Corpus callosum and motor function I – Computer-aided investigation of mirror movements

Background: Children, adolescents and adults have mirror movement with a characteristic progression of maturation. In view of the relation of function and structure in the central nervous system, we have investigated these phenomenologically using a computer-aided measurement method (Esslinger/Danek 2001). Objective: Quantitative comparison of mirror movements of the non-dominant hand in the three age groups. Methods: In each hand, the subjects (healthy right handers, mean age and SD: children: 8.3±1.5, n=11; adolescents: 15.6±0.8, n=10; adults: 25.9±2.6, n=10) held a cylindrical force transducer in a pinch grip between thumb and index finger with which the isometric force of the finger muscles was recorded (resolution: 0.1N; sampling rate: 100Hz). The right hand conducted force changes with two frequencies (1Hz and the highest possible frequency) and two forces (both depending on the subject's maximum force), while the left hand held the force transducer without voluntary movements and using the lowest force necessary. The mirror movement ratio (MM ratio) was calculated from the quotients of corresponding maximum values of the force measured in left and right hand, with a high MM ratio representing strong mirror movements. Results: All subjects showed mirror movements. The MM ratios of children and adults differed significantly in three experiments depending on force and frequency (Mann-Whitney test [MWT]: low frequency, low force: p=0.06; low frequency, high force: p=0.001; high frequency, low force: p=0.02; high frequency, high force: p=0.03). Children and adolescents showed significant differences in all four experiments (MWT, experiments as above: p=0.04; p=0.003; p=0.006; p=0.03). In none of the experiments, a significant difference between adolescents and adults was found (MWT, experiments as above: p=0.97; p=0.1; p=0.2; p=0.8). Conclusions: The computer-aided motor function test employed in the experiments allows a more precise assessment of the age dependancy of mirror movements than clinical tests and provides a quantitative parameter for further research on the corpus callosum. Regarding these mirror movements, we assume that transcallosal pathways mediate an inhibition of the contralateral hemisphere which increases with progressing microstructural changes and leads to a decrease of mirror movements up to adulthood.