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The Journal of Neuroscience, April 4, 2007, 27(14):3807-3812; doi:10.1523/JNEUROSCI.5348-06.2007

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Behavioral/Systems/Cognitive
Timing-Dependent Modulation of Associative Plasticity by General Network Excitability in the Human Motor Cortex

Michael A. Nitsche, Amelie Roth, Min-Fang Kuo, Anja K. Fischer, David Liebetanz, Nicolas Lang, Frithjof Tergau, and Walter Paulus

Georg-August-University, Department for Clinical Neurophysiology, 37075 Goettingen, Germany

Correspondence should be addressed to Michael A. Nitsche at the above address. Email: mnitsch1{at}gwdg.de

Associative neuroplasticity, which encompasses the modification of synaptic strength by coactivation of two synaptic inputs, has been linked to learning processes. Because unlimited plasticity destabilizes neuronal networks, homeostatic rules were proposed and experimentally proven that control for the amount and direction of plasticity dependent on background network activity. Accordingly, low background activity would enhance facilitatory plasticity, whereas high background activity would inhibit it.

However, the impact of background excitability on associative plasticity has not been studied so far in humans. Facilitatory associative plasticity was induced by paired associative stimulation (PAS) in the human motor cortex, whereas background activity was enhanced or diminished by transcranial direct current stimulation (tDCS). When applied before PAS, excitability-enhancing tDCS also boosted the efficacy of PAS, whereas excitability-diminishing tDCS turned it into inhibition. Thus, previous background activity does not influence associative plasticity homeostatically. When tDCS and PAS were applied simultaneously, now in accordance with homeostatic rules of neuroplasticity, reduced background activity resulted in a prolonged excitability enhancement by PAS, whereas enhanced background activity turned it into inhibition. We conclude that background network activity can influence associative plasticity homeostatically. However, only simultaneous modulation of both parameters is in accordance with homeostatic concepts. These findings might be of importance for the development of plasticity-inducing stimulation protocols supporting information processing in humans.

Key words: homeostatic plasticity; paired associative stimulation; tDCS; TMS; motor cortex; human

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Received Dec. 12, 2006; revised March 4, 2007; accepted March 5, 2007.

Correspondence should be addressed to Michael A. Nitsche at the above address. Email: mnitsch1{at}gwdg.de

This article has been cited by other articles:

				M.-F. Kuo, J. Grosch, F. Fregni, W. Paulus, and M. A. Nitsche Focusing Effect of Acetylcholine on Neuroplasticity in the Human Motor Cortex J. Neurosci., December 26, 2007; 27(52): 14442 - 14447. [Abstract] [Full Text] [PDF]

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