This study aimed to investigate language-related WM structural connectivity alterations in nvASD individuals compared to matched verbal ASD (vASD) and typical development (TD) individuals. Manual DWI deterministic tractography was used for reconstruction of the main WM fiber tracks associated to language processing and measured by computing fractional anisotropy (FA) and volume measures. The two main findings are, firstly, a main effect of group consisting in a reduction in FA in the IFOF in nvASD relative to the TD group, and secondly, a significant interaction of hemisphere and group in the UF, which showed reduced volume in the left hemisphere when compared to the right only in the vASD group.
The reduction of FA in the IFOF in nvASD compared to TD individuals is a new finding. Although the exact involvement of the IFOF in language functions is still unclear, previous reports have demonstrated its role in reading, writing and attention (Catani and Thiebaut de Schotten 2008; Dorrichi et al. 2008), but it has been first and foremost considered as a crucial pathway subserving semantic processing (Fekonja et al. 2019; Catani and Thiebaut de Schotten 2008; Dick et al. 2014). In line with this, several lesion and tumor studies using electric stimulation have shown the relationship between IFOF integrity and proficiency in a semantic matching task (Sierpwoska et al. 2019), a verbal fluency task (Almairac et al. 2015), and the number of semantic paraphasias (Duffau et al. 2005; Sierpwoska et al. 2019), but not for semantic learning (Ripolles et al. 2017). In that sense, the anatomical course and terminations of the IFOF can also be of great value to understand its contribution in language processing.
Recently, both DTI and anatomical dissection studies have described the main course of the IFOF at the level of the insula and the temporal lobe (Martino et al. 2010; Catani and Thiebaut de Schotten, 2008), but more debate has been generated with respect to its anterior and posterior terminations. Sarubo and colleagues (2013) attempted to describe the frontal terminations of the IFOF by combining anatomical dissections and DWI. The authors proposed a division of the tract in two major components: a superficial one, terminating in the inferior frontal gyrus (IFG) and a deeper one, connecting with the middle frontal gyrus (MFG), dorso-lateral prefrontal cortex (DLPFC), the orbitofrontal cortex and the frontal pole. Similarly, Wu and colleagues (2016) used high resolution diffusion tensor tractography to identify five subcomponents of the IFOF based on its frontal terminations (which overlapped greatly with those described by Sarubo and colleagues, 2013). These results would support the idea of the IFOF as a “multi-function” tract, with a clear involvement in language processing due to its role in conveying information to crucial language-related regions and nearby ones (IFG, MFG, DLPFC and orbitofrontal cortex). In most cases, these are associated to semantic processing functions (Binder et al. 2009; Plaza et al. 2008). Similarly, Martino and colleagues (2010) used post-mortem anatomical dissections to investigate and describe the posterior terminations of this tract. In this case, the authors also suggested the division of the IFOF into a superficial and a deeper component based on the posterior terminations. The former would project to the superior parietal lobe and posterior parts of the superior and middle occipital gyrus, whereas the latter would be associated with terminations in the inferior occipital gyrus and the posterior temporo-basal area. Again, the terminations of the IFOF in the associative extra-striate cortex and posterior temporo-basal area would further support the involvement of this tract in semantic functions (Martino et al. 2010, Price 2000, Vilha et al. 2004).
Despite this evidence, no study until now has attempted to elucidate the role of this pathway in a disorder with a clear semantic impairment such as individuals with nvASD. In standardized settings, language comprehension measures in this group have yielded scores far below those expected by individuals’ CA (Chenausky et al. 2019; DiStefano et al. 2016; Garrido et al. 2015; Slusna et al. 2021), and caregiver reports consistently document a lack of understanding or following of complex linguistic constructions (e.g., three-step instructions) in individuals with nvASD (Skwerer et al. 2016). Although children with nvASD show variation in how many single words they produce, there is evidence that those words are not semantically understood as carrying referential meaning (Preissler 2008), unlike what is seen already even in very young neurotypical infants (Marno et al. 2015). In line with this, experimental assessments using EEG have uncovered anomalous patterns of lexico-semantic neural processing in a mixed group of nonverbal and preverbal children with ASD (Cantiani et al. 2016), effectively pointing to an aberrant rather than delayed language processing in line with the neural patterns observed here. Although lexical semantic anomalies are seen throughout ASD (Tek et al. 2008; Arunachalam and Luyster 2016), these certainly do not reach the level of the essential absence of neurotypical word use in nvASD, suggesting that ventral structural alterations of the IFOF may indeed be unique to nvASD.
Although it was not the original focus of this investigation, anomalies in the ventral language route were also found here for the vASD group. Specifically, higher volume of the UF on the right compared to the left hemisphere was observed in this group, a result that converges with previous findings in both children and adults with vASD (Samson et al. 2016; Li et al. 2019; Catani et al. 2016). Some of this previous work proposed that the maldevelopment of the UF, a tract connecting the lateral orbitofrontal cortex and Brodmann area 10 with the anterior temporal lobe (Von der Heide et al. 2013), is a potential neural substrate for the socio-affective deficits observed in this group (Samson et al. 2016; Li et al. 2019). Our vASD and nvASD individuals, however, shared a diagnosis and were selected so as to differ in language, not in socio-affective deficits. Further work is therefore required to corroborate what functions the UF supports. Given anomalies relating to the ventral route of language processing found in both ASD groups in our study, our results are consistent with a more localized ventral impact in vASD, as reflected by macrostructural alterations in a short and restricted associative bundle such as the UF, while nvASD shows a more global effect underpinned by a microstructural anomaly in the IFOF, a massive tract crossing the entire brain ventrally. Furthermore, as neural profiles between nvASD and vASD diverge, it is possible that nvASD should not be viewed as continuous with vASD, but as a relatively separate group within the autism spectrum, with distinct structural correlates.
In this study we capitalized on manual dissection, despite it being labor-intensive and making larger samples difficult. This method was selected as it allowed a more suitable neuroanatomic approach for the research question of this study. First, manual dissections make the tract reconstruction adaptable to individual differences, which in the present case of developing brains (children and adolescents) is crucial, since most automatic dissection tools are based on adult anatomical landmarks / atlases. Second, we wanted to combine different authors’ proposals for dissecting the IFOF, a complex tract for which both anterior and posterior terminations are highly controversial. Despite the multiple possible frontal terminations discussed for this tract, all the streamlines are compacted when passing through the external/extreme capsule, so a first region of interest placed in this bottleneck should include all of the tract’s fibers, as suggested by Catani and Thiebaut de Schotten (2008). However, the posterior ROI proposed by these authors is a lot more restrictive as it does not encompass some of the parietal and superior occipital terminations observed postmortem by other authors, such as Martino and colleagues (2010). Hence, we opted for a more inclusive ROI in the middle temporal gyrus, anterior to the radiation of the corpus callosum (Fekonja et al. 2019), comprising all the fibers coming from the temporal isthmus before they spread into their final cortical destination. The aim of this approach was to be as comprehensive as possible when selecting fibers, to ensure a complete and anatomically reliable characterization of the structural connectivity of this tract, which seems to be crucial for the understanding of this disorder. Nonetheless, very similar results were obtained when using the two ROIs proposed by Catani and Thiebaut de Schotten (2008) for the dissection of the IFOF as compared to the more comprehensive approach (see Online Resource 1).
Unlike in the case of our predictions for the ventral language pathway, our findings did not confirm our predictions based on previous literature in nvASD for structural alterations of the dorsal language pathway (Wan et al. 2012). These predictions were based on the study by Wan and colleagues (2012), who compared volume lateralization of the Arcuate Fasciculus between five completely non-verbal ASD and five TD children. Their results showed a rightward laterality (instead of the typical leftward asymmetry) in nvASD, which the authors argue could be critical for the language deficits observed in this group. Several factors could explain the divergence between theirs and our results: a difference in the selection of the tractography method (probabilistic vs. deterministic) or sample size (five vs nine participants per group), or even the inclusion criteria applied (completely vs minimally verbal ASD children). While not ruling out dorsal route involvement, our results do not support that the severe language problems in nvASD can be due only to problems of sensory-motor integration related to the AF and the dorsal processing route. Instead, they point to a deficit involving anomalous comprehension and semantic language processing.
Limitations of the current study include different scanning sites and protocols, although generalized scanner artifacts seem unlikely given the specificity of the patterns observed. Also, the fact that dissections were performed in native space for every participant, extracting individual values from selected tracts, implies less methodological issues than voxel-based techniques performed at a group-level. Another limitation may be the reduced sample size, which prevents us from extracting definitive conclusions from our results. Although limited, the sample used in this study is similar to the ones recruited in previous studies on nvASD, which makes evident the difficulty of scanning and working in the lab with this population, therefore supporting the value of the present results. Finally, as previously discussed, manual dissection was used for this study, but future work should try to expand the sample size and complement the analyses with other tractography methods like TRACULA (a global probabilistic approach - Yendiki et al. 2011), AFQ (an automated deterministic method - Yeatman et al. 2012), or tract-based spatial statistics (TBSS, to compare at group and voxel-based-like levels - Smith et al. 2006). This would help to better understand the neurobiological basis of this extreme side of the ASD spectrum, from which we know so little in terms of structural neural underpinnings despite its prevalence.