Figure 1.
Sagittal (top) and coronal (bottom) myelin stained sections in a human cadaver brain (adapted from Riley, 1943).
Radial fibers (RF) connecting the caudate nucleus (CN, circles) and the external globus pallidus (GPe) are visible (yellow arrows). In the right panel, connections from the CN to the neocortex are highlighted by red arrows. Anatomical structures are labeled: Put = putamen, GPi = internal globus pallidus, Thal = thalamus.
Figure 2.
Direction-coded color images of the main diffusion direction derived from diffusion tensor imaging (DTI; blue: inferior-superior; red: left-right; green: posterior-anterior), averaged across participants, superimposed on sagittal anatomical slices of the typical subject (in Talairach space) of the left and right caudate nucleus (CN; see upper panels).
In a sagittal view, the radial fibers (RF) of the CN (lower panels) can be seen as inferior-superior fiber bundles that connect the CN with the globus pallidus (GP) crossing the internal capsule (IC). Lower left: Fiber orientation map from polarized light imaging (PLI) of a sagittal section through the CN comparable to the section shown with DTI. Fiber directions within the image plane are color-coded and fiber inclination is saturation-coded (see color circle in the inset). Lower right: Maximum intensity map of the same section, which shows the maximum intensity of the polarization sequence and encodes the inclination of the fiber directions. White pixels mark myelinated fibers (dependent on their inclination, the steeper the darker), while dark pixels denote unmyelinated or perpendicular structures. Anatomical structures are labeled: CN = caudate nucleus, Put = putamen, GPe = external globus pallidus, GPi = internal globus pallidus, Thal = thalamus. Note that there is high coherence between the RFs in the CN, as visualized with PLI, and the fiber directions seen in DTI (circles). All individual subjects show similar patterns, see Figure S1.
Figure 3.
Complex fiber orientation structure visualized as orientation density functions overlaid to the direction-coded color image derived from diffusion tensor imaging (DTI).
This sagittal image of a representative participant shows that the principal diffusion directions as defined by the diffusion tensors represent subtle but consistent imbalances within the complex fiber orientation profile of the caudate nucleus (CN) gray matter. While fibers running along the axis of the CN outweigh the fibers entering from the adjacent white matter of the internal capsule (IC) in the posterior portion, in the anterior portion, it seems that fibers entering from the IC dominate the diffusion profile. The middle portion with inferior-superior principal directions (blue area) is characterized by additional fibers restricted to the caudate and pointing in the direction of the external globus pallidus (GPe). All individual subjects show similar patterns, see Figure S2.
Figure 4.
Segmentation of the caudate nucleus according to the principal diffusion orientation.
Left: sagittal slice through one example caudate nucleus, showing the principal diffusion directions (little dashes). The blue curve represents the longitudinal spline (see Methods section) along which the directions are sampled. As a general pattern, the diffusion direction is largely tangential (with respect to the longitudinal spline) in the caudal part of the caudate nucleus. At some point the orientation turns into predominantly radial (red dashed line at approx. 45°). Further anterior, there is again a section with tangential orientation (green dashed line at maximally tangential position), and towards the anterior tip they become radial again. Right: Radiality index (angle of principal diffusion direction with longitudinal spline) plotted against distance along the longitudinal spline, for all subjects and hemispheres. The red discs denote the 45° points (equivalent to red dashed line in left panel), while the green discs indicate the local minimum of the radiality index (i.e. the points with maximally tangential orientation; equivalent to green dashed line in left panel).
Figure 5.
Major fiber populations connecting the CN as reconstructed using streamline tractography based on spherical deconvolution.
Fibers from all 13 subjects are overlaid to T1 slices of on subject. Color coding: Longitudinal fiber pathways running along the head and body of the CN (red); fiber pathways following the tail of the CN (purple); fiber pathways leaving the head of the CN towards the frontal pole (blue); a thin ventral bundle of fibers connecting the inferior CN head region with temporal and occipital cortices (green); several fiber pathways running in parallel to the radiating fibers in the internal capsule appear to penetrate the CN (orange); fiber pathways from the thalamus enter the mid portion of the CN (yellow).
Figure 6.
Three-dimensional rendering of the fiber populations of the caudate (see Fig. 5 for details).
For an interactive version of this figure see Figure S3.
Figure 7.
Cortical targets of fibers originating in the caudate subregions.
Center columns: Cumulative projection sites from all subjects, overlaid to T1 image of one subject (brain surface distance map). Outer columns: Surfaces of caudate nuclei (green) and respective subregion (red) for one subject.