Contribution of somatosensory input to mechanisms of movement suppression during action observation

Execution of everyday movements as for example grasping of an object appears and feels effortless but requires a sophisticated mechanical machinery of muscles and joints and a complex control system including spinal cord and brain circuits. There is accumulating evidence that large part of this control system is active during movement observation. More than twenty years ago Prof Rizzolatti and colleagues discovered single neurons in area F5 of macaque monkeys brain responding not only to monkey grasping but also to observation of the same grasp performed by human experimenter.

These neurons were called "mirror neurons". Our group made a major contribution to the field of mirror neurons by discovering that in area F5 and in primary motor cortex even neurons directly connected to the spinal motor neurons can be modulated just by observation of an action. This apparent paradox of the motor system being active during action observation without production of any movement challenges our understanding of how movements are generated and prompts research to identify what separate movement execution from action observation.

An obvious candidate is somatosensory system that responds to changes to the surface or internal state of the body. Somatosensory input is clearly very different between observation of a grasp when hand is at rest and grasp execution when hand is moving and touching an object. This difference might be an important contributing factor to why movements do not occur during action observation despite mirror neuronal activity present in motor areas.

A dramatic examples of how powerful the somatosensory input is for perception is the "rubber hand" illusion: light stroking of subject hand combined with observation of synchronous stroking of an artificial hand forces subjects to believe that an artificial rubber hand is their own and that they can control it.

To understand the possible contribution of somatosensory input to the mechanisms of movement suppression during action observation, we suggest to investigate the somatosensory properties of the mirror neuron system (MNS). This includes responses of single mirror neurons to light touch, hair brushing and passive hand movement. Some neurons in area F5, where mirror neurons where discovered, have been shown to respond to such stimulation but it was not so far tested for mirror neurons.

And more specifically we will investigate differences in somatosensory properties of facilitation and suppression mirror neurons discovered in our laboratory.

It is well established that during movement we are less sensitive to the sensory input, eg touch. This reduced sensitivity manifests itself in a smaller neuronal signal in response to electrical stimulation of the nerve in comparison to the response at rest. This phenomenon is called sensory attenuation but it was not tested for mirror neurons.

Non-invasive studies aiming at revealing sensory attenuation during action observation are contradictory. Some studies report attenuation while others enhancement. To resolve this controversy it is critical to investigate sensory attenuation on the level of local neuronal signals.

In classical studies of the MNS, subjects just passively observe the actors' movements. In real life, it is quite often an active observation. While we execute an action we simultaneously observe a similar action, e.g. in a shared motor task such as two surgeons working together.

Here action observation happens in the presence of somatosensory input, which resembles somatosensory input of an actor. What happens to the MNS simultaneously driven by action execution and action observation? To answer this question we will combine invasive neurophysiological investigation of non-human primates with non-invasive transcranial magnetic stimulation (TMS) studies of human volunteers.