Welcome to Quantum Magazine's podcast. Each episode, we bring you stories about developments in science and mathematics.

I'm Susan Vallett. How do you know the difference between something real and something imagined?

New experiments show that the brain distinguishes between perceived and imagined mental images

by checking whether they cross a reality threshold.

That's next.

Quantum Magazine is an editorially independent online publication supported by the Simon's Foundation

to enhance public understanding of science.

The brain constantly processes streams of visual signals from the eyes and purely mental pictures from our imagination. Brain scan studies have

repeatedly found that seeing something and imagining it evoke similar patterns of neural activity.

Yet for most of us, the subjective experiences they produce are clearly different. Thomas

Nazillaris is an associate professor at the University

of Minnesota. You have two signals that represent completely independent content. Like, I can

look outside my window right now, and if I want to, I can imagine a unicorn walking down the street.

And my brain has to represent those two things simultaneously.

It's very clear to me which of those two sensory representations corresponds to

reality outside of my brain.

We also know that when I'm seeing the world and I'm imagining the unicorn walking through it, it's

the same brain regions, at least to the level that we've been able to resolve with

the fMRI, you've got the same brain regions that are representing both sources of content.

The knowledge that unicorns are mythical barely plays into that.

A simple imaginary white horse would seem just as unreal.

Nadine Dykstra is a postdoctoral fellow at University College London.

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