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The Journal of Neuroscience, June 20, 2007, 27(25):6815-6822; doi:10.1523/JNEUROSCI.0598-07.2007

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Behavioral/Systems/Cognitive
Focal Stimulation of the Posterior Parietal Cortex Increases the Excitability of the Ipsilateral Motor Cortex

Giacomo Koch,^1,2,3 Miguel Fernandez Del Olmo,^1,4 Binith Cheeran,¹ Diane Ruge,¹ Sven Schippling,^1,5 Carlo Caltagirone,^2,3 and John C. Rothwell¹

¹Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, Queen Square, London WC1N 3BG, United Kingdom, ²Laboratorio di Neurologia Clinica e Comportamentale, Fondazione Santa Lucia, 00179 Rome, Italy, ³Clinica Neurologica, Dipartimento di Neuroscienze, Università di Roma Tor Vergata, 00133 Rome, Italy, ⁴Institute of Physical Education and Sport, La Coruña, Spain, and ⁵Department of Neurology, University Medical Centre Hamburg-Eppendorf, Germany

Correspondence should be addressed to Prof. John C. Rothwell, Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, Queen Square, London WC1N 3BG, United Kingdom. Email: J.Rothwell{at}ion.ucl.ac.uk

Paired-pulse transcranial magnetic stimulation (TMS) has been applied as a probe to test functional connectivity within distinct cortical areas of the human motor system. Here, we tested the interaction between the posterior parietal cortex (PPC) and ipsilateral motor cortex (M1). A conditioning TMS pulse over the right PPC potentiates motor evoked-potentials evoked by a test TMS pulse over the ipsilateral motor cortex, with a time course characterized by two phases: an early peak at 4 ms interstimulus interval (ISI) and a late peak at 15 ms ISI. Activation of this facilitatory pathway depends on the intensity of stimulation, because the effects are induced with a conditioning stimulus of 90% resting motor threshold but not at lower or higher intensities. Similar results were obtained testing the ipsilateral interaction in the left hemisphere with a slightly different time course. In control experiments, we found that activation of this facilitatory pathway depends on the direction of induced current in the brain and is specific for stimulation of the caudal part of the inferior parietal sulcus (cIPS) site, because it is not observed for stimulation of adjacent scalp sites. Finally, we found that by using poststimulus time histogram analysis of single motor unit firing, the PPC conditioning increases the excitability of ipsilateral M1, enhancing the relative amount of late I wave input recruited by the test stimulus over M1, suggesting that such interaction is mediated by specific interneurons in the motor cortex. The described facilitatory connections between cIPS and M1 may be important in a variety of motor tasks and neuropsychiatric disorders.

Key words: connectivity; facilitation; motor control; motor cortex; motor-evoked potential; parietal

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Received Feb. 11, 2007; revised April 27, 2007; accepted May 1, 2007.

Correspondence should be addressed to Prof. John C. Rothwell, Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, Queen Square, London WC1N 3BG, United Kingdom. Email: J.Rothwell{at}ion.ucl.ac.uk

This article has been cited by other articles:

				G. Koch, M. F. Del Olmo, B. Cheeran, S. Schippling, C. Caltagirone, J. Driver, and J. C. Rothwell Functional Interplay between Posterior Parietal and Ipsilateral Motor Cortex Revealed by Twin-Coil Transcranial Magnetic Stimulation during Reach Planning toward Contralateral Space J. Neurosci., June 4, 2008; 28(23): 5944 - 5953. [Abstract] [Full Text] [PDF]

				J. Reis, O. B. Swayne, Y. Vandermeeren, M. Camus, M. A. Dimyan, M. Harris-Love, M. A. Perez, P. Ragert, J. C. Rothwell, and L. G. Cohen Contribution of transcranial magnetic stimulation to the understanding of cortical mechanisms involved in motor control J. Physiol., January 15, 2008; 586(2): 325 - 351. [Abstract] [Full Text] [PDF]

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