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1 Human Cortical Physiology Laboratory, Department of Neurology, University of Rostock, D-18147 Rostock, Germany
2 Human Cortical Physiology and Motor Control Laboratory, Department of Neurology, University of Würzburg, D-97080 Würzburg, Germany
Animal experiments suggest that cortical sensory representations may be remodelled as a consequence of changing synaptic efficacy by timing-dependent associative neuronal activity. Here we describe a timing-based associative form of plasticity in human somatosensory cortex. Paired associative stimulation (PAS) was performed by combining repetitive median nerve stimulation with transcranial magnetic stimulation (TMS) over the contralateral postcentral region. PAS increased exclusively the amplitude of the P25 component of the median nerve-evoked somatosensory-evoked potential (MN-SSEP), which is probably generated in the superficial cortical layers of area 3b. SSEP components reflecting neuronal activity in deeper cortical layers (N20 component) or subcortical regions (P14 component) remained constant. PAS-induced enhancement of P25 amplitude displayed topographical specificity both for the recording (MN-SSEP versus tibial nerve-SSEP) and the stimulation (magnetic stimulation targeting somatosensory versus motor cortex) arrangements. Modulation of P25 amplitude was confined to a narrow range of interstimulus intervals (ISIs) between the MN pulse and the TMS pulse, and the sign of the modulation changed with ISIs differing by only 15 ms. The function describing the ISI dependence of PAS effects on somatosensory cortex resembled one previously observed in motor cortex, shifted by 
7 ms. The findings suggest a simple model of modulation of excitability in human primary somatosensory cortex, possibly by mechanisms related to the spike-timing-dependent plasticity of neuronal synapses located in upper cortical layers.
(Received 10 February 2005;
accepted after revision 18 April 2005;
first published online 21 April 2005)
Corresponding author J. Classen: Human Cortical Physiology and Motor Control Laboratory, Department of Neurology, Bayerische Julius-Maximilians Universität, Josef-Schneider Str. 11, 97080 Würzburg, Germany. Email: classen_j{at}klinik.uni-wuerzburg.de
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