New insights in the limbic modulation of visual inputs: The role of the inferior longitudinal fasciculus and the Li-Am bundle

Abstract

Recent anatomical and DTI data demonstrated new aspects in the subcortical occipito-temporal connections. Although a direct (inferior longitudinal fasciculus, ILF) pathway has been previously described, its fine description is still matter of debate. Moreover, a fast and direct subcortical connection between the limbic system and the occipital lobe has been previously recognized in many functional studies but it still remains poorly documented by anatomical images. We provided for the first time an extensive and detailed anatomical description of the ILF subcortical segmentation. We dissected four human hemispheres with modified Klingler's technique, from the basal to the lateral occipito-temporal surface in the two steps, tracking the ILF fibers until their cortical termination. Pictures of this direct temporo-occipital pathway are discussed in the light of recent literature regarding anatomy and functions of occipito-temporal areas. The dissection confirmed the classical originating branches of ILF and allowed a fine description of two main subcomponent of this bundle, both characterized by separate hierarchical distribution: a dorsal ILF and a ventral ILF. Moreover, a direct pathway between lingual cortex and amygdala, not previously demonstrated, is here described with anatomical images. Even if preliminary in results, this is the first fine description of ILF's subcomponents. The complex but clearly segregated organization of the fibers of this bundle (dILF and vILF) supports different level of functions mediated by visual recognition. Moreover, the newly described direct pathway from lingual to amygdala (Li-Am), seems involved in the limbic modulation of visual processing, so it may support physiological conditions the crucial role of this connection in human social cognition. In pathological conditions, on the other hand, this may be one of the hyperactivated pathways in temporo-occipital epileptic and nonepileptic syndromes.

The white matter tracts of the cerebrum in ventricular surgery and hydrocephalus

“…11A, 11B, 11D, and 11E; Tables 1 and 2). 53,54 To our knowledge, this is the first report that shows the Li-Am fasciculus in both MR tractography and fiber dissection.…”

“…The Li-Am fasciculus, connecting the limbic and visual systems, has been linked to developmental and individual differences in social cognition, and damage to this structure has been implicated in the propagation of occipital lobe epilepsy and deterioration of social cognition. 53,54,93 Damage to the anterior part of the optic radiations in this approach may result in superior homonymous quadrantanopsia. 124 Paramedian Supracerebellar-Transtentorial Approach.…”

“…104,107 This approach requires a parahippocampal gyrus incision to reach the hippocampus and amygdala, which can damage the cingulum, fornix, and Li-Am fasciculus and may cause limbic system dysfunction with behavioral and cognitive changes (Table 3). 53,54,72,88 …”

The use of a cerebral perfusion and immersion–fixation process for subsequent white matter dissection

“…All the major associative bundles were dissected and isolated from the lateral surface of the brain until their cortical terminations as already reported by many authors on both DTT and dissection studies (Ture et al, 2000;Schmahmann and Pandya, 2006;Thiebaut de Schotten, 2008, 2012;Fernandez-Miranda et al, 2008;Martino et al, 2010;Maldonado et al, 2013;Latini, 2015) (Fig. 2).…”

“…The specimens were dissected in a stepwise manner, from lateral surface to the medial structures and from the basal surface to the ventricle, with a modified fibre dissection technique in respect to the technique described by Klingler (Latini, 2015). Microscopic metal dissectors and thin wooden spatulas were used in the initial steps of the dissection, to split or partially peel away the brain cortex, preserving the most superficial intra-cortical and subcortical fibres of the lateral and basal brain surfaces.…”