Dr Adam Zeman didn’t give much thought to the mind’s eye until he met someone who didn’t have one. In 2005, the British neurologist saw a patient who said that a minor surgical procedure had taken away his ability to conjure images.

Over the 16 years since that first patient, Dr Zeman and his colleagues have heard from more than 12,000 people who say they don’t have any such mental camera. The scientists estimate that tens of millions of people share the condition, which they’ve named aphantasia, and millions more experience extraordinarily strong mental imagery, called hyperphantasia.

In their latest research, Dr Zeman and his colleagues are gathering clues about how these two conditions arise through changes in the wiring of the brain that join the visual centres to other regions. And they’re beginning to explore how some of that circuitry may conjure other senses, such as sound, in the mind. Eventually, that research might even make it possible to strengthen the mind’s eye — or ear — with magnetic pulses.

“This is not a disorder as far as I can see,” said Dr Zeman, a cognitive scientist at the University of Exeter in Britain. “It’s an intriguing variation in human experience.”

The patient who first made Dr Zeman aware of aphantasia was a retired building surveyor who lost his mind’s eye after minor heart surgery. To protect the patient’s privacy, Dr Zeman refers to him as M.X.

When M.X. thought of people or objects, he did not see them. And yet his visual memories were intact. M.X. could answer factual questions such as whether former Prime Minister Tony Blair has light-coloured eyes. (He does.) M.X. could even solve problems that required mentally rotating shapes, even though he could not see them.

To better understand aphantasia, Dr Zeman and his colleagues invited their correspondents to fill out questionnaires. One described the condition as feeling the shape of an apple in the dark. Another said it was “thinking only in radio”.

The vast majority of people who reported a lack of a mind’s eye had no memory of ever having had one, suggesting that they had been born without it. Yet, like M.X., they had little trouble recalling things they had seen.

On the other hand, people with aphantasia don’t do as well as others at remembering details of their own lives. It’s possible that recalling our own experiences — known as episodic memory — depends more on the mind’s eye than does remembering facts about the world.

To their surprise, Dr Zeman and his colleagues were also contacted by people who seemed to be the opposite of M.X. — they had intensely strong visions, a condition the scientists named hyperphantasia.

Joel Pearson, a cognitive neuroscientist at the University of New South Wales, Australia, who has studied mental imagery since 2005, said hyperphantasia could go far beyond just having an active imagination. “It’s like having a very vivid dream and not being sure if it was real or not,” he said.

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

Now Dr Zeman and 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, Canada, 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.

“This really is a global human phenomenon,” Ebeyer said.

Pearson has developed ways to study aphantasia and hyperphantasia without relying solely on surveys. In one experiment, he took advantage of the fact that our pupils automatically constrict when we look at bright objects. When Pearson and his colleagues asked most people to picture a white triangle, their pupils also shrank.

But most people with aphantasia whom they studied didn’t have that response. Their pupils stayed open.

In another experiment, Pearson took advantage of the fact that people’s skin becomes more conductive when they see frightening scenes. He and his colleagues monitored the skin of volunteers as they read scary stories that were projected on a screen in front of them. When most people read about frightening experiences such as being attacked by a shark, they experienced a spike in skin conductance. But people with aphantasia did not.

In a study published in May, Dr Zeman and his colleagues scanned the brains of 24 people with aphantasia, 25 people with hyperphantasia and 20 people with neither condition.

The scientists had the volunteers lie in the scanner and let their minds wander. Those with hyperphantasia had stronger activity in regions linking the front and back of the brain. They may be able to send more potent signals from decision-making regions of the front of the brain to the visual centres at the back.

For those used to seeing things with their mind’s eye, aphantasia might seem like a debilitating condition. But Dr Zeman’s research doesn’t suggest that to be the case; aphantasia may even have some advantages over hyperphantasia.

Hyperphantasia creates images that seem so real that it may open the way to false memories. Similarly, people with no mind’s eye may escape some of the burdens caused by reliving traumatic experiences, because they don’t have to visually replay them.

NYTNS