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

Using biphasic magnetic stimuli, paired-pulse transcranial magnetic stimulation (TMS) at short interstimulus intervals (ISIs) was employed to investigate age-related changes in the balance between intracortical inhibition and facilitation. In 26 right-handed healthy individuals, motor evoked potentials were recorded from the relaxed right first dorsal interosseus muscle after 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 reduced in elderly individuals, whereas no age effect was observed for intracortical paired-pulse facilitation at ISIs of 11-15 ms. This finding demonstrates that normal aging is associated with a relative decrease in the excitability of intracortical inhibitory circuits. In conclusion, paired-pulse TMS provides a non-invasive means of studying age-related functional changes in the motor cortex.