Vigneswaran, G; (2013) The role of mirror neurons in movement suppression. Doctoral thesis , UCL (University College London).

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Abstract

The characteristic feature of mirror neurons is that they modulate their firing rate during both a monkey’s own action and during observation of another individual performing a similar action. Some premotor (F5) mirror neurons have also been shown to be corticospinal neurons, meaning that spinal targets are also influenced during action observation. Simultaneous electromyography (EMG) recordings from hand and arm muscles provide important evidence that the activity of these cells cannot be explained by any covert movement on the part of the monkey. The question arises as to how output cells (pyramidal tract neurons, PTNs) that are classically involved in the generation of movement can be modulated without any resulting movement. Since there are many more PTNs in primary motor cortex (M1) compared with F5, it is important to assess whether PTNs in M1 also have mirror activity. We recorded activity of identified PTNs in areas M1 and F5 of two macaque monkeys during action execution and observation of a skilled grasping action. We found evidence of modulation of PTNs in M1 during action observation in over half the recorded units. However, the depth of modulation was much smaller during action observation compared with action execution. In a separate analysis we investigated whether it is possible to assign mirror neuron activity to different cell types on the basis of extracellular spike duration. Surprisingly, we found considerable overlap between identified pyramidal cells and putative interneurons and provide evidence 4 that spike duration alone is not a reliable indicator of cell type in macaque motor cortex. In a separate series of studies we used non-invasive transcranial magnetic stimulation (TMS) in human volunteers to measure the corticospinal excitability during the same task. Taken together, although we found evidence of modulation of PTN activity during action observation in M1, the level of activity was greatly reduced during action observation and may not be sufficient to produce overt muscle activity.

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