Reproducibility of primary motor cortex somatotopy under controlled conditions

Abstract

Background and purpose: The somatotopic organization of the contralateral primary motor cortex (M1) and its intra- and intersubject reproducibility has been the subject of many investigations and controversies. A potential explanation for a least some of the conflicting results could be the lack of movement control in the studies performed. The purpose of this study was to investigate these issues under controlled experimental conditions.

Methods: Functional MR imaging was performed in 12 healthy volunteers performing hand, finger, wrist, elbow, foot, and tongue movements. Two experimental sessions were separated by 2 weeks. Controlled conditions were achieved by means of a custom-designed arm and hand manipulandum providing standardization of the movements within and across subjects.

Results: The experiments revealed a clear large-scale somatotopy of the contralateral M1 with distinct subregions controlling the foot, arm, and tongue. Despite considerable overlap of the volumes, geometric centers of gravity (COGs) showed statistically significant differences in coordinates between the elbow, wrist, fingers, and hand. COGs showed a high degree of intra- and interindividual reproducibility, particularly for the upper limb movements, in contrast to the activation volumes that proved to be unreliable parameters, despite the controlled conditions.

Conclusion: These findings support the existence of a gross-scale somatotopic organization yet also demonstrate a clear, fine-scale somatotopy of the within-arm representations. Furthermore, they reveal high reproducibility of the COGs when standardized conditions are applied. This observation highlights the need for movement control to allow for intra- and intersubject comparison.

Fig 1.

Activation in contralateral M1 (arrows) displayed in axial sections for one subject in the first session. The right side of the sections corresponds to the left hemisphere, and the numbers in the color bar correspond to t values. A and B, The fingers (A) and hand (B) are in almost identical locations (z plane, +58). C and D, The wrist (C) and elbow (D) representations are located more medially, superior and posterior along the course of M1 (z plane, +59 and +61, respectively). E and F, Note the considerable overlap of activated volumes within the arm and the clear separation of the foot (E) and tongue (F) (z plane, +66 and +28, respectively).

Fig 2.

Activation of contralateral M1 in the second session in the same subject as in Figure 1. A comparison with the images in Figure 1 reveals a high degree of reproducibility in the somatotopy despite variations in the activated volumes. A and B, Fingers (A) and hand (B). C and D, Wrist (C) and elbow (D). E and F, Foot (E) and tongue (F).

Fig 3.

Two-dimensional scatter plots of the COGs in the 12 subjects (two sessions per subject) in the contralateral M1. Small dots represent individual COGs, and large dots indicate the mean COGs. Note the separate subdivisions for the foot, arm, and tongue and the clear somatotopic gradients within the arm representations in both the axial and coronal planes. The x, y, and z coordinates corresponding to those in Talairach space (21). Left, Axial plane with approximate contour of the precentral gyrus. Right, Coronal plane with the cortical surface and limited to the white matter.

Fig 4.

Two-dimensional scatter plots of within-forearm COGs in contralateral M1 in the 12 subjects. COGs plane for the fingers, hand, wrist, and elbow are displayed in the axial plane. A, First experimental session. The approximate contour of the precentral gyrus is outlined (as in Fig 3 left image, with the same colors as in Fig 3). The width x of each rectangle is 20 mm and the height y is 18 mm in Talairach space. Note the preserved somatotopic gradient in all 12 individual hand and forearm representations. B, Second experimental session. Note the highly similar distribution and preserved somatotopic gradient of within-forearm COGs in almost all subjects compared with those of first session.