Age-related decrease in paired-pulse intracortical inhibition in the humanprimary motor cortex

Using biphasic magnetic stimuli, paired-pulse transcranial magnetic stimula tion (TMS) at short interstimulus intervals (ISIs) was employed to investig ate age-related changes in the balance between intracortical inhibition and facilitation. In 26 right-handed healthy individuals, motor evoked potenti als were recorded from the relaxed right first dorsal interosseus muscle af ter paired-pulse TMS of the left primary motor hand area. The magnitude of intracortical paired-pulse inhibition at Isis of 1-5 ms was markedly reduce d in elderly individuals, whereas no age effect was observed for intracorti cal paired-pulse facilitation at Isis of 11-15 ms. This finding demonstrate s that normal aging is associated with a relative decrease in the excitabil ity of intracortical inhibitory circuits. In conclusion, paired-pulse TMS p rovides a non-invasive means of studying age-related functional changes in the motor cortex. (C) 2001 Elsevier Science Ireland Ltd. All rights reserve d.