Functional Anatomy of the Inferior Longitudinal Fasciculus: From Historical Reports to Current Hypotheses

Abstract

The inferior longitudinal fasciculus (ILF) is a long-range, associative white matter pathway that connects the occipital and temporal-occipital areas of the brain to the anterior temporal areas. In view of the ILF’s anatomic connections, it has been suggested that this pathway has a major role in a relatively large array of brain functions. Until recently, however, the literature data on these potential functions were scarce. Here, we review the key findings of recent anatomic, neuromodulation, and neuropsychological studies. We also summarize reports on how this tract is disrupted in a wide range of brain disorders, including psychopathologic, neurodevelopmental, and neurologic diseases. Our review reveals that the ILF is a multilayered, bidirectional tract involved in processing and modulating visual cues and thus in visually guided decisions and behaviors. Accordingly, sudden disruption of the ILF by neurologic insult is mainly associated with neuropsychological impairments of visual cognition (e.g., visual agnosia, prosopagnosia, and alexia). Furthermore, disruption of the ILF may constitute the pathophysiologic basis for visual hallucinations and socio-emotional impairments in schizophrenia, as well as emotional difficulties in autism spectrum disorder. Degeneration of the ILF in neurodegenerative diseases affecting the temporal lobe may explain (at least in part) the gradual onset of semantic and lexical access difficulties. Although some of the functions mediated by the ILF appear to be relatively lateralized, observations from neurosurgery suggest that disruption of the tract’s anterior portion can be dynamically compensated for by the contralateral portion. This might explain why bilateral disruption of the ILF in either acute or progressive disease is highly detrimental in neuropsychological terms.

Early childhood development of white matter fiber density and morphology

“…We also observed rapid increases in FC of the inferior longitudinal fasciculi, which may be related to higher-order visual, emotional and reading capabilities (Herbet et al, 2018).…”

Reverse-Engineering the Cortical Architecture for Controlled Semantic Cognition

“…The progression from unimodal perceptual representations to multimodal conceptual representations occurs in a graded fashion across many cortical areas 55,58 corresponding well to a deep network. Long-distance white matter tracts, such as the inferior longitudinal fasciculus 59 , traverse temporal cortex, providing pathways for sparse shortcut connectivity. Recent evidence suggests specifically that posterior fusiform connects to the ATL both via gradual progression along the anterior fusiform and a ‘direct’ white matter route 60 .…”

White matter pathways supporting individual differences in epistemic and perceptual curiosity

“…The ILF, which connects ventral aspects of ATL, occipito-temporal, and occipital cortex (Herbet et al, 2018), appears critical for bidirectional interactions between an ATL-based bilateral semantic ‘hub’ and representations supported by occipital and middle/posterior temporal regions (Patterson et al, 2007; Lambon Ralph et al, 2017; Chen et al, 2017). In addition to demonstrations of altered ILF microstructure in semantic dementia (Agosta et al, 2010), recent studies report associations between ILF microstructure and individual differences in semantic learning (Ripollés et al, 2017) and memory (Hodgetts et al, 2017).…”

“…Here, we used multi-shell diffusion MRI and spherical deconvolution tractography to investigate whether individual differences in ILF and fornix microstructure would be associated with individual differences in trait curiosity. Given the importance of ILF to semantic cognition (Hodgetts et al, 2017; Ripollés et al, 2017; Herbet et al, 2018), we predicted an association between ILF microstructure and EC but not PC. In contrast, given that hippocampal circuitry supports novelty detection, exploratory behaviour and information seeking in many domains (O’Keefe and Nadel, 1978; Kumaran and Maguire, 2009; Murray et al, 2017; Voss et al, 2017) we predicted an association between fornix microstructure and both EC and PC.…”

“…Studies investigating higher-order personality traits subsuming curiosity, however, provide a fruitful starting point to investigate the neuroanatomical connections underlying trait curiosity (DeYoung, 2014; Woo et al, 2014). For example, Privado et al (2017) found an association between ‘openness to experience’ and microstructure of the inferior longitudinal fasciculus (ILF), a ventral, temporo-occipital association tract implicated in semantic memory (Herbet et al, 2018; Hodgetts et al, 2015, 2017). Additionally, Cohen et al (2009) showed that individual differences in novelty seeking were associated with microstructure of the fornix, a key pathway that connects the hippocampus - involved in novelty detection, exploration, information seeking and episodic memory (O’Keefe and Nadel, 1978; Kumaran and Maguire, 2009; Murray et al, 2017; Voss et al, 2017) - to the thalamus, ventral striatum, amygdala and prefrontal cortex (Saunders and Aggleton, 2007; Aggleton et al 2015).…”