Confirmation of functional zones within the human subthalamic nucleus: patterns of connectivity and sub-parcellation using diffusion weighted imaging

Abstract

The subthalamic nucleus (STN) is a small, glutamatergic nucleus situated in the diencephalon. A critical component of normal motor function, it has become a key target for deep brain stimulation in the treatment of Parkinson's disease. Animal studies have demonstrated the existence of three functional sub-zones but these have never been shown conclusively in humans. In this work, a data driven method with diffusion weighted imaging demonstrated that three distinct clusters exist within the human STN based on brain connectivity profiles. The STN was successfully sub-parcellated into these regions, demonstrating good correspondence with that described in the animal literature. The local connectivity of each sub-region supported the hypothesis of bilateral limbic, associative and motor regions occupying the anterior, mid and posterior portions of the nucleus respectively. This study is the first to achieve in-vivo, non-invasive anatomical parcellation of the human STN into three anatomical zones within normal diagnostic scan times, which has important future implications for deep brain stimulation surgery.

Supplementary Material 1

Delineation of an STN shown as the hyperintense region on the R2* images (left — green, right — red), progressing from the most superior tip (top) to inferior aspect (bottom) in 1 mm slices. The corresponding MT image is also shown, which helped identify the surrounding boundaries by using ITK-SNAP multisession view (STN is not visible on these images). The following boundaries were used in addition to direct visualisation on the R2* images: Inferiorly — The mid-point of the red nucleus, superior aspect of the optic tract and substantia nigra; Anteriorly — The posterolateral wall of the hypothalamus, defined as the grey matter between the mamillothalamic tract posteriorly and fornix anteriorly; Medially — The white matter situated between the thalamus and red nucleus; Laterally — The internal capsule; Superiorly — The anterior commissure.

Supplementary Material 3

Renderings displaying the spatial distributions of sub-cortical connectivity from the ipsilateral sub-segmented STN. Regions were identified as areas where the corresponding tractography overlapped in 3 or more subjects at a tractography threshold of 1%. The individual voxel values of the groups were defined according to the STN sub-region they were maximally assigned to across all subjects. Colour coded according to the calculated STN regions shown previously (red = anterior “Limbic” STN, green = middle “associative” STN, blue = posterior “motor” STN). Outline of the region of interest shown in transparent grey. Substantia nigra and red nucleus not shown as these areas were strongly connected to all regions.

Supplementary Material 4

Regional ipsilateral cortical connectivity from sub-segmented STN. Regions with connectivity present in ≥ 25% of subjects shown (see Table 1). Bars represent the proportion of all connected voxels within a cortical region that are connected to a corresponding STN zone (limbic, motor or associative). Overlap regions are shown, defined as a voxel that meets threshold for two or more STN sub-distributions.

Supplementary Material 5

GPe–putamen–thalamic relay ring showing unique functional topological gradient, defined as projections to all STN-sub regions respecting a somatotopic gradient. These were only present in these areas, and provide a morphological correlate for closed reciprocal, open non-reciprocal spiral loops.

Supplementary Material 6

STN segmentations from 6 individuals.

Fig. 1

Methodological pipeline, refer to text for details. Abbreviations: CSF = Cerbrospinal fluid, CN = Caudate nucleus, GPe = External segment of the globus pallidus, GPi = Internal segment of the globus pallidus, MT = Magnetic transfer, NA = Nucleus accumbens, NNMF = Non-negative matrix factorisation, RN = Red nucleus, SN = Substantia nigra, STN = Subthalamic nucleus.

Fig. 2

Circular ideogram summarising a literature review of the STN afferent and efferent connectivity in mammals. For comparison, reported parasubthalamic connections are included but uniquely labelled. STN afferents are shown in red and efferents in blue. Para-STN afferents are in orange and efferents in pale blue. The width of each connecting ribbon represents the normalised proportion (as a percentage) of the respective connections. Refer to the Methods for details on how this was calculated. A summary of the data and articles reviewed can be found in Supplementary Material 2.

Fig. 3

Cortical regions of common ipsilateral STN connectivity across the group. Dark grey regions demonstrate cortico-tractography intersections. Each hemisphere is shown on the corresponding side. Views, clockwise from top-left: Left medial surface, anterior surface, right medial surface, right lateral surface, right lateral insula, inferior surface, left lateral insula, left lateral surface. Superior surface central image. See Table 1 for details of cortical regions.

Fig. 4

Example elbow plot demonstrating optimal cluster number given probabilistic tractography data for a single STN. Clear “elbow” shown at n = 3 clusters.

Fig. 5

Rendering of group averaged sub-segmented STN regions. The voxel borders of the group were defined according to which cluster they were maximally assigned to across all subjects. Anterior “limbic” cluster = red, middle “associative” cluster = green, posterior “motor” cluster = blue. Left lateral, superior and anterior views demonstrated above. The relationship to group averaged renderings of the red nuclei (dark red) and substantia nigra (grey) are shown. Similar segmentation patterns were achieved on an individual subject basis.

Fig. 6

Overlap of group averaged projections from sub-segmented STN regions. Overlap regions are defined by group averaged tractography distributions in standard space for each STN subregion, and then classifying each ROI brain voxel according to the combination of these average distributions that is connected with it. This is summarised in the top left legend. These demonstrate that the associative regions previously shown (Supplementary Material 2) represent an overlapping network between distinctive motor and limbic networks, sharing regions common to both.

Fig. 7

Overlap between published isolated lesions causing non-STN hemiballismus (top row) and motor-STN projections defined in the current study (bottom row). Top row images reproduced with kind permission from Springer Science and Business Media (left) and Elsevier (centre and right) and with the corresponding authors' permission.