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Neural correlates of encoding and expression in implicit sequence learning
Authors:R?D?Seidler  A?Purushotham  S-G?Kim  K?Ugurbil  D?Willingham  Email author" target="_blank">J?AsheEmail author
Institution:(1) Department of Neuroscience, Psychiatric and Anaesthesiological Sciences, University of Messina, Messina, Italy;(2) Dipartimento di Scienze Neurologiche and Istituto Neurologico Mediterraneo Neuromed IRCCS, Universita degli Studi di Roma ldquoLa Sapienzardquo, Roma, Italia;(3) Department of Physiology and Pharmacology, City University of New York Medical School, New York, USA;(4) Department of Neurology, Christian-Albrechts-University, Kiel, Germany;(5) Present address: Clinica Neurologica 2, Policlinico Universitario, 98125 Messina, Italy
Abstract:It has been shown that high-frequency repetitive transcranial magnetic stimulation (rTMS) to the human primary motor hand area (M1-HAND) can induce a lasting increase in corticospinal excitability. Here we recorded motor evoked potentials (MEPs) from the right first dorsal interosseus muscle to investigate how sub-threshold high-frequency rTMS to the M1-HAND modulates cortical and spinal excitability. In a first experiment, we gave 1500 stimuli of 5 Hz rTMS. At an intensity of 90% of active motor threshold, rTMS produced no effect on MEP amplitude at rest. Increasing the intensity to 90% of resting motor threshold (RMT), rTMS produced an increase in MEP amplitude. This facilitatory effect gradually built up during the course of rTMS, reaching significance after the administration of 900 stimuli. In a second experiment, MEPs were elicited during tonic contraction using weak anodal electrical or magnetic test stimuli. 1500 (but not 600) conditioning stimuli at 90% of RMT induced a facilitation of MEPs in the contracting FDI muscle. In a third experiment, 600 conditioning stimuli were given at 90% of RMT to the M1-HAND. Using two well-established conditioning-test paradigms, we found a decrease in short-latency intracortical inhibition (SICI), and a facilitation of the first peak of facilitatory I-waves interaction (SICF). There was no correlation between the relative changes in SICI and SICF. These results demonstrate that subthreshold 5 Hz rTMS can induce lasting changes in specific neuronal subpopulations in the human corticospinal motor system, depending on the intensity and duration of rTMS. Short 5 Hz rTMS (600 stimuli) at 90% of RMT can selectively shape the excitability of distinct intracortical circuits, whereas prolonged 5 Hz rTMS (ge900 stimuli) provokes an overall increase in excitability of the corticospinal output system, including spinal motoneurones.
Keywords:Corticospinal excitability  Human motor cortex  Paired pulse TMS  Repetitive transcranial magnetic stimulation  Transcranial electrical stimulation
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