Complex organization of human primary motor cortex: a high-resolution fMRI study

Abstract

A traditional view of the human motor cortex is that it contains an overlapping sequence of body part representations from the tongue in a ventral location to the foot in a dorsal location. In this study, high-resolution functional MRI (1.5x1.5x2 mm) was used to examine the somatotopic map in the lateral motor cortex of humans, to determine whether it followed the traditional somatotopic order or whether it contained any violations of that somatotopic order. The arm and hand representation had a complex organization in which the arm was relatively emphasized in two areas: one dorsal and the other ventral to a region that emphasized the fingers. This violation of a traditional somatotopic order suggests that the motor cortex is not merely a map of the body but is topographically shaped by other influences, perhaps including correlations in the use of body parts in the motor repertoire.

FIG. 1.

Hemodynamic response function based on data from subject 1. Signals from 40 voxels throughout motor cortex were used. Each individual time series was normalized to a peak value of 1. The individual, normalized time series were averaged to obtain the mean hemodynamic response function. Error bars are SD.

FIG. 2.

Cortical activations evoked by 10 different movement types in subject 1. Shown on an inflated cortical sheet are β values obtained by regression against the 10 movement types, thresholded at P < 0.0001 uncorrected; only positive β values are shown. Warm colors indicate greater β values and cold colors indicate smaller β values. The β values are expressed in units of percent signal change. Thus a β value of 1 indicates that, according to the regression analysis, the movement caused a peak-to-peak change in signal intensity of 1%. The scale was truncated at 2%. All displays are of a lateral view of the left hemisphere except for I, which shows a dorsal view.

FIG. 3.

Somatotopy in subject 1 shown in a “winner-take-all” map. A: data from 1st scan session. B: data from 2nd scan session, 2 mo later. Voxels were selected if the regression was positive and significant at the P < 0.0001 level. These selected voxels were assigned to the movement that resulted in the largest F value. The results were projected onto the inflated cortical sheet, in which dark shading indicates the floor of a sulcus and light shading indicates the crown of a sulcus. The map shows the overall somatotopic progression in the primary motor cortex (anterior to the floor of the central sulcus) and in the primary somatosensory cortex (posterior to the floor of the central sulcus).

FIG. 4.

Somatotopy displayed in a “winner-take-all” map. A–D: data from subjects 2–5.

FIG. 5.

Time series data for subject 1. The cortical map shows a close-up of the winner-take-all display of the motor cortex (see Fig. 3A). A–F: hemodynamic time series for the 10 movement conditions, for each of the indicated points on the cortex. Time series were based on unsmoothed data and were a mean of 5 adjacent voxels within the cortical slab. The percent signal change was normalized to the initial value in the plotted time interval. Error bars are SD. The blue rectangle indicates the time during which the movement was performed.

FIG. 6.

Time series data for subjects 2–5. Each column shows data from a different subject. Each row shows data from a different location along the motor cortex map. A: data from a dorsal location in the upper wrist/forearm representation. B: data from a transitional location between the upper wrist/forearm representation and the finger representation. C: data from the center of the finger representation. D: data from a transitional location between the lower wrist/forearm representation and the finger representation. E: data from a ventral location in the lower wrist/forearm representation. The color scheme for indicating specific movement types is the same as in Fig. 5.

FIG. 7.

Core-and-surround organization of the hand and arm representation in 5 individual subjects and in a group average of subjects. Each panel shows a close-up of the flattened motor cortex. The width of each panel represents ∼4 cm. A: those voxels that showed hemodynamic responses to the fingers, wrist, or forearm, and that were significant at P < 0.0001, were selected for further analysis. For each selected voxel, a finger-vs.-arm index was calculated in which −1 indicated a response to the wrist and forearm but not to the fingers, +1 indicated a response to the fingers but not to the wrist and forearm, and 0 indicated an equal response to both. The results were projected onto an inflated cortical surface, with red indicating +1 and dark blue indicating −1. The white dotted line shows the floor of the central sulcus, with the primary motor cortex to the left of the line. B–E: same for remaining subjects. F: mean result obtained by 1st aligning the finger representations of the 5 subjects and then averaging the finger-vs.-arm indices.

FIG. 8.

Result for subject 1 when signal in primary somatosensory cortex was removed before analysis. For all voxels in S1 or within 2 mm of S1, the measured hemodynamic signal was replaced with noise that had the same SD and mean but no temporal structure. After smoothing and regression analysis, a similar result was obtained in M1, although the amount of signal in M1 was reduced (cf. Fig. 3A). The result suggests that the findings in M1 were not the result of signal contamination across the sulcus from S1.