Mapping finger movements by transcranial magnetic stimulation

Abstract

We used transcranial magnetic stimulation (TMS) to characterise, in detail, the kinematic properties of finger movements evoked by stimulation of the motor cortex. Finger movements evoked by TMS at 30 different stimulation sites on the scalp (5×6 grid, 15 stimuli per site, 1.3–1.4 times resting motor threshold) were recorded at rest using an instrumented glove equipped with 10 sensors (60Hz sampling rate) at the two most proximal finger joints. Electromyographic recordings were obtained from the abductor pollicis brevis muscle (APB, 10 subjects) and the abductor digiti minimi muscle (ADM, 8 subjects). Across subjects, movements were released from 22±3 (mean±S.D.) scalp stimulation sites in 39±14% of all trials. Movements occurred both around single and multiple fingers. Individual thumb movements were evoked most frequently (14±13% of all movements) followed by index (10±10%), middle (7±7%), little (4±4%) and ring finger (2±2%) movements. The center of gravity (CoG) of finger movement representations on the scalp followed a subtle latero-medial gradient. The CoG-positions of the thumb and the index finger were located most laterally followed by the middle, ring, and little finger. The thumb-CoG was located 2.3±3.4mm (p=0.011, two-tailed paired t-test) more laterally than the CoG-position of the little finger. The CoG-positions of the APB and ADM muscles closely matched the CoG-positions of the corresponding finger. Scalp representations of individual fingers overlapped widely. Both flexion and extension synergies were evoked and composite finger movements frequently resembled natural movements. Their scalp distribution did not exhibit a systematic topographic pattern. Similar composite finger movements resulted from stimulation of spatially separate sites and dissimilar movements could be evoked from stimulation of the same site. Properties of TMS-evoked finger movements lacked topographical organization on a macroscopic scale, while following a fine somatotopical gradient on a mesoscopic scale. Therefore, they shared idiosyncratic physiological properties emerging from functional activation studies of voluntary movements. Mapping of finger movements by TMS may considerably extend the options of physiological description of the corticospinal output system.