Depth Electrode Recordings: Human Mirror Neurons

For decades, mirror neurons were known only from the monkey brain. These remarkable cells fire both when an individual performs an action and when it watches someone else perform the same action, and they have been proposed as a building block of imitation, empathy and social understanding. Thanks to a rare clinical opportunity, researchers were finally able to record single neurons directly inside the living human brain, providing the first direct evidence that mirror neurons exist in our own species.

Who is Marco Iacoboni

Marco Iacoboni, MD PhD, is a neurologist and neuroscientist originally from Italy. He is based at the David Geffen School of Medicine at UCLA, where he serves on the faculty of the Department of Psychiatry and Biobehavioral Sciences and directs the Transcranial Magnetic Stimulation laboratory of the Ahmanson-Lovelace Brain Mapping Center. Iacoboni’s lab is widely regarded as one of the leading groups in human mirror neuron research, and he has long collaborated with Giacomo Rizzolatti, in whose laboratory mirror neurons were originally discovered in monkeys.

The discovery of mirror neurons in the monkey

In the monkey premotor cortex, Rizzolatti and colleagues discovered cells that fire not only when the monkey performs goal-oriented actions, but also when it observes the same action performed by somebody else. These cells were named mirror neurons. They are thought to be the evolutionary precursors of neural mechanisms supporting several aspects of social behavior, from imitation to empathy. The core idea is striking in its simplicity: watching an action and doing it share part of the same neural code, so that observing another individual is, in a sense, a way of internally rehearsing what they are doing.

Why the jump to humans was so difficult

The many evolutionary steps between small apes and humans suggested that mirror neurons might also have evolved from the monkey brain to the human brain. Demonstrating this directly, however, was far from easy. Investigations of the human brain typically do not allow researchers to study individual cells, because inserting recording electrodes into healthy brain tissue would be unacceptable. Most evidence in humans therefore came from indirect methods such as functional imaging or brain stimulation, which measure the activity of large populations of neurons rather than single cells. Indirect data can be suggestive, but they cannot prove that an individual neuron behaves in a mirror-like way.

Depth electrode recordings: a rare clinical window

The breakthrough came from depth electrode recordings carried out in patients who were already undergoing intracranial monitoring for clinical reasons. In some forms of drug-resistant epilepsy, neurosurgeons implant fine electrodes deep inside the brain to locate the precise origin of seizures before possible surgery. While the electrodes are in place for clinical monitoring, and with the patients’ informed consent, they offer an exceptional and ethically acceptable opportunity to study how individual human neurons respond.

Using this rare clinical opportunity, the research team recorded single-unit and multi-unit spiking activity from human neurons while participants both performed and observed actions. This is the level of resolution that had previously been available only in animals: the firing of one identifiable cell at a time, measured on the scale of milliseconds.

What the human recordings revealed

The data provided several novel findings. First, they offered direct evidence for the existence of mirror neurons in the human brain, confirming what indirect methods had long suggested. Second, the anatomical distribution of these neurons turned out to be broader than expected: beyond the previously reported inferior frontal and inferior parietal cortex, mirror neurons were also found in the medial frontal and medial temporal cortices, regions involved in memory and in the control of behavior.

Third, excitatory and inhibitory responses in mirror neurons were equally represented, meaning that some cells increased their firing during the mirroring condition while others decreased it. Fourth, and perhaps most intriguing, about a third of human mirror neurons showed an opposing pattern of excitation and inhibition during action observation versus action execution. In other words, a neuron might fire more strongly when the person acted and be suppressed when the person merely watched, or vice versa.

Why an opposing pattern matters

This opposing pattern is more than a technical curiosity. A neural feature of this kind may help preserve the sense of being the owner of an action during mirroring, and may exert control on unwanted imitation. When we watch someone reach for a cup, our motor system partly simulates the same movement, yet we do not actually grab the cup ourselves. Cells that respond in opposite directions to observation and execution could provide exactly the kind of internal “brake” that lets us understand and resonate with another person’s action without automatically copying it.

Taken together, these findings suggest that mirror neurons form a multimodal system for the flexible integration of the perceptual and motor aspects of actions, both our own and those of others. Rather than being a rigid copy-machine, the human mirror system appears to be a finely balanced network that links seeing and doing while keeping self and other distinct.

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What are mirror neurons?

Mirror neurons are cells that fire both when an individual performs a specific action and when it observes the same action carried out by someone else. They are thought to support imitation, action understanding and aspects of empathy.

How were mirror neurons recorded directly in humans?

By using depth electrodes implanted in patients with drug-resistant epilepsy for clinical seizure monitoring. With the patients’ consent, researchers recorded the activity of single neurons while the participants observed and performed actions, something that is not possible in healthy volunteers.

Where in the human brain are mirror neurons located?

In addition to the inferior frontal and inferior parietal regions identified earlier, single-neuron recordings found mirror neurons in the medial frontal and medial temporal cortices, areas linked to memory and behavioral control.

Why do some mirror neurons respond in opposite ways during observation and execution?

This opposing excitation-inhibition pattern may help us tell apart our own actions from those we observe, preserving the sense of being the agent of our movements and preventing automatic, unwanted imitation.

Depth electrode recordings in epilepsy patients allowed researchers to capture, for the first time, the activity of individual human mirror neurons. The findings confirm that these cells exist in our species, show that they are distributed more widely than expected, and reveal that many of them respond in opposite ways to observing and executing an action. This balance of excitation and inhibition may be the mechanism that lets us share another person’s experience while still knowing where we end and the other begins.

Depth Electrode Recordings in the Brain