Important for you to empathize with the pain of others is a triangular region of the brain called the insula, deep under the corrugations of the outer cortex. But the degree to which neural activity in the insula versus input from related regions of the brain encodes empathy, and the timeline of such encodings, remain enigmatic.

A study in eLife analyzed the contribution of the insula in our ability to understand the pain of others by recording from electrodes embedded in the human brain that measured neuronal activity in individual cells in epilepsy patients as they viewed short movies of an actress in pain. The study improves our understanding of the inherent differences in people’s ability to empathize and react to the pain of others, as well as potential mechanisms for the lack of empathy in certain neuropsychiatric conditions.

The study analyzed intracranial electroencephalographic (iEEG) recordings and single-cell recordings from the human insulae. The researchers recorded neural activity in seven epilepsy patients as they watched short movie clips of either painful facial expressions, or a hand being slapped by a belt. The patients rated the intensity of the pain as they watched the videos. The movies varied in how much pain the actress in the movies was experiencing. This allowed the team to explore whether movies in which the patients perceived others to be in more pain are those where the insular neurons show more activity.

“We found activity in the broadband 20-190 Hz range correlated with the trial-by-trial perceived intensity in the insula for both types of stimuli,” the authors noted.

Some locations in the insula showed neural activity for facial expressions but not for a hand being slapped, while other regions showed neural activity for the hand stimuli but not for facial expressions, and other regions in the insula were activated by both stimuli. Throughout the insula, the researchers recorded electrical activity that scaled with the pain the participants reported perceiving in the movies. This was true in the local field potentials, and in individual neurons, providing the first evidence, that a brain region involved in our own pain, contains a fine-grained representation of the pain of others.

When the participant was shown the facial expression that unfolded from a neutral expression to one of varying degree of pain, analysis of the electrical responses in the insula and the muscle movements of the actress in the movies revealed that the brain uses the shape of the actresses contracted eyes to perceive the pain and not the movement of the face.

On the other hand, when the participant was shown the hand being hit, the brain appeared to deduce the amount of pain from the movement of the hand under the slap of the belt. This indicates that the responses to the video with facial expressions was best inferred from shape, while that of the hand being hit was best inferred from the speed of the movement of the hand. This reveals how flexibly the human brain transforms what we see others do into a fine-grained perception of their inner states.

Lead author of the study, Efe Soyman, PhD, Assistant Professor at Koç University in Turkey said, “Other people’s suffering can be inferred from a variety of indicators: a painful expression, the intensity of the event that inflicts pain in them, etc. With this incredibly valuable data we collected from the patients, we see how the human insula might tune into whichever is available among these various cues when we experience the pain of other people.”

The researchers then compared broadband activity in the iEEG signal with neuronal spiking activity from a few neurons and an fMRI (functional magnetic resonance) experiment with similar stimuli. This showed a consistent spatial organization of neuronal activity with the type of painful stimuli that the patients viewed.

The study was made possible through the cooperation of patients with epilepsy that is untreatable with drugs. Such patients have electrodes that are surgically implanted into the brain to allow doctors to identify the neural origins of their epileptic seizures. This involves hospital stays to enable clinical monitoring and some patients consent for the use of this unique opportunity to better understand brain function. In the current study, researchers leveraged this opportunity to understand the role of neurons in the insula in mirroring the pain of others.

Cosenior author of the study, Christian Keysers, PhD, a neuroscientist at the Netherlands Institute of Neuroscience who aims to understand what makes us social, said, “As neuroscientists, our dream is to understand how neurons make us who we are. What these patients do, by allowing us to record from these electrodes, is to make that dream come true: we could see in real time, how the pain of someone else is mirrored in the neurons of an observer. After decades of working on empathy, we could see empathy unfold in the human insula.”

Using advanced data analytics, the team could predict a patient’s level of empathy, i.e., the degree to which they perceived the pain of others, from the degree of electrical activity in the insula during each movie. In future studies, the team plans to combine the data from all recorded electrodes to develop a map of brain regions responsible for the nuanced empathy we can have for other people’s emotions, and pinpoint areas responsible to individual differences in the ability for empathy.

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