Reductions in the length of the paracingulate sulcus may be associated with hallucinations in people with schizophrenia, according to research published in Nature Communications.

The paracingulate sulcus (PCS) is a fold in the prefrontal cortex that has been associated with a process known as “reality monitoring” which allows healthy individuals to distinguish between real and imagined information. In this study, researchers sought to identify brain structural differences between patients with schizophrenia who have hallucinations and those who do not.

Jane R. Garrison,PhD, of the University of Cambridge, and colleagues used structural MRI to compare PCS length in 3 matched groups: patients with schizophrenia with a history of hallucinations (n=79); patients with schizophrenia with no history of hallucinations (n=34); and healthy controls (n=40). PCS length was characterized as “prominent” if it was >40 mm, “absent” if it was <20 mm, and “present” if length was between those two limits.

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Comparisons revealed that patients with schizophrenia who experienced hallucinations had significantly smaller PCS length compared to schizophrenia patients without hallucinations (mean reduction=19.2 mm, t(111)=2.531, P=0.013, d=0.519) and healthy controls (mean reduction=29.2 mm, t(117)=4.149, P<0.001, d=0.805). PCS length between schizophrenia patients without hallucinations and healthy controls did not differ significantly (t(72)=1.07,P=0.288, d=0.246).

Ultimately, a 1 cm reduction in PCS length resulted in a 20% increase in risk for hallucinations.

“We think that the PCS is involved in brain networks that help us recognize information that has been generated ourselves,” Dr. Garrison said. “People with a shorter PCS seem less able to distinguish the origin of such information, and appear more likely to experience it as having been generated externally.”

The researchers also observed lateral differences in PCS length, as length was greater in the left hemisphere compared to the right hemisphere across all patient groups (t(152)=2.959, P=0.004, d=0.317). However, patients with schizophrenia who experienced hallucinations exhibited reduced PCS length in both hemispheres compared to healthy controls (t>2.636, P<0.01, d>0.546). The difference in PCS length between patients with schizophrenia who experienced hallucinations and those who did not was only significant in the left hemisphere (t(111)=2.464, P=0.015, d=0.505).

The effect of PCS length was observed regardless of hallucination modality, confirming the role of reality monitoring impairment in hallucinations.

“Hallucinations are very complex phenomena that are a hallmark of mental illness and, in different forms, are also quite common across the general population,” Dr. Garrison said. “There is likely to be more than one explanation for why they arise, but this finding seems to help explain why some people experience things that are not actually real.”


  1. Garrison JR, Fernyhough C, McCarthy-Jones S, Haggard M, Simons JS. Paracingulate sulcus morphology is associated with hallucinations in the human brain. Nat Commun. 2015; doi:10.1038/ncomms9956.