Age-related changes in short-latency motor cortex inhibition

This study examined whether short-latency intracortical inhibition (SICI) and/or facilitation (ICF) changes with ageing, and if this can be attributed to age-related changes in the inhibition and/or corticospinal stimulus-response curves. SICI/ICF was studied in 17 "old" (63.1 +/- 4.2 years) and 13 "young" males (20.0 +/- 2.0 years) in both hemispheres using a paired-pulse transcranial magnetic stimulation paradigm at four interstimulus intervals (1, 3, 10 and 12 ms). Motor-evoked potentials were recorded from the first dorsal interosseous muscle at rest, with a conditioning intensity set at 5% stimulator output below the active threshold (aMT). Regardless of age, SICI was greater in the left compared with the right hemisphere. SICI was increased in old men at 3 ms in the left hemisphere and at 1 ms in the in both hemispheres, but ICF was not altered. However, aMT, and hence the conditioning stimulus intensity, was higher in old men. Comparisons of pairs of young and old men with the same aMT, and of SICI curves constructed relative to aMT, failed to show any age-related increase in SICI, although age-related changes in aMT accounted for less than 20% of the variability. Corticospinal stimulus-response characteristics did not influence SICI/ICF and appear not to be altered by ageing in men. When measured in resting muscles, SICI/ICF appears unaltered by age. But it remains unknown if, when assessed during movement preparation or movement, there are changes in SICI related to functional motor changes commonly associated with ageing, such as slowing of movement.