title: Effects of action observation on corticospinal excitability: Muscle specificity, direction, and timing of the mirror response.
abstract: Many human behaviours and pathologies have been attributed to the putative mirror neuron system, a neural system that is active during both the observation and execution of actions. While there are now a very large number of papers on the mirror neuron system, variations in the methods and analyses employed by researchers mean that the basic characteristics of the mirror response are not clear. This review focuses on three important aspects of the mirror response, as measured by modulations in corticospinal excitability: (1) muscle specificity; (2) direction; and (3) timing of modulation. We focus mainly on electromyographic (EMG) data gathered following single-pulse transcranial magnetic stimulation (TMS), because this method provides precise information regarding these three aspects of the response. Data from paired-pulse TMS paradigms and peripheral nerve stimulation (PNS) are also considered when we discuss the possible mechanisms underlying the mirror response. In this systematic review of the literature, we examine the findings of 85 TMS and PNS studies of the human mirror response, and consider the limitations and advantages of the different methodological approaches these have adopted in relation to discrepancies between their findings. We conclude by proposing a testable model of how action observation modulates corticospinal excitability in humans. Specifically, we propose that action observation elicits an early, non-specific facilitation of corticospinal excitability (at around 90ms from action onset), followed by a later modulation of activity specific to the muscles involved in the observed action (from around 200ms). Testing this model will greatly advance our understanding of the mirror mechanism and provide a more stable grounding on which to base inferences about its role in human behaviour.
authors.affiliation: School of Psychology and Clinical Language Sciences, University of Reading, Earley Gate, Whiteknights, Reading RG6 6AL, UK; Centre for Integrative Neuroscience and Neurodynamics, University of Reading, Earley Gate, Whiteknights, Reading RG6 6AL, UK; Department of Psychology, Neuroscience & Behaviour, McMaster University, 1280 Main Street West, Hamilton, ON, Canada L8S 4L8. Electronic address: naishek@mcmaster.ca.
authors.forename: Katherine R
authors.initials: KR
authors.lastname: Naish
authors.affiliation: University of Reading Malaysia, Menara Kotaraya, Level 7, Jalan Trus, Johor Bahru, Malaysia 80000. Electronic address: c.houston-price@reading.ac.uk.
authors.forename: Carmel
authors.initials: C
authors.lastname: Houston-Price
authors.affiliation: Department of Psychology, Goldsmiths, University of London, New Cross, London SE14 6NW, UK. Electronic address: a.bremner@gold.ac.uk.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Bremner
authors.affiliation: School of Psychology and Clinical Language Sciences, University of Reading, Earley Gate, Whiteknights, Reading RG6 6AL, UK; Centre for Integrative Neuroscience and Neurodynamics, University of Reading, Earley Gate, Whiteknights, Reading RG6 6AL, UK. Electronic address: npholmes@neurobiography.info.
authors.forename: Nicholas P
authors.initials: NP
authors.lastname: Holmes
mesh_terms: D019054:Evoked Potentials, Motor / Q000502:physiology*; D006801:Humans; D059167:Mirror Neurons / Q000502:physiology*; D009043:Motor Activity / Q000502:physiology; D009044:Motor Cortex / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D010775:Photic Stimulation; D011712:Pyramidal Tracts / Q000502:physiology*; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation; D014796:Visual Perception / Q000502:physiology*
keywords: Action execution; Action observation; Corticospinal excitability; Mirror neuron system; Motor evoked potential; Transcranial magnetic stimulation