Special issue: Editorial

What is a disconnection syndrome?

The anterior commissure (AC) is one of the main commissural fibers of the brain. The commissural fibers are involved in bilateral integration and coordination of any normal brain activity. The AC is an important interhemispheric structure that forms a bidirectional communication channel between the frontal, temporal, parietal, and occipital lobes bilaterally.

In the present study, we focused on describing the morphology, relationships, and distribution of the AC using diffusion spectrum imaging-based fiber tracking. The tractograms were compared with the findings from gross anatomical dissection of the AC of adult brains.

Our findings suggest that the AC found using tracking methods is larger than that found by dissection.

The use of tractography added extensions to the main AC structure.

Patients with temporal lobe epilepsy (TLE) exhibit both volumetric and structural connectivity abnormalities relative to healthy controls. How these abnormalities inter-relate and their mechanisms are unclear. We computed grey matter volumetric changes and white matter structural connectivity abnormalities in 144 patients with unilateral TLE and 96 healthy controls. Regional volumes were calculated using T1-weighted MRI, while structural connectivity was derived using white matter fibre tractography from diffusion-weighted MRI. For each regional volume and each connection strength, we calculated the effect size between patient and control groups in a group-level analysis. We then applied hierarchical regression to investigate the relationship between volumetric and structural connectivity abnormalities in individuals. Additionally, we quantified whether abnormalities co-localised within individual patients by computing Dice similarity scores. In TLE, white matter connectivity abnormalities were greater when joining two grey matter regions with abnormal volumes. Similarly, grey matter volumetric abnormalities were greater when joined by abnormal white matter connections. The extent of volumetric and connectivity abnormalities related to epilepsy duration, but co-localisation did not. Co-localisation was primarily driven by neighbouring abnormalities in the ipsilateral hemisphere. Overall, volumetric and structural connectivity abnormalities were related in TLE. Our results suggest that shared mechanisms may underlie changes in both volume and connectivity alterations in patients with TLE.

The development of scanners with ultra-high gradient strength, spearheaded by the Human Connectome Project, has led to dramatic improvements in the spatial, angular, and diffusion resolution that is feasible for in vivo diffusion MRI acquisitions. The improved quality of the data can be exploited to achieve higher accuracy in the inference of both microstructural and macrostructural anatomy. However, such high-quality data can only be acquired on a handful of Connectom MRI scanners worldwide, while remaining prohibitive in clinical settings because of the constraints imposed by hardware and scanning time. In this study, we first update the classical protocols for tractography-based, manual annotation of major white-matter pathways, to adapt them to the much greater volume and variability of the streamlines that can be produced from today's state-of-the-art diffusion MRI data. We then use these protocols to annotate 42 major pathways manually in data from a Connectom scanner. Finally, we show that, when we use these manually annotated pathways as training data for global probabilistic tractography with anatomical neighborhood priors, we can perform highly accurate, automated reconstruction of the same pathways in much lower-quality, more widely available diffusion MRI data. The outcomes of this work include both a new, comprehensive atlas of WM pathways from Connectom data, and an updated version of our tractography toolbox, TRActs Constrained by UnderLying Anatomy (TRACULA), which is trained on data from this atlas. Both the atlas and TRACULA are distributed publicly as part of FreeSurfer. We present the first comprehensive comparison of TRACULA to the more conventional, multi-region-of-interest approach to automated tractography, and the first demonstration of training TRACULA on high-quality, Connectom data to benefit studies that use more modest acquisition protocols.

The ability to string together words into a structured arrangement capable of conveying nuanced information is key to speech production. The assessment of the neural bases for structuring sentences has been challenged by the need of experts to delineate the aberrant morphosyntactic structures in aphasic speech. Most studies have relied on focused tasks with limited ecological validity. We characterized syntactic complexity during connected speech produced by patients with chronic post-stroke aphasia. We automated this process by employing Natural Language Processing (NLP). We conducted voxel-based and connectome-based lesion-symptom mapping to identify brain regions crucially associated with sentence production and syntactic complexity. Posterior-inferior aspects of left frontal and parietal lobes, as well as white matter tracts connecting these areas, were essential for syntactic complexity, particularly the posterior inferior frontal gyrus. These findings suggest that sentence structuring during word production depends on the integrity of Broca’s area and the dorsal stream of language processing.

Neurosurgeons should know the anatomy required for safe temporal lobe surgery approaches. The present study aimed to determine the angles and distances necessary to reach the temporal stem and temporal horn in surgical approaches for safe temporal lobe surgery by using a 3.0 T magnetic resonance imaging technique in post-mortem human brain hemispheres fixed by the Klingler method. In our study, 10 post-mortem human brain hemisphere specimens were fixed according to the Klingler method. Magnetic resonance images were obtained using a 3.0 T magnetic resonance imaging scanner after fixation. Surgical measurements were conducted for the temporal stem and temporal horn by magnetic resonance imaging, and dissection was then performed under a surgical microscope for the temporal stem. Each stage of dissection was achieved in high-quality three-dimensional images. The angles and distances to reach the temporal stem and temporal horn were measured in transcortical T1, trans-sulcal T1–2, transcortical T2, trans-sulcal T2–3, transcortical T3, and subtemporal trans–collateral sulcus approaches. The safe maximum posterior entry point for anterior temporal lobectomy was measured as 47.16 ± 5.00 mm. Major white-matter fibers in this region and their relations with each other are shown. The distances to the temporal stem and temporal horn, which are important in temporal lobe surgical interventions, were measured radiologically, and safe borders were determined. Surgical strategy and preoperative planning should consider the relationship of the lesion and white-matter pathways.

A critical concept in neurology is cortical disconnection, in which seemingly normal gray matter can have reduced function due to loss of white matter (WM) connections. White matter damage has been extensively described in temporal lobe epilepsy (TLE), but the anatomical distribution of cortical disconnection in TLE is not fully characterized. Here, we studied 221 participants (64 left-TLE, 55 right-TLE, 102 controls) from three different epilepsy treatment centers. We employed a group connectometry diffusion imaging tractography approach to identify WM fibers with reduced integrity in TLE. We then assessed the anatomical distribution of the gray matter endpoint projections of abnormal fibers to map the anatomical pattern of disconnections. As expected, left- and right-TLE were associated with multiple WM pathways with reduced integrity, which were associated with extensive cortical disconnection involving predominantly limbic structures. Controlling for medial temporal gray matter atrophy, cortical disconnection of the left cingulum and the thalamus as well as disconnection of the bilateral putamen and the amygdala was associated with lower verbal memory immediate recall.

In conclusion, our results support that cortical disconnection is an underappreciated but pervasive phenomenon in TLE, and cortical disconnection of limbic structures beyond the medial temporal regions is related to verbal memory performance.