Increased intracortical inhibition in middle-aged humans; a study using paired-pulse transcranial magnetic stimulation.

Using single and paired-pulse transcranial magnetic stimulation we compared the cortical excitability in two different age groups of healthy subjects (mean+/-SD age: 28.5+/-5.2 vs. 56.1+/-4.9 years). Motor evoked potentials were recorded from right extensor and flexor carpi radialis muscles. The effect of paired-pulse stimulation was assessed by the ratio conditioned/unconditioned response area with interstimulus intervals of 3 and 13 ms to test for intracortical inhibition and facilitation, respectively. To test the influence of sensory input the experiments were conducted without and with vibration of the extensor carpi radialis muscle. The intracortical inhibition was significantly greater in older subjects; however, during muscle vibration this difference between the two groups vanished. The different effect of vibration favors compensatory mechanisms to be responsible for a different paired-pulse excitability in middle-aged subjects.