The brain's insula has its own body map, and it could make epilepsy surgery safer

Mayo Clinic

Up to 30% of patients who undergo epilepsy surgery in or near the insula experience temporary problems afterward: difficulty speaking, trouble swallowing, loss of hand coordination. Surgeons have known for years that this deep, folded brain region was involved in movement somehow, but they lacked the detailed maps needed to predict which patients would be affected and how.

A study published in the Proceedings of the National Academy of Sciences changes that. A team at Mayo Clinic has discovered that the insula contains its own organized map of the body, with distinct zones for the hands, tongue, and feet. The finding gives surgeons a practical tool for planning safer operations.

A region hidden under other brain folds

The insula sits several centimeters beneath the brain's surface, tucked under the temporal and frontal lobes. Its inaccessibility has made it one of the harder brain regions to study. For a long time, researchers thought of it as a general-purpose integrator, a region that activated during many tasks without much internal organization.

Lead author Panos Kerezoudis, a neurosurgery resident at Mayo, wanted to test whether that view was wrong. The opportunity came from 18 patients with medically refractory epilepsy who had thin recording electrodes placed deep in their brains as part of their clinical care. The electrodes were already there. The researchers just needed to ask patients to move.

Hand here, tongue there

While hospitalized, patients performed simple tasks: opening and closing a hand, moving their tongue, flexing a foot. The electrodes recorded electrical activity with millisecond precision in both the insula and the primary motor cortex, the brain's main movement-control center.

The results were clear. Hand movements activated one area of the insula, tongue movements activated another, and foot movements activated a third, though less prominently. This was not random activation. There was spatial order.

The timing data added another layer. The primary motor cortex fired first, then the insula, then the movement occurred. That sequence rules out the possibility that the insula was simply reacting to sensory feedback after a movement had already happened. It was active before the movement, suggesting it plays a role in generating or coordinating it.

Connections that respect the map

In a subset of patients, researchers went further. They delivered brief, safe electrical pulses to the motor cortex and measured responses in the insula. Stimulating a hand-related area in the motor cortex triggered a response in the matching hand zone of the insula. The same pattern held for the tongue. The connections between the two regions respected the body map.

Senior author Kai Miller, a Mayo neurosurgeon, frames this as evidence of a distributed movement network that is more tightly integrated than previously appreciated. Movement, it appears, is not solely the province of the motor cortex. The insula participates, and it does so in an organized, body-part-specific way.

What this means for epilepsy patients

The clinical applications are immediate. If surgeons can identify where hand and speech functions are located in each patient's insula before operating, they can plan their approach to minimize the chance of hitting those zones. Neurologists interpreting seizure symptoms, such as involuntary hand contractions or facial movements, can now correlate those symptoms with specific insular locations to better localize the seizure focus.

Beyond epilepsy, the findings may eventually inform treatment for stroke survivors with speech or hand difficulties. If recovery from motor deficits depends on a network that includes both the motor cortex and the insula, rehabilitation strategies and brain stimulation therapies might need to target both regions.

The study was conducted with 18 patients, all of whom had epilepsy severe enough to require invasive brain monitoring. Whether the same organizational pattern holds in healthy brains has not been directly tested, though there is no strong reason to expect it would differ. The mapping was limited to three body parts, and finer-grained organization within the insula remains to be explored.