Dr. Adam Zeman didn’t think much about the mind’s eye until he met someone who didn’t have one. In 2005, the British neurologist saw a patient who said minor surgery had robbed him of his ability to conjure up images.

In 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 such a mental camera. Scientists estimate that tens of millions of people have the disease they have called aphantasia, and millions more experience an exceptionally powerful mental image called hyperphantasia.

In their latest research, Dr. Zeman and colleagues suggest how these two conditions arise from changes in the wiring of the brain that connect the visual centers to other regions. And they begin to explore how some of these circuits can invoke other senses, such as sound, into the mind. Finally, this research could even make it possible to strengthen the mind’s eye – or ear – with magnetic pulses.

“As far as I can tell, this is not a disorder,” said Dr. Zeman, a cognitive scientist at the University of Exeter in the UK. “It’s a fascinating variation on the human experience.”

The patient who was Dr. Aphantasia was first introduced to Zeman when he was a retired building surveyor who lost his mind’s eye after undergoing minor heart surgery. In order to protect the patient’s privacy, Dr. Zeman as MX

When MX thought of people or objects, he didn’t see them. Yet his visual memories were intact. MX could answer factual questions, such as whether former Prime Minister Tony Blair has light eyes. (He does.) MX was even able to solve problems that required mentally rotating shapes even though he couldn’t see them.

I came across the MX case study in 2010 and wrote a column about it for Discover Magazine. After that, I got emails from readers who had the same experience but were noticeably different from MX: They had never had an eye initially.

I forwarded the news to Dr. Zeman, who interviewed 21 of my readers. In a 2015 report of these findings, he and his colleagues suggested that these readers all had the same condition, which the researchers called Aphantasia. I reported on this second study for the New York Times, as well as other journalists in their own publications. The growing attention turned Dr. Zeman’s email trickle into a stream.

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 “only thought on the radio”.

The vast majority of people who reported a lack of spiritual eye had no memory of ever having one, suggesting they were born without it. But like MX, they had no trouble remembering things they had seen. When asked whether, for example, grass or pine needles are a darker shade of green, they answered correctly that the needles are.

On the other hand, people with aphantasia cannot remember details of their own lives as well as others. It is possible that retrieving our own experiences – known as episodic memory – depends more on the mind’s eye than remembering facts about the world.

To their surprise, Dr. Zeman and his colleagues also contacted by people who appeared to be the opposite of MX: They had intensely powerful visions, a state that scientists called hyperfantasy.

Joel Pearson, a cognitive neuroscientist at the University of New South Wales who has been studying mental imagery since 2005, said hyperphantasia can go well beyond an active imagination. “It’s like having a very vivid dream and not sure whether it was real or not,” he said. “People watch a movie and then they can watch it again in their minds and it’s indistinguishable.”

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

Now, Dr. Zeman and Dr. Pearson created an even larger group of people who experience extreme mental imagery. One of the original 21 people with Aphantasia identified by Dr. Zeman, Thomas Ebeyer of Kitchener, Ontario, created a website called the Aphantasia Network, which has become a hub for people with the condition and for researchers studying it. Visitors to the site can take an online psychological survey, read about the condition, and participate in discussion forums on topics ranging from dreams to relationships. To date, more than 150,000 people have taken the surveys, and over 20,000 had scores suggestive of aphantasia.

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

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

While such surveys can be informative, Dr. Pearson that they could only offer a rough, subjective look at people’s thoughts as they relied on volunteers to score themselves. “Your three and my four could be the same,” he said.

Dr. Pearson has developed methods to study aphantasia and hyperfantasy without relying solely on surveys. In an experiment he made use of the fact that our pupils automatically narrow when we look at bright objects. When Dr. Pearson and his colleagues asked most people to imagine a white triangle, their pupils also contracted.

But most of the people with aphantasia they studied didn’t have that reaction. Their pupils remained open no matter how hard they tried to visualize the white triangle.

In another experiment, Dr. Pearson, the fact that people’s skin becomes more conductive when they see scary scenes. He and his colleagues watched the skin of volunteers as they read creepy stories projected onto a screen in front of them. When most people read about scary experiences like a shark attack, they saw an increase in skin conductivity. But people with aphantasia didn’t.

The study suggests that the mind’s eye acts as an emotional amplifier, reinforcing both the positive and negative feelings generated by our experiences. People with aphantasia may have the same feelings from their experiences, but they don’t reinforce them with mental imagery later.

Researchers are also starting to use brain scans to find the circuits that cause aphantasia and hyperphantasia. So far, this work suggests that mental images arise from a network of brain regions that speak to each other.

Decision-making regions on the front of the brain send signals to regions on the back that are usually useful for information from the eyes. These top-down signals can cause the visual regions to produce images that are not there.

In a study published in May, Dr. Zeman and colleagues scanned the brains of 24 people with aphantasia, 25 people with hyperphantasia, and 20 people without the disease.

The scientists left the volunteers in the scanner and let their thoughts wander. The people with hyperfantasy had more activity in regions that connect the front and back of the brain. You may be able to send stronger signals from decision-making areas on the front of the brain to the visual centers on the back.

The strength of the spiritual eye can have a subtle influence on people’s lives. The questionnaires from Dr. Zeman showed that people with aphantasia are more likely than average to have a job that involves science or math. Genome pioneer Craig Venter even claimed that Aphantasia helped him as a scientist by removing distractions.

But that is anything but a hard and fast rule. Charles Darwin left writings that suggested hyper-imagination: when asked to remember the objects that were on his breakfast table that morning, he said they were “as clear as if I had photographs in front of me.”

Likewise, people with vivid mental images do not have a monopoly on creative work. Ed Catmull, the former president of Pixar, announced in 2019 that he was suffering from Aphantasia.

To those who are used to seeing things with the mind’s eye, aphantasia may seem like a debilitating disease. But dr. Zeman’s research does not suggest this. In fact, Aphantasia can even have some advantages over hyperphantasia.

Hyperphantasia creates images that seem so real that they can pave the way for false memories. Similarly, people without a mind’s eye can escape some of the stresses of reliving traumatic experiences by not having to visually repeat them.

“Anecdotally, they’re really good at moving on,” said Dr. Zeman. “One wonders if that is because they are less concerned about the images that come to mind to many of us that evoke regret and longing.”

Dr. Pearson said that one day it might be possible to give people with aphantasia a spiritual eye that they never had. He found that delivering non-invasive magnetic impulses to the visual centers in the brain of average people makes their mental images more vivid. He suspects that the impulses calm the activity of the visual centers, making them more receptive to inquiries from the front of the brain.

In theory, magnetic impulses combined with cognitive training could enable people without a mind’s eye to strengthen the circuits necessary for mental imagery. But dr. Pearson is unsure whether it would be right to undertake such a procedure. If a person regretted such a surge in intrusive images, the scientist may not be able to close the mind’s eye again. “There is a dark side to that,” he said.

For his part, Mr Ebeyer said he would use Dr. Only consider Pearson if his mind’s eye only lasted a few days. He’s not interested in being plagued by unwanted visions.

“If it were an experience where you could take that pill and imagine yourself forever, I probably wouldn’t risk it,” he said.