title: Probing Corticospinal Recruitment Patterns and Functional Synergies with Transcranial Magnetic Stimulation.
abstract: BACKGROUND
  On the one hand, stimulating the motor cortex at different spots may activate the same muscle and result in a muscle-specific cortical map. Maps of different muscles, which are functionally coupled, may present with a large overlap but may also show a relevant variability. On the other hand, stimulation of the motor cortex at one spot with different stimulation intensities results in a characteristic input-output (IO) curve for one specific muscle but may simultaneously also activate different, functionally coupled muscles. A comparison of the cortical map overlap of synergistic muscles and their IO curves has not yet been carried out.
  
  
  OBJECTIVE
  The aim of this study was to probe functional synergies of forearm muscles with transcranial magnetic stimulation by harnessing the convergence and divergence of the corticospinal output.
  
  
  METHODS
  We acquired bihemispheric cortical maps and IO curves of the extensor carpi ulnaris, extensor carpi radialis, and extensor digitorum communis muscles by subjecting 11 healthy subjects to both monophasic and biphasic pulse waveforms.
  
  
  RESULTS
  The degree of synergy between pairs of forearm muscles was captured by the overlap of the cortical motor maps and the respective IO curves which were influenced by the pulse waveform. Monophasic and biphasic stimulation were particularly suitable for disentangling synergistic muscles in the right and left hemisphere, respectively.
  
  
  CONCLUSIONS
  Combining IO curves and different pulse waveforms may provide complementary information on neural circuit dynamics and corticospinal recruitment patterns of synergistic muscles and their neuroplastic modulation.
authors.affiliation: Division of Functional and Restorative Neurosurgery, Eberhard Karls University TübingenTübingen, Germany; Centre for Integrative Neuroscience, Eberhard Karls University TübingenTübingen, Germany.
authors.forename: James
authors.initials: J
authors.lastname: Mathew
authors.affiliation: Division of Functional and Restorative Neurosurgery, Eberhard Karls University TübingenTübingen, Germany; Centre for Integrative Neuroscience, Eberhard Karls University TübingenTübingen, Germany.
authors.forename: Angelika
authors.initials: A
authors.lastname: Kübler
authors.affiliation: Division of Functional and Restorative Neurosurgery, Eberhard Karls University TübingenTübingen, Germany; Centre for Integrative Neuroscience, Eberhard Karls University TübingenTübingen, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Bauer
authors.affiliation: Division of Functional and Restorative Neurosurgery, Eberhard Karls University TübingenTübingen, Germany; Centre for Integrative Neuroscience, Eberhard Karls University TübingenTübingen, Germany.
authors.forename: Alireza
authors.initials: A
authors.lastname: Gharabaghi
keywords: abnormal synergies; cortical motor map; forearm muscles; motor-evoked potential; neurorehabilitation; plasticity; stimulus–response curve; stroke