MNI NORMALIZATION OF TMS MOTOR MAPS: PROBING WITHIN-LIMB SOMATOTOPY OF THE PRIMARY HAND MOTOR CORTEX
A within-limb somatotopy of the motor cortex remains a highly debated research topic. Transcranial magnetic stimulation (TMS) is a promising approach of non-invasive motor cortex mapping which is suitable for studying this topic in humans. One of the problems for interpreting TMS mapping results is that MNI normalization, which is routinely used for other brain mapping techniques, is still quite rarely applied for TMS mapping results. Thus, the aim of this work was to develop an algorithm for MNI normalization of TMS maps and to assess the somatotopy gradient for intrinsic and extrinsic hand muscles both at the individual and averaged TMS motor maps. 17 healthy young male right-handed volunteers underwent a TMS motor mapping procedure that included an investigation of three upper limb muscles: abductor pollicis brevis (APB), abductor digiti minimi (ADM) and extensor digitorum communis (EDC). We developed a TMS map normalization algorithm based on SPM8 and implemented it into the TMSmap software. Comparison of the locations of the centers of gravity and hotspots for ADM and EDC TMS representations relative to the APB TMS-map in MNI space revealed a more medial location of ADM and EDC centers of gravity and hotspots: 3 and 8 mm, respectively. For weighted normalized cortical TMS-representations, the ADM area was equivalent to 81%, and the EDC area was equivalent to 115% of the APB area. Thus, in this work, we showed the presence of a within-limb somatotopic gradient in the upper limb primary motor cortex using MNI-normalized TMS motor maps.