Limited impact of homeostatic plasticity on motor learning in humans.
Neuroplasticity is the adaptive modification of network connectivity in response to environmental demands and has been identified as a major physiological correlate of learning. Since unrestricted neuroplastic modifications of network connectivity will result in a de-stabilization of the system, metaplastic modification rules have been proposed for keeping plastic connectivity changes within a useful dynamic range. In this connection, the modification threshold to achieve synaptic strengthening is thought to correlate negatively with the history of activity of the respective neurons, i.e. high previous activity enhances the threshold for synaptic strengthening and vice versa. However, the relevance of metaplasticity for actual learning processes has not been tested so far. We reduced or enhanced motor cortex excitability before performance of the serial reaction time task (SRTT), a sequential motor learning paradigm, and a reaction time task (RTT) by transcranial direct current stimulation (tDCS). If homeostatic rules apply, excitability-diminishing cathodal tDCS should improve subsequent motor learning, especially if combined with the partial NMDA receptor-agonist d-cycloserine, which selectively enhances efficacy of active receptors, while excitability-enhancing anodal tDCS should reduce it. Only the results for anodal tDCS, when combined with d-cycloserine, were in accordance with the rules of homeostatic plasticity. We conclude that homeostatic plasticity, as tested here, has a limited influence on implicit sequential motor learning.
Kuo
Min-Fang
MF
Department of Clinical Neurophysiology, University of Goettingen, Robert Koch Str. 40, 37099 Goettingen, Germany.
Unger
Mandy
M
Liebetanz
David
D
Lang
Nicolas
N
Tergau
Frithjof
F
Paulus
Walter
W
Nitsche
Michael A
MA
D000328:Adult; D000704:Analysis of Variance; D000963:Antimetabolites / Q000494:pharmacology; D003523:Cycloserine / Q000494:pharmacology; D004558:Electric Stimulation / Q000379:methods; D019054:Evoked Potentials, Motor / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D005260:Female; D006801:Humans; D007858:Learning / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D008297:Male; D009044:Motor Cortex / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D009473:Neuronal Plasticity / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D011597:Psychomotor Performance / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D011930:Reaction Time / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D050781:Transcranial Magnetic Stimulation
D000963:Antimetabolites; D003523:Cycloserine