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Received February 10, 2005
Revised March 8, 2005
Accepted after revision April 18, 2005
1 Dept. Neurology, University of Rostock
2 Dept. Neurology, University of Wuerzburg
3 University of Wuerzburg
* To whom correspondence should be addressed. E-mail: classen_j{at}klinik.uni-wuerzburg.de.
Animal experiments suggest that cortical sensory representations may be remodeled 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 likely generated in 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. Findings suggest a simple model of modulation of excitability in human primary somatosensory cortex, possibly by mechanisms related to spike-timing dependent plasticity of neuronal synapses located in upper cortical layers.
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