The Inferior Frontal Occipital Fasciculus (IFOF) is a major anterior-to-posterior white matter pathway in the ventral human brain. It connects occipital, parietal and posterior temporal regions, implicated in word and object recognition, to prefrontal areas associated with cognitive control, semantic retrieval and speech production (Caverzasi et al., 2014; Duffau et al., 2013; Martino et al., 2010). In line with this functional anatomy, IFOF has long been associated with language and semantic cognition (Almairac et al., 2015; Binder et al., 2009; Han et al., 2013; Sierpowska et al., 2019). Stimulation of IFOF during neurosurgery elicits semantic errors in picture naming tasks (Duffau et al., 2008, 2005), while damage to IFOF is associated with semantic impairment (Han et al., 2013; Souter et al., 2022; Surbeck et al., 2020). Similarly, variation in the strength of IFOF in healthy participants is associated with performance on lexical-orthographic tasks (Vandermosten et al., 2012) and bilingual aspects of language (Mohades et al., 2012). Nevertheless, there are many unresolved questions about the contribution of IFOF to cognition, including whether this tract supports both conceptual and visually-guided (non-conceptual) aspects of cognition, whether there are differences between semantic tasks employing pictures and words, and whether there are functional distinctions across subdivisions of IFOF.
Contemporary theoretical accounts emphasize that semantic cognition and language emerge from interacting component processes, which have dissociable neurocognitive bases (Lambon Ralph et al., 2017) and which might draw on different white matter tracts (Duffau et al., 2009, 2013; Moritz-Gasser et al., 2013; Rauschecker, 2012). For example, a dorsal language pathway is thought to support acoustic-motor mappings and articulatory sequences, while a ventral language pathway supports semantic processing (Saur et al., 2008): by this account, conceptual retrieval draws on the interaction of heteromodal conceptual representations in an anterior temporal ‘hub’ with unimodal ‘spoke’ representations in visual and auditory cortex (Patterson et al., 2007). The inferior longitudinal fasciculus (ILF) links occipital cortex to anterior temporal regions and is consequently well-placed to support rapid and automatic access to heteromodal conceptual knowledge from visual inputs (Duffau et al., 2013; Herbet et al., 2018; Saur et al., 2008; Turken and Dronkers, 2011). However, this pathway is thought to be insufficient for semantic cognition since we need to retrieve conceptual information in a flexible fashion to generate adaptive thoughts and behaviour and this is thought to involve the interaction of semantic knowledge with control processes. Recent studies have suggested that a distributed network including posterior middle temporal gyrus and anterior inferior frontal cortex is important for our capacity to focus on semantic information that is relevant to the evolving context or our current goals (Davey et al., 2016; Gao et al., 2021; Jackson, 2021). This ‘semantic control network’ shows the strongest recruitment in fMRI studies when non-dominant aspects of knowledge are needed, or when there is ambiguity or conflict between concepts (Badre et al., 2005; Jackson, 2021; Noonan et al., 2013; Thompson-Schill et al., 1997). Since IFOF connects posterior temporal to prefrontal regions, it is likely to play a critical role in the controlled application of knowledge during semantic cognition (Duffau et al., 2013; Giampiccolo and Duffau, 2022; Nugiel et al., 2016). Patients with poor control of semantic cognition in the context of stroke aphasia have highly consistent damage to IFOF and ILF despite highly variable lesions affecting left inferior prefrontal or posterior temporal areas (Souter et al., 2022). Moreover, for phonological and semantic verbal fluency tasks with high executive demands, IFOF integrity is correlated with performance (Almairac et al., 2015; Nugiel et al., 2016).
IFOF also shows broader connectivity beyond semantic control regions, connecting occipital and parietal regions to broad swathes of lateral, anterior and ventral frontal regions (Catani and Thiebaut de Schotten, 2008; Duffau et al., 2013) linked to cognitive control, decision-making and speech production. Consequently, IFOF is likely to be important for controlled visually-guided cognition beyond the semantic domain; for example, Walsh et al. (2011) found that reduced microstructural integrity of the IFOF is associated with poor object working memory performance. Given this connectivity with visual cortex, it is also important to establish if the functional role of IFOF varies across semantic tasks involving pictures and words. If IFOF is critical for linking visual input regions to prefrontal cortex, this tract might play a greater role in semantic tasks that utilize picture inputs, such as picture naming and picture association. Alternatively, IFOF may support connections critical to the putative heteromodal semantic control network, linking posterior middle temporal gyrus to anterior portions of inferior frontal cortex: both of these regions respond to control demands across word and picture semantic tasks (Krieger-Redwood et al., 2015).
Research has also revealed dorsal and ventral subdivisions within IFOF, which may have different functional associations (Martino et al., 2010; Rollans and Cummine, 2018; Roux et al., 2021; Sarubbo et al., 2013). Martino et al. (2010) found the superficial dorsal subcomponent of IFOF connects superior occipital regions associated with visually-guided action, plus posterior superior temporal and parietal regions, to posterior parts of inferior frontal gyrus: this pathway is potentially suited to the control of action and visual-spatial processing. In contrast, a deep ventral pathway was found to connect inferior occipital and ventral temporal regions implicated in object recognition, plus heteromodal posterior middle temporal gyrus, to diverse frontal lobe regions, including anterior and ventral regions within the default mode network and dorsolateral prefrontal regions associated with cognitive control: this pathway is therefore suited to supporting visually guided decision-making and heteromodal semantic control. Sarubbo et al. (2013) corroborated this division of the IFOF into dorsal and ventral components, using a combination of the Klingler method and single-subject DTI, showing that the dorsal aspect terminates in the posterior inferior frontal gyrus, while the ventral component can be further divided in three minor bundles terminating in middle frontal gyrus and dorsolateral prefrontal cortex, lateral orbitofrontal cortex, and frontal pole. Almairac et al. (2015) proposed that the ventral pathway might be important for semantic access, while the dorsal pathway supports phonology; the dorsal route is also implicated in reading and writing (Motomura et al., 2014). However, there are alternative functional interpretations. Rollans and Cummine (2018) found that Fractional Anisotropy (FA) in dorsal and ventral IFOF was linked to Go/No-Go and naming tasks respectively, suggesting the dorsal pathway may be crucial for controlled action driven by visual input beyond the domain of language or semantic cognition, while the ventral pathway supports more abstract conceptual behaviour.
Finally, given that language and semantic control are thought to be highly left-lateralised (Frost et al., 1999; Gonzalez Alam et al., 2019; Josse and Tzourio-Mazoyer Nathalie, 2004), while visually-guided non-semantic cognition and domain-general executive processes are more bilateral (Bartolomeo and Seidel Malkinson, 2019; Duncan, 2010; Hellige and Michimata, 1989; Hugdahl et al., 2015), there may be important functional dissociations between left and right IFOF. Using tract-based spatial statistics (TBSS), Rollans et al. (2017) found that IFOF was associated with gross picture naming differences in the left hemisphere, but more subtle differences in naming performance in the right hemisphere. Herbet et al. (2017) showed that damage to the IFOF in the right hemisphere is associated with poorer performance in non-verbal semantic processing. Moreover, in a meta-analysis performed by Vigneau et al. (2011), the cognitive load of semantic tasks determined the contribution of right IFOF: when the task involved working memory to manipulate verbal content, or the capacity to switch between categories in a verbal fluency test, the association with right IFOF was stronger.
In the current study, we investigated these diverse hypotheses about the functional relevance of subdivisions of left and right IFOF using a semantic Go/No-Go task. In semantic conditions, decisions to produce or withhold a prepotent button press response were made on the basis of conceptual content (whether a visually-presented stimulus was an animal or a man-made object), while in non-semantic conditions, these decisions were based on the shape of the box enclosing scrambled images, such that semantic access was not necessary for the task. The semantic conditions also compared written words and pictures, to allow us to consider the extent to which effects related to language, transmodal aspects of semantic cognition or visual semantic processes necessary only for pictorial tasks. Semantic Go/No-go efficiency should show an association with tracts that link visual inputs to conceptual regions needed to select an appropriate response on the basis of meaning, particularly for verbal trials, since written word inputs do not contain any superficial cues to the behaviorally-relevant categorical distinction, unlike pictures. In contrast, behavioural efficiency for both semantic and non-semantic Go/No-Go tasks is expected to relate to tracts that connect visual regions to dorsal parts of prefrontal cortex supporting action selection and inhibition. In summary, given the distinct anatomy of dorsal and ventral IFOF, we expected dorsal IFOF to be relevant to visually-guided Go/No-Go behaviour across semantic and non-semantic domains, while ventral IFOF was expected to be associated with controlled semantic behaviour.