Interindividual variability and age-dependency of motor cortical plasticity induced by paired associative stimulation
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Paired associative stimulation (PAS) can increase motor cortical excitability, possibly by long-term potentiation (LTP)-like mechanisms. As the capability of the cortex for plasticity decreases with age, we were interested here in testing interindividual variability and age-dependency of the PAS effect. Motor-evoked potentials (MEPs) were recorded from the resting right abductor pollicis brevis muscle before and for 30 min after PAS in 27 healthy subjects (22–71 years of age). PAS consisted of 225 pairs (rate, 0.25 Hz) of right median nerve stimulation followed at an interval equaling the individual N20-latency of the median nerve somatosensory-evoked cortical potential plus 2 ms by transcranial magnetic stimulation of the hand area of left primary motor cortex (PASN20+2). The PASN20+2-induced changes in MEP amplitude (ratio post PAS/pre PAS) were highly variable (1.00 ± 0.07, range 0.36–1.68). Fourteen subjects showed the expected LTP-like MEP increase (responders) while 13 subjects showed a long-term depression (LTD)-like MEP decrease (non-responders). Responders had a significantly lower resting motor threshold (RMT) and minimum stimulus intensity to elicit MEPs of 1 mV (MEP1 mV) than non-responders. RMT and MEP1 mV correlated significantly negatively with the PASN20+2 effect. The absolute PASN20+2 effect size irrespective of its direction decreased with age (r = −0.57, P = 0.002), i.e., LTP-like and LTD-like plasticity were large in young subjects but substantially smaller in elderly subjects. In conclusion, measures of motor cortical excitability (RMT, MEP1 mV) and age determine direction and magnitude of PAS effects in individual subjects.
KeywordsAge Motor cortex Paired associative stimulation Plasticity Transcranial magnetic stimulation
We thank B. Bliem for constructive comments on the manuscript. This work was supported by grant ZI 542/4-1 from the German Research Foundation (DFG). A similar study has been published during the review process on this manuscript (Tecchio et al. (2008) Clin Neurophysiol Jan 4 [Epub ahead of print]).
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