Mirror neurons were first identified by a team of researchers led by Giacomo Rizzolatti at the University of Parma in the early 1990s. While studying the premotor cortex of macaque monkeys, the team noticed something remarkable: certain neurons became active not only when the monkey performed an action (such as grasping an object) but also when the monkey observed another individual—monkey or human—performing the same action. These specialized cells came to be called mirror neurons because they “mirror” the action taking place in front of the observer.

Over the decades, scientists have explored how mirror neurons might form a neural basis for imitation, empathy, and social understanding. The big question, however, is how do researchers determine that these neurons are truly responding to another’s behavior rather than reflecting our own planned or imagined actions?

1. The Original Monkey Studies

In the foundational experiments with macaques, researchers placed electrodes in the premotor cortex (specifically area F5) to measure neural firing. They noticed:

• Active During Action: Certain neurons would fire when the monkey itself reached for an object.
• Active During Observation: Those same neurons would also fire when the monkey merely watched a researcher or another monkey perform the same reach.

What ruled out the possibility that these neurons were simply a reflection of the monkey’s internal plan to move its own arm was that the monkey did not move in these observation scenarios—nor was it preparing to act. Additionally, no reward or direct benefit came from observing the action, yet the neuron still fired. This suggested the neuron specifically coded the observed action.

2. Distinguishing Self vs. Other

To confirm that mirror neurons respond to another’s actions rather than the observer’s internal imagery, scientists designed controlled experiments:

• Action Observation Without Execution: By preventing the monkey from moving (or giving it no reason to move), the neuron’s activation when seeing another’s action indicates the cell responds to the external event. If the neuron’s firing was strictly about the monkey’s own movement plan, it would not consistently activate in these purely observational contexts.
• Hidden Movement Conditions: In certain tests, part of the action was obscured. For instance, a monkey might see someone’s arm reach behind a screen where the actual grasping happened out of view. If the monkey knew (from prior trials) that an action was taking place behind the screen and the neuron still fired, it meant that the cell was responding to the inferred action of another rather than directly seeing or planning its own movement.
• Timing and Matching: The neural firing patterns closely match the phase of the observed action (e.g., grasping) rather than any movement the observer might be executing or preparing to execute themselves at the same time.

3. Human Studies and Brain Imaging

In humans, ethical and technical limitations prevent researchers from inserting electrodes in the same way. However, neuroscientists use noninvasive methods—like functional Magnetic Resonance Imaging (fMRI), magnetoencephalography (MEG), and transcranial magnetic stimulation (TMS)—to observe mirror-like responses in areas of the brain analogous to those in monkeys.

• fMRI Studies: When participants watch actions performed by others, brain regions associated with motor planning and execution (such as parts of the premotor cortex and parietal lobe) show increased activity—even though the participants themselves remain still.
• TMS Studies: Delivering a magnetic pulse to motor areas while participants observe actions can modulate motor excitability in a way that mirrors the observed motion. This indicates a direct mapping of the external action onto the observer’s motor system, rather than the observer simply “thinking about” moving themselves.

By carefully controlling conditions so that participants are not moving or expecting to move, scientists confirm that these activations reflect the observation of another individual’s action.

4. Implications for Social Understanding

This body of research suggests that mirror neurons—and the larger mirror mechanism in the brain—may help us intuitively understand others’ actions, intentions, and even emotional states. When you see someone pick up a glass of water or wince in pain, your brain partially simulates that experience. This does not mean you are about to pick up a glass or feel their pain literally, but that your neural circuits respond in a way that relates to the act or emotion you’re observing in someone else.

Conclusion

Scientists distinguish “self” from “other” in mirror neuron responses through rigorous experimental setups that rule out actual or intended movements from the observer. By showing that these neurons activate purely from the observation (or inferred observation) of someone else’s action, researchers have demonstrated that mirror neurons play a crucial role in how we interpret and empathize with the behaviors of those around us. This discovery not only broadens our understanding of motor and social cognition but also opens doors to exploring how empathy and understanding between individuals might be strengthened or rehabilitated in clinical contexts.