Reality Check: Why Some Brains Can’t Tell Real From Imagined

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How do you know what’s real? A new study suggests that people’s ability to distinguish between what really happened and what was imagined may be determined by the presence of a fold at the front of the brain that develops late in pregnancy, and is missing entirely in 27% of people.

Although the study sounds like it sprouted from the musings of stoned undergraduates or the abstruse pursuits of basic-neuroscience geeks, its findings may prove important for the understanding of schizophrenia, a disorder which often includes confusion between real and imagined voices.

The key brain structure identified by the study is called the paracingulate sulcus (PCS), a fold in part of the prefrontal cortex, the region that is involved with planning, thought and judgment. The size of the PCS varies greatly in normal people, and some people a PCS only on one side of their brain, while others have one on both. Compared to the quarter or so of healthy people who are completely missing the PCS, 44% of people with schizophrenia do not have it, suggesting that its absence could play a role in the disease.

But the new study found that even normal people without the PCS have difficulty distinguishing between what they remembered and what they imagined.

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The researchers studied 53 healthy volunteers, chosen because brain scans showed they had a prominent PCS, no PCS, or were missing it on either side of the brain. Participants were presented with either recognizable word-pairs like “Laurel and Hardy” or the first word in a common word-pair, such as “Laurel and ?” When only half the word-pair was provided, participants were asked to imagine the second word. Sometimes, the volunteers were then asked to read the words out loud. In other cases, the words were read to them by an experimenter.

Afterward, all participants were given a memory test in which they were asked whether they had seen or imagined the second word of each word-pair and also whether it was they or the experimenter who had read the words aloud. They also estimated how confident they were in the accuracy of their memories.

Researchers found that people missing the PCS performed significantly worse than the other participants on remembering who had read the words, but they were unaware of their inaccurate memories. And they expressed the same level of confidence in their real or imagined memories as those with a PCS on at least one side of their brains. (Performance on whether they imagined or read the second word was unaffected).

The authors, led by Jon Simons of Cambridge University, write that the differences were “particularly striking,” given that the participants were healthy adults with “typical educational backgrounds and no reported history of cognitive difficulties.”

“It is exciting to think that these individual differences in ability might have a basis in a simple brain folding variation,” Simons said in a statement.

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Of course, imaging studies like this cannot determine whether having a small or absent PCS causes difficulties with reality recognition memories, or whether the effect works the other way around. However, because structural differences in this region of brain arise in utero, it is likely that brain size affects memory performance in this case.

The findings may also lead to clues about the origins of schizophrenia. Although the disorder doesn’t typically appear until adulthood, it is believed to have its roots in the womb. Having an absent PCS may well contribute to schizophrenia by blurring the line between real and imaginary. “Hallucinations [in schizophrenia] are often reported whereby, for example, someone hears a voice when nobody’s there. Difficulty distinguishing real from imagined information might be an explanation for such hallucinations,” said Simons.

But since 27% of normal people are missing a PCS but only about 1% of the population has schizophrenia, clearly the disease has other contributors. Further research on people with schizophrenia is needed to figure out what goes awry in the disorder and why their problems with recognizing reality arise.

The study was published in the Journal of Neuroscience.

Maia Szalavitz is a health writer at Find her on Twitter @maiasz. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter @TIMEHealthland.