Within-digit functional parcellation of Brodmann areas of the human primary somatosensory cortex using functional magnetic resonance imaging at 7 tesla

Abstract

The primary somatosensory cortex (S1) can be subdivided cytoarchitectonically into four distinct Brodmann areas (3a, 3b, 1, and 2), but these areas have never been successfully delineated in vivo in single human subjects. Here, we demonstrate the functional parcellation of four areas of S1 in individual human subjects based on high-resolution functional MRI measurements made at 7 T using vibrotactile stimulation. By stimulating four sites along the length of the index finger, we were able to identify and locate map reversals of the base to tip representation of the index finger in S1. We suggest that these reversals correspond to the areal borders between the mirrored representations in the four Brodmann areas, as predicted from electrophysiology measurements in nonhuman primates. In all subjects, maps were highly reproducible across scanning sessions and stable over weeks. In four of the six subjects scanned, four, mirrored, within-finger somatotopic maps defining the extent of the Brodmann areas could be directly observed on the cortical surface. In addition, by using multivariate classification analysis, the location of stimulation on the index finger (four distinct sites) could be decoded with a mean accuracy of 65% across subjects. Our measurements thus show that within-finger topography is present at the millimeter scale in the cortex and is highly reproducible. The ability to identify functional areas of S1 in vivo in individual subjects will provide a framework for investigating more complex aspects of tactile representation in S1.

Figure 1.

Experimental paradigms used to assess within-finger somatotopy. The asterisks in the hand legend represent locations of stimulation. A, Traveling-wave paradigm: the index finger is stimulated sequentially either from the palm to the tip or in the reverse order. Shaded bars represent 6 s delivery of intermittent stimulation. B, Block paradigm. Blocks of 12 s duration (consisting of 24 bursts of 0.4 s vibrotactile stimulation separated by 0.1 s gaps) were presented in a randomized order. Blocks consisting of 12 s rest periods were also presented.

Figure 2.

A, Results from a previous traveling-wave experiment (Besle et al., 2010) in which all fingertips of the left hand were stimulated (subject 1). Phase maps, thresholded at a coherence value of 0.25, are displayed on an inflated 3D model of the right hemisphere cortical surface (left) and flattened cortical patch (right). Dark gray, Areas of negative curvature (sulci); light gray, areas of positive curvature (gyri); shaded area on the 3D model, location of the cortical flat patch. The orderly representation of the fingers is found in the posterior bank of the CS (white line) and the postcentral gyrus (dashed black line), corresponding to S1. The black line of the inset image represents the delineation of the index finger ROI, which consists of phase values encoded by the green color. B, Results of the traveling-wave paradigm across sites of the index finger for subject 1. Coherence (left) shown with index fingertip ROI overlaid in black, and phase map (right) for the expanded ROI, thresholded at a coherence value of 0.25, displayed over the same patch as inset in A.

Figure 3.

Within-finger traveling-wave data for all subjects. Each row shows data from one subject. A, Coherence maps shown with index fingertip ROI overlaid in black. B, Somatotopic (phase) within-finger maps. Displayed voxels correspond to the index finger ROI, expanded to encompass voxels showing preferential responses to proximal locations. White segments (a, c) and black and white dashed segments (b, d) emphasize phase reversals at the tip and base of the index finger, respectively. C, Histogram of phase values for voxels displayed in the expanded index finger ROI shown in B.

Figure 4.

Reproducibility of within-finger somatotopic maps for a subject with mirror finger representations, subject 1 (top), and a subject with no mirror representations, subject 2 (bottom). A, Phase activation maps from data acquired in two separate scanning sessions. Black line shows index fingertip ROI with data shown for the expanded ROI. B, Corresponding histograms of voxelwise phase differences between data acquired in two traveling-wave scanning sessions.

Figure 5.

MVPA classifier response from the block-design data. Classification of test patterns from each stimulation site for the expanded index finger ROI (data averaged across subjects). Each curve represents the classifier responses to one stimulation site (palm, base, middle, and tip). Filled circles, Proportion of correct responses; error bars, SD. Dashed line, Chance level (25%).