In many of the original mirror neuron studies, researchers looked for neurons in monkeys that activated when they watched other monkeys perform motor actions. But, says psychologist Christian Keysers, PhD, who works at the University of Groningen in the Netherlands, some of the most intriguing questions raised by mirror neurons aren’t answered by motor neurons alone–researchers want to understand how we perceive the emotions and sensations associated with other people’s actions as well.
Observation of a motor act
Observation of a motor act requires attention, memory and other cognitive elements. The activation of monkey neuron reflects these elements in the cortex.
We trained monkeys to observe videos displaying a biological movement performed by a monkey or a human actor (Biological motion, BM) and the motion of an object (Object motion, OM). For this purpose we developed a task in which they had to keep their hand on a resting position and were presented with different videos showing monkey grasping actions in first person perspective (Monkey Grasping I, MGI) or third person perspective (Monkey Grasping III, MGIII) and human grasping or mimicking these actions (Human Grasping, HG), or simply extending his forelimb in front of himself (Biological movement, HM).
In addition, the phase of video was manipulated: during the observation of a grasping action in first person, only the initial phases of the movement were visible. Hence, only the most relevant parts of the movement were recorded for the purpose of analyzing the neural responses to this type of observation.
A majority of HS neurons showed a stimulus-specific activity, responding best or exclusively to one of the presented stimuli (Fig. 3a). The highest percentage of HS neurons coded goal-directed actions, most of them performed by monkeys, while human goal-directed actions were coded by a lower number of neurons.
The response of a few HS neurons was also affected by the obscuration of part of the observed action: those that responded to a hand grasping in first person exhibited their preferred discharge during Video epoch 2 while those that responded to this type of action in third person shifted their preferred discharge to Video epoch 1.
These results indicate that a neuron activation is not only triggered by a visual cue. It may also be based on the context of the observed action, which gives enough information to the monkey about its beginning and its outcome. This is in line with the known properties of prefrontal neurons that exploit contextual information for predicting and planning behavior.
Among the neurons tested in this study, 12% of them were found to discharge when the monkey performed a grasping action. This is in contrast to the 4% of them that did not discharge when this action was performed. This indicates that they are responsible for maintaining the visual representation of the motor act in the connected parietal/premotor and inferotemporal areas.
Observation of an object
Several studies have shown that there are specific neurons which activate not only when an animal is executing a motor act, but also when they observe the same action performed by another. The activation of such a neuron is known as mirror-neuron activity and has been observed in the ventral premotor cortex and parietal lobe [29,30].
In order to examine this phenomenon more thoroughly, the researchers performed an experiment in which they gave monkeys different objects, such as peanuts, to handle, while recording the F5 nerves in both areas. The results were astonishing: when the monkeys watched other monkeys or human experimenters grasp the object, the same neuron that fired when the monkey itself had gripped it would fire as well.
These neurons have been named mirror-neuron because they discharge when a monkey observes the same motor act performed by another, even if the object is hidden from view. In addition, some mirror neurons are selective for particular aspects of the action, coding the context or predicting its beginning and outcome.
This property of these mirror neurons resembles those found in the orbitofrontal cortex and in the lateral prefrontal cortex, which are sensitive to the identity of other monkeys and can predict their choices. However, unlike the orbitofrontal neurons, these mirror neurons do not show a massive activation of the motor system.
The epoch preference of these mirror neurons is important for understanding their role in observation, since it reveals whether the monkey can code the information provided by the video in a certain phase. In most cases, the majority of HS neurons responded during Video epoch 2, while 13 showed a slightly higher discharge for Video epoch 1.
A few HS neurons, however, exhibited a peculiar behavior, in that they shifted their discharge from the first to the second epoch when the video was obscured in the first or the second phase. This could indicate that these neurons rely on the presence of visual information in the first epoch, even if this information is not present in the second one.
This is in line with the findings of Michaels and Scherberger, who observed that many of these F5 neurons respond equally to both hands. However, some of them showed a hand preference.
Observation of a person’s action
When a person sees another person performing a particular action, it can cause them to imitate that act. This is a type of imitation known as model learning. It requires an observer to pay attention to the model, then form a mental representation of the behavior, and then remember it later. This may involve a combination of simple attention and a structured learning process that can include mnemonics or a daily practice routine.
The brain areas typically activated during a model-learning task are the superior temporal cortex, the inferior parietal lobule and the ventral premotor cortex (PMv). These areas are also involved in directing motor actions such as reaching and grasping. Activation of these areas during a model-learning task is mediated by the mirror neuron system [56, 58].
To determine whether neurons in VLPF respond to videos representing hand-object interaction and biological movements, we used videos that showed goal-directed monkey or human actions, such as reaching, grasping and object movement. We trained monkeys to view these videos and recorded the activation of VLPF neurons in real time while they watched them.
We found that many VLPF neurons discharged during different phases of the observed videos, with a majority responding during Video epoch 2 and a few responding during Epoch 1. These results are in line with previous studies on mirror neurons, where some neurons were selective for coding specific aspects of a particular movement or its outcome (e.g. hand-object interaction), while others could also predict the beginning and outcome of an action in advance.
In addition, some HS neurons discharged during the same epoch as the movement onset. This may be related to the possibility that these neurons code the context or predict the beginning and outcome of an action, which would help the monkey plan its next move.
To further examine the lateralized contribution of the two hemispheres to the processing of an action, we asked monkeys to perform a visual discrimination task in which they had to distinguish between two pictures depicting a human actor and a monkey. The latencies of the LC neurons and behavioral RTs increased as the difficulty of the discrimination task increased.
Observation of a mirror action
Mirror neurons are sensory-motor cells that activate when an individual performs an action or observes another person performing the same action. These neurons are of interest in the study of social behaviours such as empathy and imitation. They have also been shown to play a role in a number of neurorehabilitation therapies, including action observation treatment (AOT).
These neurons are located in the ventral premotor area F5 of the monkey brain and are activated during the execution of a motor act and the observation of that same act performed by another monkey or a human experimenter. This is a result of the fact that the observed motor act is similar to the goal-directed motor acts coded by the same mirror neurons during execution of the motor act.
During the observation of a motor act performed by a human actor, these neurons are activated with an intensity that is proportional to the magnitude of the observed movement. This activity is most intense before the onset of the movement and it continues until the end of the movement. However, it is less intense during the reaching and lifting phases of the movement.
In addition to this, the observed direction of the motion also modulates the activity of these neurons. For example, 65% of the recorded neurons showed a preference for the direction toward which the motor act was executed. This preference can be interpreted as a form of directional selection, although it is possible that the preferred direction was a combination of a preference for the left and the right side.
These results indicate that the mirror mechanism is not a single individual neuron, but a complex neural machinery constituted of many different cell types that work together to encode other-related information and to provide inputs to more executive, self-related areas, which are necessary for the planning and control of behavioral responses. These results have important implications for understanding social learning and behavior coordination. They suggest that these circuits are more flexible and extensively articulated than previously thought.