How does your brain know something is real?

It’s a foggy day in New York City. Through my computer screen, I believe I’m seeing and hearing neuroscientist Nadine Dijkstra. But how can I be sure?

“We are as much creating our perception of reality as we are perceiving it,” she explains.

Rather than rudely asking her to perform a battery of tests to prove she exists, I trust my eyes and ears. It’s an efficient way to live. While neuroscientists might argue the details, most agree that perception—essentially, how we process sensory information to create a coherent experience—involves the active construction of a reality, as opposed to the passive reception of the world around us. 

For instance, when you see a busy road, you’re actively creating that reality, combining information from your senses (the sights and sounds of whooshing cars ) with past experiences (knowing you’ve walked along this popular boulevard before). Quickly understanding that the automobiles speeding down the street are real helps keep you safe.

This model for experiencing reality is efficient, but not foolproof: Sometimes our brain still gets things wrong. That dissonance is something Dijkstra, who works as the principal investigator at the Imagine Reality Lab at University College London, examines in her latest study, recently published in Neuron. 

How an early 20th-century psychologist fooled brains

Much of Dijkstra’s work is inspired by the groundbreaking psychologist Mary Cheves West Perky. In a seminal 1910 paper on imagination and perception, Perky asked subjects to visualize objects—a red tomato, a green leaf, etc.—on a blank wall. Secretly, in that seemingly empty space, Perky projected barely visible images of those same objects on the wall. 

The subjects were none the wiser, attributing the perceived objects to their imaginations instead of the projections. It appeared, Perky mused, that “the image of imagination must have much in common with the perception of everyday life.”

More than a century later, many researchers concur, believing that imagination and perception work together to create our sense of reality. But how does our brain know what’s real and what’s not? Dijkstra’s new research may have the answer.

Testing brains in the 21st century

“We expected the results to be more complicated and nuanced,” says Dijkstra.

Instead, brain activity measured by functional magnetic resonance imaging (fMRI) scans told Dijkstra a clear story: the level of activity in the fusiform gyrus could predict whether or not someone believed an image was real. The region, located on both sides of the brain behind the temples, plays an important role in recognizing faces and objects, but its ability to potentially sort out real from fake is something neuroscientists weren’t aware of before.

The study was a modern twist on Perky’s experiment. Instead of projecting fruit and other objects on a wall, Dijkstra and her colleagues asked participants to imagine sets of diagonal lines on a screen. Those lines were then projected into the fMRI machine via a mirror. (Using simple shapes, like diagonal lines, made it easier to predict what subjects might visualize. Ask people to imagine a leaf, and they might envision a plethora of shapes and colors.) The diagonal lines were displayed against a noisy background—think TV static—to make it more difficult to distinguish reality from imagination. 

When someone saw real projected lines, activity in the fusiform gyrus was stronger than when they knew they were simply imagining the diagonal lines. At the front of the brain, the anterior insula of the prefrontal cortex, which acts as a kind of hub between brain networks, also showed increased activity when participants saw the projected lines.

However, when someone confused imagined lines for real ones, essentially having a mild hallucination, both the fusiform gyrus and anterior insula regions lit up—as if they’d seen the real thing. 

The brain’s “reality threshold”

These results led Dijkstra and her team to conclude that imagined and perceived signals combine to create a “reality signal.” If strong enough, that signal crosses a “reality threshold” and we accept what we perceive as an objective reality. 

While she believes activity in the fusiform gyrus determines whether something passes the reality threshold, Dijkstra said her research was in its early stages. It could be “the other way around,” she notes, with activity in the prefrontal cortex deciding “whether something is real or not based on some other signal” and then feeding that “back to the fusiform gyrus to boost your experience or make things feel more vivid.”

Looking beyond the brain scans

How the reality threshold is passed matters. Proving a causal link between activity in the fusiform gyrus and hallucinations, for example, might allow medical practitioners in the future to stimulate that part of the brain to treat symptoms of schizophrenia and other brain disorders.

Not only can this research shed light on why humans see things that don’t exist, but it can also explain why we sometimes don’t believe our eyes. When she first moved to London from the Netherlands, Dijkstra saw a creature in the distance while walking in her neighborhood. She assumed it was a dog, even though it was wandering alone. “I was really surprised. I was like, ‘Where’s the owner?’ I really saw a dog.” If she had turned away and not questioned her reality, she might not have realized what she was actually seeing was a fox, one of the 10,000 or so that called her new city home. Dijkstra perceived something that didn’t match her past experiences and, for a moment, saw something that didn’t exist.   

As for the future of her research, there are so many unanswered questions about perception, says Dijkstra, such as whether people with vivid imaginations are more likely to hallucinate. In this field, it’s important to consistently challenge what you believe is real. “You can have this really cool idea that makes a lot of sense and it seems to be explaining so many things, and then it turns out to be totally wrong,” she says. “And that’s OK, we still make progress.”

 

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