Imagination is not just replaying what we’ve seen and heard

When you imagine a waterfall, what comes to mind? Maybe a misty cascade into a blue pool surrounded by trees. Perhaps the roar of water splashing down, as well. It’s a big mystery how brain activity differs when imagining a waterfall versus seeing or hearing one in real life. Brain scans now suggest some activity when you imagine something is similar to what happens when you see or hear the real thing.

But that overlap did not appear in brain areas that deal with single senses. Instead, it showed up in high-level brain networks. These are ones that handle inputs from multiple types of senses.

Researchers shared these findings March 31 in Neuron.

Imagining a scene such as this one activates various areas of the brain, a new study shows. sara_winter/iStock/Getty Images Plus

Neuroscientist Rodrigo Braga led the research. “When I was a teenager,” he says, “I remember the first time realizing that there’s like a voice I can hear in my head.” This “internal monologue” narrates one’s own thoughts.

“That’s really strange,” Braga recalls thinking. “Why is it that we experience those thoughts and sensations almost as if we’re perceiving them?”

He’s spent years trying to answer that question. He now works at Northwestern University’s Feinberg School of Medicine. It’s in Chicago, Ill. In a new study, his team asked eight people to imagine scenes, faces, someone else speaking, their own internal monologues and other sounds. All the while, an MRI machine scanned their brains.

Having hours of MRI data on all eight people allowed the team to create individualized brain maps for each. These showed how each person’s brain activity changed during imagination.

The imagination prompts that people got were open-ended. One was: “Imagine a castle on a hill.” Another: “Imagine a rock song playing on the radio.” After each prompt, the scientists asked people what they saw and heard in their heads. The questions probed how vivid each imagined experience had been — how clear and realistic it felt.

After completing the brain scans, Braga’s team asked follow-up questions. These explored what made some mental images more vivid than others.

Under the hood of imagination

The researchers grouped their data into two buckets. In one, they put the data on imagined locations and events. In the other, they grouped the data on imagined speech and language.

Scientists Say: Aphantasia

When people were thinking about locations or events, they reported highly vivid sights in their mind’s eye. Activity also went up in their brains’ “default network A.” This is a system of nerve cells in the brain — neurons — known to process real spatial information.

When people thought about speech or language, they reported hearing highly vivid sounds in their heads. Activity also spiked in their brains’ language network. This system of neurons is usually involved when someone reads or listens to speech.

In short, imaginary sights and sounds both enlisted parts of the brain used to process real experiences.

But imagined sensations did not activate sight- and sound-specific parts of the brain. Rather, the default network A and language network respond to new information — no matter what sense it had entered through.

Explaining the data

Other studies have found that when someone imagines a certain object they’ve just seen, vision-specific brain regions light up. In this study, prompts to imagine more general scenes did not do that.

That might be because the basic visual areas of the brain respond to details like edges, colors and lines, says Nathan Anderson. Those fine details might matter when someone is asked to picture a real object they saw. But, he notes, “people don’t necessarily imagine fine details when they are imagining a [general] scene.” So the neurons that handle specific visual details aren’t needed. Anderson is a neuroscientist who took part in the study. He works at Brigham Young University in Provo, Utah.

These findings don’t surprise Stephen Kosslyn. Since the prompts didn’t ask people to imagine detailed sights or sounds, he says it makes sense that the brain regions linked to those tasks didn’t light up. Kosslyn is a neuroscientist at Harvard University in Cambridge, Mass. He did not take part in the new work.

Open-ended prompts are a strength of the study, says Alfredo Spagna. A psychologist, he works at John Cabot University in Rome, Italy, and did not take part in the study. Everyday mental imagery is likely closer to imagining a castle on a hill than it is to picturing the tiny details of something you just saw, he says. So the open-ended prompts used here probably closely mimic what’s happening in our brains day-to-day.

But there’s still much to learn about how the brain conjures sights and sounds that feel so real. “This is one of the many papers that will come out in the next years that try to break down this obscure concept of vividness,” says Spagna.