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

Objective: To study the effect of age on the physiological threshold asymme tries for the motor evoked potential (MEP) and silent period (SP) following transcranial magnetic stimulation. Methods: We studied 63 right-handed sub jects and 13 young left-handed subjects (19-39 years). The right-handers we re 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 thres holds at rest and during voluntary contraction (VC), and the SP thresholds from the right and left abductor pollicis brevis (APB) muscles. We also mea sured 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 subject s, all of the MEP and SP thresholds for the right APE were significantly lo wer than those for the left APE in the right-handers, and the reverse was t rue 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 fin d any asymmetries of the F wave in the subjects of any age group. Conclusio n: We speculate that the age-dependent threshold asymmetries are preferenti ally related to functional asymmetries at the cortical level. (C) 1998 Else vier Science Ireland Ltd. All rights reserved.