One of the features of schizophrenia used in diagnosis is an incapacity to recognize metaphors. Polish researchers have used MRI scans to identify the parts of the brain responsible for metaphor processing and observed how these differ between people with the disease and those without.

The use of metaphor is fundamental to human communication. Like all science communicators, at IFLScience we frequently rely on it to explain difficult concepts using comparisons with more familiar examples. Many metaphors are so familiar we barely notice them. Since these tend to vary from language to language, however, they cause endless confusion, and not a little humor, for people learning a new tongue confused by expressions like “give us a hand” or “break a leg”.

People with schizophrenia can find metaphors even more daunting than those struggling with a new language, making statements that are harmless to everyone else misleading or even frightening. To understand the way this works on a neurological level Martin Jáni of the Jagiellonian University, Poland had 60 volunteers, half with schizophrenia, read very short stories while undergoing MRI scans.

A third of the stories each person read ended with someone using a common metaphor, a third ended with a straightforward literal ending, and a third turned to nonsense or absurdity. They were asked to identify which were which. The areas of the brain that responded to the metaphorical endings were different in the participants with schizophrenia than in the controls.

People without schizophrenia showed strong responses in the prefrontal cortex and left amygdala when encountering metaphor. Those with schizophrenia, however, showed strong activation in places not usually associated with metaphor processing.

Jáni presented his work at the 32nd European College of Neuropsychopharmacology conference. Although not the first to seek to observe the differences in metaphor processing between schizophrenics and the rest of the population, he thinks he and his collaborators have added something new.

"Previous researchers studied brain areas that are connected to impaired metaphor understanding in schizophrenia, so comparing metaphors with literal statements,” Jáni said in a statement. “However, by adding the absurd punchline, we were able to explore the stage at which the deficit occurs. We also used everyday metaphors, which would be easily understood.”

Jáni reported the scans suggest the areas that normally process metaphor do not respond properly in people with schizophrenia, and their brains attempt to bypass them and process the information elsewhere. Sometimes this works, but creates an extra cognitive load on the brain, making other activities harder. At other times the work-arounds fail, leaving people thinking an author or person they are speaking to is being literal.

It's easy to see how, when so many of our common metaphors refer to wars or other forms of violence, this undermines a sense of safety, and failure to understand more harmless metaphors can still be socially isolating. It is hoped the work will lead to ways to train the brain to recognise metaphors it might currently miss.