Functional mapping of the somatotopic organization of the supplementary motor area using navigated repetitive transcranial magnetic stimulation and computer vision-based analysis

TL;DR

This study used TMS and computer vision to investigate the somatotopic organization of the supplementary motor area, finding functional rather than anatomical patterns in motor control.

Contribution

A novel kinematic paradigm was introduced to objectively analyze movement phases during TMS, revealing functional SMA organization.

Findings

SMA stimulation disrupted upper extremity function, with strongest effects at posterior sites.

Lower extremity performance increased under SMA stimulation, contradicting expected somatotopy.

Orofacial effects were inconsistent and occurred mainly outside SMA stimulation.

Abstract

The supplementary motor area (SMA) is a cortical region involved in motor and language functions. Motor representations within the SMA follow a somatotopic organization: Anterior regions are linked to orofacial movements, middle regions to upper limb movements, and posterior regions to lower limb movements. SMA lesions may produce impairments that correspond to this somatotopy; therefore, preoperative assessment may aid diagnosis.

This study aimed to revise and extend a protocol for assessing the SMA using navigated repetitive transcranial magnetic stimulation (nrTMS), incorporating somatotopic organization and validating positive stimulation points against non-motor regions.

The dominant-hemisphere SMA of 30 healthy participants (27.1 ± 6.21 years, 18 female) was examined. After mapping of the primary motor cortex with single-pulse TMS, six predefined SMA sites were stimulated using 20 Hz nrTMS while participants performed the Nine Hole Peg Test (NHPT; 120% resting motor threshold (RMT)), the lower extremity motor coordination test (LEMOCOT; 140% RMT), and an orofacial task (130% RMT). Each test was repeated under identical parameters at non-motor control sites. Kinematic measurements were obtained using high-speed recordings.

SMA stimulation disrupted upper extremity function, with the strongest effects observed at posterior sites. In contrast, lower extremity performance was not impaired during SMA stimulation, where tapping speed increased under validation conditions. Orofacial effects were limited and inconsistent, occurring mainly during stimulation outside the SMA and showing no significant spatial pattern.

The expected somatotopic organization of the SMA could not be demonstrated using nrTMS. However, SMA-selective disruptions of upper extremity movements suggest a functional, rather than effector-specific, organization. The novel kinematic paradigm enabled detailed, objective analysis of movement phases and may benefit future TMS studies.

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