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BackgroundThe pallidofugal pathways are classically subdivided into ansa lenticularis, lenticular fasciculus, and subthalamic fasciculus. In addition to these three subsystems, we characterize an anatomical structure that connects the antero-medial pole of the subthalamic nucleus to the ventral portions of the pallidum, both related to limbic processing of information. This bundle has been previously considered to form a part of the ansa lenticularis, however, it shows striking differences on histology and MRI features compared to the ansa lenticularis, and therefore we suggest to denominate it ansa subthalamica. ObjectivesTo describe the ansa subthalamica as a different structure than the ansa lenticularis, that can be recognized by different methods (histology, high-field MRI and connectome tractography), including current 3T clinical imaging. MethodsA complete human brain was histologically processed and submitted to registration procedures to correct for tissue deformations and normalization to MNI space. Coordinates of histological structures were then comparable to high-field (7T) post-mortem and in vivo MRIs, 13 pre-operative 3T imaging of parkinsonian patients and normative connectome tractography. Mean intensity gray values for different structures were measured in Susceptibility-Weighted Images. ResultsIt was possible to characterize this structure with different methods and there was significant difference in signal intensity in the ansa subthalamica (hypointense), compared to the ansa lenticularis (hyperintense). ConclusionsThe ansa subhtalamica may represent the anatomical pathway that connects limbic regions of the STN and pallidum, and should be investigated as a possible substrate for limbic effects of stereotactic surgery of the subthalamic region.

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