The WM anatomy of the temporal, parietal, occipital lobe (Burks et al. 2017; Kadri et al. 2017; Ripollés et al. 2017; Saalasti et al. 2019; Jitsuishi and Yamaguchi 2020; Briggs et al. 2021; Egemen et al. 2021) and junction (Biceroglu et al. 2019) has been extensively studied in the literature. Previously, Yagmurlu et al. (2015) divided AG as upper and lower parts with evaluating only the SLF II and AF. In our study, the AG was divided into two parts (superior and inferior), and the WM fibers associated with its parts were compared. Consequently, SLF II and MdLF are associated with sAG; the AC, OR with iAG; the AF, IFOF, and Tp with both parts. To the best of our knowledge, this is the first anatomical study dividing the AG into two parts to evaluate all fiber tracts according to these parts.
A functional MRI study (Seghier et al. 2010) divided left AG into three parts: mid, dorsomedial, and ventrolateral regions. It has been stated that regardless of the presence of the stimulus, the mid-region is involved in semantic associations, the dorsomedial region is in search of semantics in all visual stimulations, and the ventrolateral region is in the conceptual identification of visual inputs. According to the anatomical classification in our study, approximately the dorsomedial region corresponds to sAG and the ventrolateral and mid-region to iAG. A tractography-based parcellation study divided IPL into five clusters (Mars et al. 2011). The activity was recorded when people redirected visuospatial attention from one location to another in tractography corresponding to the anterior part of AG and when people successfully retrieved memories in the posterior part of AG. The classification in this study was not correlated with the classification in our study. In another study (Wang et al. 2017), AG was parcelled as dorsal and ventral parts. The dorsal part had associations with the frontal pole, superior frontal gyrus, MFG, pre-supplementary motor area, middle MTG, and dorsal lateral prefrontal cortex (DLPFC). The ventral part had associations with DLPFC, anterior MTG, precuneus, medial prefrontal cortex, anterior and posterior parahippocampus. In the tractography-based study conducted by Wang et al. (2012), IPL was divided into 5 clusters, but the cluster containing AG was also included the ventral premotor cortex and IFG. All these studies in the literature evaluated AG as functional, but our study is the first to classify AG according to cortical anatomy and determine the WM connections of sAG and IAG in detail.
There is no agreement in the terminology of SLF. The frontoparietal network was first classified as SLF I, II, and III by Petrides and Pandya (1984). There are publications stating that SLF is a single entity (Christiaens et al. 2015; Lerma-Usabiaga et al. 2019), or that questioning the existence of SLF I (Wang et al. 2016), or that SLF II and III are entirely independent entities from SLF I (Komaitis et al. 2019). Furthermore, the clear distinction between SLF and AF has not been resolved yet (Schurr et al. 2020). Besides, Nakajima et al. (2019) proposed classification as ventral SLF (SLF III, AF anterior), dorsal SLF (SLF II), posterior SLF (SLF-temporoparietal segment and AF posterior), and AF (AF long fibers). In a recent study (Schurr et al. 2020), SLF II and III boundaries were tried to be separated in tractography using three different datasets. They focused on SLF III and pointed that the SLF III projects to AG, especially in the right hemisphere. They didn't divide AG into parts; however, the SLF III covers only the sAG on their tractographic demonstrations. In contrast, we didn't notice any connections between SLF III and the AG based on our dissections. The classically defined SLF II relationship with sAG and iAG has not been noted so far. Recent publications (Nakajima et al. 2019; Schurr et al. 2020) and our findings reveal that SLF II is associated with sAG. Because of SLF II affects oculomotor function and spatial aspects, Gerstmann syndrome and Spatial hemineglect are observed in its disconnection (Chechlacz et al. 2013; Yagmurlu et al. 2015). (Table 2) The AF connects STG and MTG to MFG and IFG and therefore is in direct contact with both parts of AG.
While fiber dissection studies have shown that the parietal connections of the MdLF are almost completely terminated at the SPL (Maldonado et al. 2013; Wang et al. 2013), DTI studies have also shown connections to the IPL (Menjot de Champfleur et al. 2013, Makris et al. 2017). In current studies, MdLF has been divided into many different functional units, but the AG was considered a single unit (Latini et al. 2021; Makris et al. 2017; Kalyvas et al. 2020). A recent study (Latini et al. 2021) divided the MdLF into anterior (deeper brunch) and posterior (superficial branch) parts. Anterior MdLF connects anterior STG and planum polare. Posterior MdLF connects posterior STG, anterior transverse temporal gyrus, planum temporale, and the parieto-occipital region. For the first time in the literature, it has been described that when the AG is divided into two parts, the MdLF fibers progress at the level of the superior AG and form the roof of the iAG. MdLF takes part in processing the spatial features of sounds, but the disconnection syndrome is unknown (Wang et al. 2013; Güngör et al. 2017). (Table 2)
The IFOF originates in parietal and occipital lobes and terminates in the inferior frontal lobe (Motomura et al. 2014). Adjacent to the inferolateral insula, the IFOF courses along with the UF (Kier et al. 2004). Based on its relationship with the central insular sulcus, the IFOF is divided into two parts: anterior and posterior (Güngör et al. 2017). To reach the occipital lobe, the anterior part runs lateral to the inferior of frontal horn, and the posterior part runs lateral to the temporal horn and inferior to the atrium. The anterior and posterior parts run roughly deep into the sAG and iAG. The IFOF takes part in lexical-semantic and visual spatial processing. The disconnection of IFOF fibers include semantic paraphasias and nonword reading (Duffau et al. 2005; Rollans et al. 2017). (Table 2)
The OR fibers lie deep to the AC and SS fibers and courses lateral to the temporal horn and inferior two-thirds of the atrium. The OR fibers will be at the iAG level when evaluated according to the cortical surface. The Tp, consisting of splenial callosal fibers surrounding the lateral wall and roof of the atrium, and occipital and temporal horns, is located between the ventricular ependyma and the OR fibers (Yakar et al. 2018). Tp fibers run deep in both parts of the AG. The functions and disconnection syndromes of all relevant fibers to AG are summarized in Table 2.
In conclusion, AG was divided into sAG and iAG based on STS in this study. Although it has been shown in previous studies that there are functionally different subunits with AG parcellation, here, for the first time, different functions of the subunits have been revealed with cadaveric dissection and tractography images. Therefore, in surgical interventions for AG, this parcellation of AG will be more efficacious in predicting related complications.