Anatomy and functional role of the inferior longitudinal fasciculus: a search that has just begun

The inferior longitudinal fasciculus (ILF) was classically defined as a direct connection from the occipital cortex to the temporal lobe. But even after the advent of diffusion tensor imaging (DTI) this view was challenged by several authors. Using DTI and tractography, which allows for a non-invasive assessment of the microstructural organization of white matter pathways, Catani et al. elegantly confirmed the direct pathway between the occipital cortex and temporal lobe in a group average, as well as in eleven individual normal controls. The ILF fiber bundle connects the occipital lobe with the anterior part of the temporal lobe, running laterally and inferiorly above optic radiation fibers. However, in addition to the ILF there is a second major fiber bundle connecting the occipital cortex to the frontal brain known as the inferior fronto-occipital fasciculus (IFOF) that runs medially and above the optic pathways and spatially overlaps with the ILF along part of their pathways. The IFOF is known as a direct pathway, one that connects the occipital, posterior temporal, and the orbitofrontal areas. The ILF, on the other hand, is considered to be an indirect pathway essentially connecting similar brain areas and anteriorly joins the uncinate fasciculus to relay information to the orbito-frontal brain. In a recent report, Wahl et al. studied whether specific patterns of correlation and overlap among the major brain fibers bundles exist, using DTI scalar parameters such as fractional anisotropy, and axial, radial, and mean diffusivity. The only significant overlaps they found were between the IFOF and the ILF, and between the IFOF and the uncinate fasciculus. Such overlap may indeed reflect functional similarity as well as spatially sharing overlapping paths. The ability to track white matter bundles and access detailed anatomical information about white matter broadens our investigative ability to uncover disease-related abnormalities. However, even though advances in DTI have enabled us to better study the anatomy of the ILF, the understanding of the functional role of the ILF is still poorly understood. Thus the study by Ortibus et al. is of great importance as it broadens our knowledge of ILF functionality. This study is the first to report compromised ILF integrity in children with visualperceptual impairment and found correlations between decreased fractional anisotropy of the ILF with impaired object recognition. Although the authors clearly showed the relationship between ILF integrity and object recognition, based on strong correlations between the ILF and IFOF, it is likely that the IFOF also plays a role in the execution of such processes. As stated by Ortibus et al., when comparing mean fractional anisotropy values in impaired versus non-impaired patients, decreased fractional anisotropy in the ILF should not be considered a biomarker for visual perception, as the ILF has been shown to be involved in multiple functions and various diseases. For example, lower fractional anisotropy in the ILF was also reported in a group of adolescent patients with schizophrenia with visual hallucinations. A recent review by Chanraud et al. provides a comprehensive review of diseases with ILF involvement and other major white matter fiber bundles. As discussed by Chanraud et al., the ILF has functionally been correlated with thought disorders, visual emotion, cognitive impairments, and other symptoms. As a whole, our knowledge of the functional influences of white matter pathways continues to progress rapidly by virtue of the non-invasive nature of DTI and the insight it provides into the brain’s white matter microstructures. It is important, however, to bear in mind that abnormality in a specific fiber bundle may also be reflected in other fiber bundles integrating or sharing the functions of multiple affected cortical regions.