HANDEDNESS AND ASYMMETRY OF HAND REPRESENTATION IN HUMAN MOTOR CORTEX

The cortical representation of five simple hand and finger movements i n the human motor cortex was determined in left-and right-handed peopl e with whole-head magnetoencephalography. Different movements were fou nd to be represented by spatially segregated dipolar sources in primar y motor cortex. The spatial arrangement of neuronal sources for digit and wrist movements was nonsomatotopic and varied greatly between subj ects. As an estimator of hand area size in primary motor cortex, we de termined the smallest cuboid volume enclosing the five dipole sources within the left and right hemisphere of each subject. Interhemispheric comparison revealed a significant increase of this volume in primary motor cortex opposite to the preferred hand. This asymmetry was due to a greater spatial segregation of neuronal dipole generators subservin g different hand and finger actions in the dominant hemisphere. Mean E uclidean distances between dipole sources for different movements were 10.7 +/- 3.5 mm in the dominant and 9.4 +/- 3.5 mm in the nondomi nan t hemisphere (mean +/- SD; P = 0.01, two-tailed t-test). The expansion of hand representation in primary motor cortex could not simply be at tributed to a greater number of pyramidal cells devoted to each partic ular movement as inferred from current source amplitudes. The degree o f hemispheric asymmetry of hand area size in the primary motor cortex was correlated highly with the asymmetry of hand performance in a stan dardized handedness test(r = -0.76, P < 0.01). These results demonstra te for the first time a biological correlate of handedness in human mo tor cortex. The expansion of hand motor cortex in the dominant hemisph ere may provide extra space for the cortical encoding of a greater mot or skill repertoire of the preferred hand.