Superior longitudinal fasciculus

The superior longitudinal fasciculus is a major long association fibre system that principally connects frontal and parietal/perisylvian cortical regions within the same cerebral hemisphere. It is one of the largest association fibre systems of the cerebrum.

Traditionally, the superior longitudinal fasciculus (SLF) is divided into three longitudinal components, i.e. SLF I, SLF II and SLF III, that are arranged from dorsal to ventral.

• SLF I is the most dorsal component and is generally described as connecting the superior parietal lobule and precuneus with superior frontal and anterior cingulate regions. It is very closely related and intermixed with the cingulum.

• SLF II occupies an intermediate position and principally connects the angular gyrus and adjacent inferior parietal regions with the posterior superior and middle frontal/caudolateral prefrontal regions.

• SLF III is the most ventral component and principally connects the supramarginal and adjacent inferior parietal regions with the ventral premotor and inferior frontal cortex.

Functional correlates

Functionally, the superior longitudinal fasciculus provides important frontoparietal connectivity involved in several distributed networks rather than serving a single function.

• The left SLF II and III are particularly associated with speech and language processing, including motor planning of speech, articulation and aspects of syntactic processing.

• Their right-sided counterparts are more strongly associated with visuospatial attention, spatial orientation and spatial awareness.

Relationship to the arcuate fasciculus

Older descriptions sometimes used superior longitudinal fasciculus and arcuate fasciculus synonymously. Janelle et al. (2022) recommend distinguishing them. The SLF is principally a frontoparietal system, whereas the arcuate fasciculus is principally a frontotemporal tract that passes through parietal white matter beneath the SLF. Nevertheless, some overlap exists between the two fibre pathways.