madagascar to South Korea to California.”

His survey has revealed how aphantasia can spread beyond vision to other senses. “If I asked you to imagine your favorite song, most people can hear the music in their mind, whereas I can’t do that.” Mr. Ebeyer said. But some people who have come to Mr. Ebeyer’s site say that they can do just that. And some can’t hear imagined sounds, but their mind’s eye works well.

While such surveys can be informative, Dr. Pearson said that they could offer only a rough, subjective look at people’s minds because they depended on volunteers’ giving themselves scores. “Your three and my four might be the same,” he said.