Age-dependent changes in physiological threshold asymmetries for the motor evoked potential and silent period following transcranial magnetic stimulation

Objective: To study the effect of age on the physiological threshold asymmetries for the motor evoked potential (MEP) and silent period (SP) following transcranial magnetic stimulation. Methods: We studied 63 right-handed subjects and 13 young left-handed subjects (19–39 years). The right-handers were classified into three age groups; 22 young (20–38 years), 20 middle-aged (40–58 years) and 21 old (61–82 years) subjects. We measured the MEP thresholds at rest and during voluntary contraction (VC), and the SP thresholds from the right and left abductor pollicis brevis (APB) muscles. We also measured the side to side differences of the F wave persistency and the F wave/M wave amplitude ratio from the same muscles. Results: Among young subjects, all of the MEP and SP thresholds for the right APB were significantly lower than those for the left APB in the right-handers, and the reverse was true in the left-handers. The results in the middle-aged right-handers were similar to those in the young right-handers, but in the old right-handers, none of the thresholds were different between the two sides. We did not find any asymmetries of the F wave in the subjects of any age group. Conclusion: We speculate that the age-dependent threshold asymmetries are preferentially related to functional asymmetries at the cortical level.