Deep
brain stimulation (DBS) is a non-pharmacological treatment for Parkinson’s disease
(PD), and its efficacy depends largely on which anatomical structure (target)
is stimulated. The subthalamic nucleus (STN) is one of the most commonly used
targets, but stimulation of new targets within the posterior sub-thalamic area
(PSA), comprising a group of white matter
fibers known as prelemniscal radiations (Raprl), as well as the caudal zonaincerta nucleus (Zic), have proven to be superior at improving certain clinical
symptoms. Despite their clinical usefulness, their anatomical connectivity has
not been completely described in humans. We performed constrained spherical deconvolution of the signal in diffusion-weighted
images and subsequent tractography as a means to non-invasively define the
connectivity of the Raprl and Zic in a group of five patients with PD. Further,
we used track-density imaging, a novel method to improve the spatial resolution
of the acquired images, in order to visualize the small subregions that comprise
the PSA with a voxel resolution of 0.2 × 0.2 × 0.2 mm3. Both Raprl
and Zic demonstrated high probability of connectivity with the dorsal brainstem,
cerebellum, subcortical nuclei (globus pallidum ventral, lateral thalamic
nuclei), and cortical areas (orbitofrontal cortex, primary and supplementary
motor cortex areas). The connectivity patterns were re-producible between
patients and were discretely organized as the tracts entered/exited the PSA,
depending on their end points. These findings indicate that the PSA is part
of the neuronal circuitry controlling movement, and the precise characterization
of its connectivity will aid in our understanding of the net-works involved in
PD and how they can be modulated with DBS in order to alleviate symptoms.