New research published in scientific progress This provides evidence that special brain cells called mirror neurons track the ongoing physical movement of an action, rather than just the end goal. The findings suggest that the brain uses a dynamic, shared network of neurons to understand what others are doing by matching their movements with our own internal blueprints.
The premotor cortex is an area near the front of the brain that helps plan and coordinate physical movements. Within this region, specific brain cells are activated both when an animal performs an action and when it watches someone else perform the same action. These special cells are known as mirror neurons.
Since their discovery, experts have debated exactly what kind of information these cells represent. Some scientists have proposed that mirror neurons focus on abstract goals of behavior, such as grasping, eating, or moving objects. This perspective suggests that the brain is interested in the end result, not the physical effort required to get there.
Vassilis Raos, professor of physiology at the University of Crete’s Faculty of Medicine and co-author of the study, noted that previous studies have provided inconsistent views on how these cells correspond to behavior. “At the same time, the ‘goals’ that these neurons are claimed to encode are loosely defined, and direct evidence about what exactly they represent is limited,” Laos said. “So we revisited an important possibility that had not been sufficiently tested: that mirror neuron activity reflects detailed kinematics, meaning how movement unfolds through space and time during the execution and observation of an action.”
Rao explained that the idea gained traction after the team’s previous work showed that mirror neurons respond to actions that do not involve objects. “This possibility became especially plausible after our previous work showed that a large proportion of mirror neurons respond not only to transitive, object-oriented actions, but also to the observation of intransitive, or non-object-oriented, actions,” he said.
Other scientists have proposed that these cells encode the specific static grips used, such as pinching tightly or grasping broadly with the entire hand. Another possibility is that mirror neurons track the kinematics of behavior. Kinematics refers to the continuous physical details of movement through space and time. This includes changes in speed, arm direction, and hand shape as you reach for objects.
Previous studies have often examined the activity of individual mirror neurons in isolation. The authors of the new study took a different approach by focusing on populations of neurons. They wanted to see whether the overall pattern of brain activity changes dynamically as behavior unfolds, rather than simply informing a single static concept like an end goal.
“Based on proposals we put forward in previous studies, we expected that the apparent discrepancy between execution and observation at the level of individual mirror neurons would be consistent with a systematic correspondence at the population level,” Raos said. “We also predicted that mirror neuron activity would be modulated by the kinematics of movement, and our findings confirmed both predictions.”
To get a complete picture of the brain region, the researchers also looked at non-mirror neurons. By incorporating cells that fire only during physical movement, researchers may be able to determine whether the ability to track the details of movement is unique to mirror neurons or a broader feature of the entire motor area.
To test these ideas, the researchers recorded electrical activity from 433 neurons in the premotor cortex of two macaque monkeys. The monkeys performed a reaching and grasping task designed to test different hand shapes. In the execution part of the experiment, the monkeys reached out and grabbed four different three-dimensional objects.
These objects include spheres, cylinders, rings, and small cubes. Each object required the monkey to use a specific grip. The sphere required grasping with the whole hand, but the small cube required a highly precise grip using only the tips of the index finger and thumb. The monkeys were trained to grasp these objects consistently and smoothly.
During the observation portion of the experiment, the monkeys simply sat still. They observed human experimenters performing exactly the same reaching and grasping movements with the same set of objects. The researchers used a camera system to make sure the monkeys were watching the action.
Of the total cells recorded, 285 fired in both the execution and observation tasks. Researchers classified these as mirror neurons. Another 148 cells fired only when the monkey actually performed the action. Scientists have classified these as non-mirror neurons.
For the main statistical analysis, the team focused on a narrowed dataset of 240 mirror neurons and 129 non-mirror neurons that had enough recorded trials for an accurate mathematical test. In separate sessions, the researchers used a three-dimensional motion capture system to record the physical movements of the monkeys and human experimenters. They tracked specific points on the index finger, thumb, and wrist. This device made it possible to measure the precise speed, position, and changing distance between fingers during movement.
The scientists found that information about grip type is widely spread throughout the population of mirror neurons. Rather than a single neuron maintaining a constant signal throughout the movement, different neurons were activated at different stages of the movement. This changing activity indicates that the brain uses a dynamically changing ensemble of cells to represent the actions that occur.
The researchers then compared the patterns of brain activity when the monkeys performed the action and when they simply observed it. At the level of individual brain cells, firing patterns during action and viewing rarely match perfectly. But when the researchers analyzed the activity of entire cell populations together, they revealed partially shared neural structures.
This shared structure means that the pattern of brain activity during observation largely overlaps with the pattern during execution. Overlapping activity was most pronounced during the hand movement and at the end of the grasping movement. Neural populations tend to align most strongly when an action reaches its most complex physical stage.
A population of mirror neurons also strongly tracked the sequential kinematics of the grasp. Neural activity systematically matched the speed, wrist position, and finger placement as the hand moved toward the object. By examining a mathematical model of brain activity, researchers were able to successfully predict the physical movements of the hand.
This prediction also worked in reverse. Researchers may be able to use body movement tracking data to accurately predict patterns of neural activity. This bidirectional relationship existed for both the monkey’s own movements and the observed movements of the human experimenter.
“Our findings show that mirror neurons are indeed ‘mirror-reflecting,’ but this correspondence is most clear when we examine the activity patterns of entire populations of neurons, rather than expecting individual neurons to respond in the same way during the execution or observation of an action,” Raos told PsyPost. “These populations contain detailed information about how behavior unfolds over time.”
He pointed out that tracking movement doesn’t mean the brain ignores the end goal. “Importantly, this sensitivity to movement kinematics is consistent with the idea that mirror neurons encode behavioral goals,” Laos explained. “Research in motor kinesiology shows that intentions and goals are expressed in the movements themselves, so the brain may infer what another person is trying to accomplish by reading how that person’s movements are performed.”
By reading these ongoing physical details, the brain can decipher deeper intentions. “Our findings suggest that mirror neurons have the necessary properties to support this process, bridging the gap between how an action is performed and why it is performed,” Raos said.
Scientists have discovered that neurons other than mirrors also show strong connections to the physical kinematics of grasping. Similar to mirror neurons, this group of cells tracked the continuous physical details of the monkey’s movements. This finding provides evidence that continuous tracking of moving structures is a widespread feature shared by different cell types in the premotor cortex.
The overlap in brain activity between seeing and doing was not completely symmetrical. A statistical model trained on brain activity during observation was able to successfully predict the monkey’s physical movements during execution. Models trained on execution data did not generalize well enough to predict observed human movements. This asymmetry suggests that performing an action involves additional, specific neural processes beyond those shared when simply observing an action.
“Our findings should not be interpreted as indicating that mirror neurons are simply analyzers of movement kinematics, or that their role is limited to detecting movement parameters,” Raos cautioned. “While our study identifies information that is systematically present in their activities, their full functional or causal role in understanding others is not established.”
Instead, Laos suggests that this physical tracking is part of a larger system. “Kinematic information contributes to broader motor processes that link observed actions to an individual’s motor repertoire, potentially helping to bridge the gap between the movements we see and the motor acts or goals they represent,” he said.
Several limitations should be noted regarding this study. Recordings were obtained from specific brain regions of macaque monkeys performing highly controlled repetitive movements. “Our experiments focused on controlled grasping movements, so further research will be needed to understand how this process works during richer, more natural social behaviors,” Laos said.
The scientists recorded neurons in separate sessions and later combined them mathematically to estimate the activity of the entire population. Because of this, it was not possible to capture how all individual brain cells interact at the exact same moment.
Also, the experimental design did not allow us to completely separate the visual shape of the object from the physical grip used to grasp it. Monkeys always used a specific assigned grip for each specific object. In this setting, it is difficult to completely rule out the possibility that the brain cells were responding partially to the visual identity of the object and not just to the kinematics of the hand.
“In this study, we mainly focused on what information is encoded in premotor neuron activity,” Raos says. “Our next goal is to investigate how and when these expressions emerge, and in particular how their temporal dynamics depend on the context in which the action occurs.”
The team plans to adjust future experiments to address object and grip limitations. “We also want to be able to disentangle the contributions of observed actions and the objects involved by manipulating them individually and determine how each shapes the activity based on the observation,” Raos said.
The study, “Dynamic collective coding of kinematic structures across executed and observed actions in primate premotor cortex,” was authored by Konstantinos Chatzimichail, Christos Paschalidis, Eleftheria Tzamali, Vassilis Papadourakis, and Vassilis Raos.
