Non-invasive differentiation of motor cortical representation of hand muscles by mapping of optimal current directions.

Non-invasive mapping of human motor cortex by stimulating different scalp positions with a magnetic coil held at a constant orientation allows differentiation of proximal and distal arm muscles. This study describes a technique for more precise mapping of closely represented muscles using different orientations of a coil that delivers nearly monopolar current pulses. EMG was recorded from abductor pollicis brevis (APB), first dorsal interosseous (FDI), abductor digiti minimi (ADM), and flexor carpi radialis (FCR) of 9 normal volunteers. Stimuli were delivered from a Dantec stimulator through an 8-shaped coil. The center of the coil was kept flat on the scalp on a given position, and the coil rotated at different angles. The amplitudes of the motor evoked potentials were used for calculation of optimal current directions in the brain for activation of each muscle in each position. The optimal current direction for FCR activation pointed antero-medially. ADM, FDI and APB mapped progressively more antero-laterally. The relationship between current directions was constant across subjects and did not change in different scalp positions. This technique improves the spatial resolution of non-invasive cortical mapping and may express the differences in orientations of interneuronal nets in the precentral gyrus.