Superior longitudinal fasciculus (SLF) I and II: an anatomical and functional review

In this review, we summarize the current knowledge regarding the Superior Longitudinal Fasciculus (SLF) I and II. These fibers represent a longitudinal association tract between the parietal and frontal lobes of the brain. We highlight the anatomical representation of the SLF I and II in the primate and in the human brain. The fibers of the SLF I extend from the superior parietal lobule and precuneus, running anteriorly to reach the superior frontal gyrus and the supplementary motor area. The anatomy of the SLF I is debated in the literature, with some Authors questioning the existence of the SLF I as an individual tract. The SLF II is located inferiorly and laterally compared to the SLF I. The fibers of the SLF II extend from the inferior parietal lobule to the middle frontal gyrus. The putative functions of these tracts are reviewed, with particular regards to intraoperative findings and their relevance in applied neurosurgery. Considered together, the two tracts link associative parietal areas with premotor and supplementary motor frontal areas. The two tracts seem therefore involved in supporting the integration of sensory information and motor planning, finalized to visuospatial attention and complex motor behavior. Finally, we discuss future directions for further study of these fiber tracts, highlighting the need for more detailed anatomical study of the SLF I and additional intraoperative tests that have been suggested to explore the function of these tracts during surgery.