Brain Connectivity: A Bidirectional Involvement of Structural Connectivity and Pathological Substrates in Neurodegeneration

It is now well established that structural and functional connectivity could be significantly influenced by different pathological substrates such as amyloid deposition, tau deposition, microglial activation, synuclein pathology, astrocyte activation, mitochondrial function, and other changes occurring in the brain in different neurological disorders. It is also significantly influenced by genetic predisposition and epigenetic influence.

The role of Brain Connectivity in evaluating neuronal integrity and its relationship with pathological substrates and genetic and epigenetic influence is going to be crucial and influential in the field of neuroscience and medicine over the coming years. Because impairment of structural and functional connectivity, either as a primary or secondary event, is implicated in neuronal damage in most brain disorders, Brain Connectivity plays a major role in research into normal brain function and a range of neurological disorders. As the field of neuroscience is constantly evolving, with multimodal imaging now considered as the preferable method of evaluating different diseases and interventions, we have expanded the breadth of research published in Brain Connectivity to ensure that we are able to include articles of a translational nature in the field of neuroscience.

With the intention of expanding the scope of our journal, I would like to invite the readers of Brain Connectivity to submit original articles and reviews describing

▪ Advances in neuroimaging using positron emission tomography (PET) and magnetic resonance imaging (MRI) in Alzheimer's disease, Parkinson's disease, and other neurodegenerative diseases

▪ Novel PET and MRI biomarkers in neurodegenerative diseases and stroke

▪ How different pathological substrates and genetic and epigenetic factors influence structural and functional connectivity in brain disorders

▪ Multimodal imaging in brain disorders in both human subjects and animal models

▪ Experimental techniques combining MRI (connectivity), electroencephalography (EEG), magnetoencephalography (MEG), PET, single photon emission computed tomography (SPECT), and other new and evolving methods

For more information about the journal, including scope and instructions for authors, please visit our website (https://mc.manuscriptcentral.com/brainconnectivity).

In this current issue, you will find the following high-quality articles by experts in their fields:

Theta Oscillations Through Hippocampal/Prefrontal Pathway: Importance in Cognitive Performances. In this review, Hamid Zangbar, Parviz Shahabi, and colleagues review experimental studies in the which hippocampal–prefrontal cortex axis was investigated using either electrophysiological assessments in rodent or integrated diffusion-weighted imaging and electroencephalography (EEG) in human subjects under memory-based tasks.

Among different hippocampal rhythms including sharp wave-ripples, gamma, and theta, theta rhythm is crucial for cognitive processing, particularly learning and memory. Theta oscillations are observable in both humans and rodents during spatial navigations. While hippocampus is well known as the generator of current rhythm, other brain areas, such as prefrontal cortex, act as core structures for the execution of diverse higher cortical functions. Prefrontal cortex is connected to the hippocampus through the hippocampal–prefrontal pathway; hereby, theta oscillations convey hippocampal inputs to the prefrontal cortex and simultaneously synchronize the activity of these two regions during memory, learning, and other cognitive tasks. This review gives a comprehensive overview of the hippocampal–prefrontal pathway in cognitive performance.

Discordant Alpha-Band Transcranial Alternating Current Stimulation Affects Cortico–Cortical and Cortico–Cerebellar Connectivity. Synchronization of oscillatory brain activity is believed to play a critical role in linking distributed neuronal populations into transient functional networks. In this study, Claudia Tesche and Jon Houck from University of New Mexico investigated alternating current stimulation over bilateral parietal cortex in a double-blind, sham-controlled study to test the notion that widespread alpha mediates causal relationships in the gamma band, both within local neuronal populations and also across distant brain regions.

Optimizing Connectivity-Driven Brain Parcellation Using Ensemble Clustering. In this study, Anvar Kurmukov and Boris Gutman address the problem of constructing a unified, topologically optimal, connectivity-based brain atlas. The proposed approach aggregates an ensemble partition from individual parcellations without label agreement, providing a balance between sufficiently flexible individual parcellations and intuitive representation of the average topological structure of the connectome. Their methods exploit a previously proposed dense connectivity representation, first performing graph-based hierarchical parcellation of individual brains and subsequently aggregating the individual parcellations into a consensus parcellation. They conclude that ensemble parcellation was found to be highly stable with respect to subject sampling, outperforming anatomical atlases and other connectome-based parcellations in classification as well as preserving global connectome properties. The hard ensemble-based parcellation also showed a degree symmetry comparable to anatomical atlases and a high degree of spatial contiguity without using explicit priors.

Cognition and Learning in Decision-Making as Compensatory Mechanism for Emotional Processing Deficit. The functional roles of ventromedial prefrontal cortex and amygdala in affecting emotional processing in decision-making have been raised in support of the somatic marker hypothesis. However, latter studies demonstrated challenges to such support based on preserved cognition in the form of reversal learning in ventromedial prefrontal cortex damaged patients tested with a shuffled variant of Iowa Gambling Task. This finding provides implications for cognitive neglect in somatic marker hypothesis with its magnified emphasis on the link between somatic markers and emotion-guided decision-making. It also suggests that cognition could compensate for emotion impairment in the absence of crucial prefrontal cortical region needed for low-risk choice and decision making. Rowena Kong makes a critical evaluation of these observations in this study.

While COVID-19 has changed our lives beyond recognition (hopefully temporarily), it is good to see that the research community is coming together in these difficult times, and finding innovative ways of remote working. The Brain Connectivity team would like to thank everyone working so hard to advance research by balancing personal and professional life.

Finally, I would like to thank all the researchers and all the staff at Mary Ann Liebert, Inc., publishers as well as editors and reviewers of Brain Connectivity working during this extremely difficult time of the COVID-19 pandemic to advance research in every corner of the world to improve our lives.

I sincerely hope that all our contributors and readers of the journal are remaining in good health.