The region called the insula is buried deep beneath the brain’s wrinkled surface (artist’s illustration).Credit: Alexander Leemans/UMC Utrecht/Science Photo Library
Scientists have mapped one of the most mysterious parts of the human brain in unprecedented detail1. Their results detail how this region, the insular cortex or insula, is organised to support the brain’s processing of bodily sensations such as pain. The mapping also details a pathway that allows for the cortex — the seat of higher-order cognition — to work with the insula to affect bodily perceptions.
One region of the insula is responsible for the feeling of pain in certain parts of the body. Another region generates sensations, such as nausea, felt by the internal organs. A third region, at the front of the insula, ‘talks’ to other brain areas involved in cognition and emotion, suggesting it influences not only sensations, but also how the brain interprets them.
Contrary to previous thinking, the insula is “not just reading the body. It is partly influencing the story about how the body should be felt,” says study co-author Josef Parvizi, a physician-scientist at Stanford University in California.
The study is “quite a feat”, says Lucina Uddin, a cognitive neuroscientist at the University of California, Los Angeles. It’s also one of the most comprehensive studies on the insula and how it produces bodily sensations, she adds.
The findings were published today in Nature.
Buried in the brain
The insula has been notoriously difficult to study in detail because it’s located deep in the brain. Previous studies have suggested that this hard-to-reach region is involved in receiving signals — such as pain or pressure — from various parts of the body, then processing them into sensations that we experience. But most research on the human insula has relied on brain-imaging techniques to capture what happens in the insula when various sensations arise, Parvizi says.
To build a more detailed picture, Parvizi and his colleagues needed to find a way to send electrical signals to various parts of the insula in people and assess the bodily sensations that occur as a result. But implanting electrodes deep in the brains of healthy people would be unethical. So, the researchers turned to 87 people who’d had electrodes implanted in their brains for medical reasons.
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In experiments that spanned more than a decade, Parvizi and his colleagues sent electrical currents through the electrodes into a variety of sites in the participants’ insulas.
Stimulating the rear of the insula produced pain, heat and other sensations — including tingling and numbness — in specific areas of the participants’ bodies, such as the arm or leg. Meanwhile, participants experienced more ‘visceral’ sensations, including nausea, pressure and feelings of fullness in their chest, when the researchers sent electrical currents to the middle of the insula.