Depressive Rumination, the Default-Mode Network, and the Dark Matter of Clinical Neuroscience

The intuitive association between self-focused rumination in major depressive disorder (MDD) and the self-referential operations performed by the brain's default-mode network (DMN) has prompted interest in examining the role of the DMN in MDD. In this article, we present meta-analytic findings showing reliably increased functional connectivity between the DMN and subgenual prefrontal cortex (sgPFC)-connectivity that often predicts levels of depressive rumination. We also present meta-analytic findings that, while there is reliably increased regional cerebral blood flow in sgPFC in MDD, no such abnormality has been reliably observed in nodes of the DMN. We then detail a model that integrates the body of research presented. In this model, we propose that increased functional connectivity between sgPFC and the DMN in MDD represents an integration of the self-referential processes supported by the DMN with the affectively laden, behavioral withdrawal processes associated with sgPFC-an integration that produces a functional neural ensemble well suited for depressive rumination and that, in MDD, abnormally taxes only sgPFC and not the DMN. This synthesis explains a broad array of existing data concerning the neural substrates of depressive rumination and provides an explicit account of functional abnormalities in sgPFC in MDD.

Resting state networks in temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is typically described as a neurologic disorder affecting a cerebral network comprising the hippocampus proper and several anatomically related extrahippocampal regions. A new level of complexity was recently added to the study of this disorder by the evidence that TLE also appears to chronically alter the activity of several brain-wide neural networks involved in the control of higher order brain functions and not traditionally linked to epilepsy. Recently developed brain imaging techniques such as functional magnetic resonance imaging (fMRI) analysis of resting state connectivity, have greatly contributed to these observations by allowing the precise characterization of several brain networks with distinct functional signatures in the resting brain, and therefore also known as "resting state networks." These significant advances in imaging represent an opportunity to investigate the still elusive origins of the disabling cognitive and psychiatric manifestations of TLE, and could have important implications for its pathophysiology and, perhaps, its therapy. Herein we review recent studies in this field by focusing on resting state networks that have been implicated in the pathophysiology of psychiatric disorders and cognitive impairment in patients with epilepsy: the default mode network, the attention network, and the reward/emotion network.

Common functional networks in the mouse brain revealed by multi-centre resting-state fMRI analysis

Preclinical applications of resting-state functional magnetic resonance imaging (rsfMRI) offer the possibility to non-invasively probe whole-brain network dynamics and to investigate the determinants of altered network signatures observed in human studies. Mouse rsfMRI has been increasingly adopted by numerous laboratories worldwide. Here we describe a multi-centre comparison of 17 mouse rsfMRI datasets via a common image processing and analysis pipeline. Despite prominent cross-laboratory differences in equipment and imaging procedures, we report the reproducible identification of several large-scale resting-state networks (RSN), including a mouse default-mode network, in the majority of datasets. A combination of factors was associated with enhanced reproducibility in functional connectivity parameter estimation, including animal handling procedures and equipment performance. RSN spatial specificity was enhanced in datasets acquired at higher field strength, with cryoprobes, in ventilated animals, and under medetomidine-isoflurane combination sedation. Our work describes a set of representative RSNs in the mouse brain and highlights key experimental parameters that can critically guide the design and analysis of future rodent rsfMRI investigations.

Identifying the default mode network structure using dynamic causal modeling on resting-state functional magnetic resonance imaging

The default mode network is part of the brain structure that shows higher neural activity and energy consumption when one is at rest. The key regions in the default mode network are highly interconnected as conveyed by both the white matter fiber tracing and the synchrony of resting-state functional magnetic resonance imaging signals. However, the causal information flow within the default mode network is still poorly understood. The current study used the dynamic causal modeling on a resting-state fMRI data set to identify the network structure underlying the default mode network. The endogenous brain fluctuations were explicitly modeled by Fourier series at the low frequency band of 0.01-0.08Hz, and those Fourier series were set as driving inputs of the DCM models. Model comparison procedures favored a model wherein the MPFC sends information to the PCC and the bilateral inferior parietal lobule sends information to both the PCC and MPFC. Further analyses provide evidence that the endogenous connectivity might be higher in the right hemisphere than in the left hemisphere. These data provided insight into the functions of each node in the DMN, and also validate the usage of DCM on resting-state fMRI data.

Development of the default-mode network during childhood and adolescence: A longitudinal resting-state fMRI study

The default-mode network (DMN) is a set of functionally connected regions that play crucial roles in internal cognitive processing. Previous resting-state fMRI studies have demonstrated that the intrinsic functional organization of the DMN undergoes remarkable reconfigurations during childhood and adolescence. However, these studies have mainly focused on cross-sectional designs with small sample sizes, limiting the consistency and interpretations of the findings. Here, we used a large sample of longitudinal resting-state fMRI data comprising 305 typically developing children (6-12 years of age at baseline, 491 scans in total) and graph theoretical approaches to delineate the developmental trajectories of the functional architecture of the DMN. For each child, the DMN was constructed according to a prior parcellation with 32 brain nodes. We showed that the overall connectivity increased in strength from childhood to adolescence and became spatially similar to that in the young adult group (N = 61, 18-28 years of age). These increases were primarily located in the midline structures. Global and local network efficiency in the DMN also increased with age, indicating an enhanced capability in parallel information communication within the brain system. Based on the divergent developmental rates of nodal centrality, we identified three subclusters within the DMN, with the fastest rates in the cluster mainly comprising the anterior medial prefrontal cortex and posterior cingulate cortex. Together, our findings highlight the developmental patterns of the functional architecture in the DMN from childhood to adolescence, which has implications for the understanding of network mechanisms underlying the cognitive development of individuals.

Alzheimer's disease disrupts alpha and beta-band resting-state oscillatory network connectivity

OBJECTIVE

Neuroimaging studies in Alzheimer's disease (AD) yield conflicting results due to selective investigation. We conducted a comprehensive magnetoencephalography study of connectivity changes in AD and healthy ageing in the resting-state.

METHODS

We performed a whole-brain, source-space assessment of oscillatory neural signalling in multiple frequencies comparing AD patients, elderly and young controls. We compared eyes-open and closed group oscillatory envelope activity in networks obtained through temporal independent component analysis, and calculated whole-brain node-based amplitude and phase connectivity.

RESULTS

In bilateral parietotemporal areas, oscillatory envelope amplitude increased with healthy ageing, whereas both local amplitude and node-to-global connectivity decreased with AD. AD-related decreases were spatially specific and restricted to the alpha and beta bands. A significant proportion of the variance in areas of peak group difference was explained by cognitive integrity, in addition to group. None of the groups differed in phase connectivity. Results were highly similar for eyes-open and closed resting-state.

CONCLUSIONS

These results support the disconnection syndrome hypothesis and suggest that AD shows distinct and unique patterns of disrupted neural functioning, rather than accelerated healthy ageing.

SIGNIFICANCE

Whole-brain assessments show that disrupted regional oscillatory envelope amplitude and connectivity in the alpha and beta bands play a key role in AD.

The cerebellum, internal models and prediction in 'non-motor' aspects of language: A critical review

The emergence of studies on cerebellar contributions in 'non-motor' aspects of predictive language processing has long been awaited by researchers investigating the neural foundations of language and cognition. Despite (i) progress in research implicating the cerebellum in language processing, (ii) the widely-accepted nature of the uniform, multi-modal computation that the cerebellum implements in the form of internal models, as well as (iii) the long tradition of psycholinguistic studies addressing prediction mechanisms, research directly addressing cerebellar contributions to 'non-motor' predictive language processing has only surfaced in the last five years. This paper provides the first review of this novel field, along with a critical assessment of the studies conducted so far. While encouraging, the evidence for cerebellar involvement in 'non-motor' aspects of predictive language processing remains inconclusive under further scrutiny. Future directions are finally discussed with respect to outstanding questions in this novel field of research.

Brain connectivity aberrations in anabolic-androgenic steroid users

Sustained anabolic-androgenic steroid (AAS) use has adverse behavioral consequences, including aggression, violence and impulsivity. Candidate mechanisms include disruptions of brain networks with high concentrations of androgen receptors and critically involved in emotional and cognitive regulation. Here, we tested the effects of AAS on resting-state functional brain connectivity in the largest sample of AAS-users to date. We collected resting-state functional magnetic resonance imaging (fMRI) data from 151 males engaged in heavy resistance strength training. 50 users tested positive for AAS based on the testosterone to epitestosterone (T/E) ratio and doping substances in urine. 16 previous users and 59 controls tested negative. We estimated brain network nodes and their time-series using ICA and dual regression and defined connectivity matrices as the between-node partial correlations. In line with the emotional and behavioral consequences of AAS, current users exhibited reduced functional connectivity between key nodes involved in emotional and cognitive regulation, in particular reduced connectivity between the amygdala and default-mode network (DMN) and between the dorsal attention network (DAN) and a frontal node encompassing the superior and inferior frontal gyri (SFG/IFG) and the anterior cingulate cortex (ACC), with further reductions as a function of dependency, lifetime exposure, and cycle state (on/off).

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Sustained AAS use has adverse behavioral consequences, including aggression, violence and impulsivity.

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We obtained r-fMRI data from 50 male users testing positive for AAS and 16 previous users and 59 controls testing negative.

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We used ICA and dual regression, and defined connectivity matrices as the between-node temporal partial correlations.

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Current users showed significantly reduced connectivity between amygdala and DMN and between DAN and a SFG/IFG/ACC node.

Gradual emergence of spontaneous correlated brain activity during fading of general anesthesia in rats: Evidences from fMRI and local field potentials

Intrinsic brain activity is characterized by the presence of highly structured networks of correlated fluctuations between different regions of the brain. Such networks encompass different functions, whose properties are known to be modulated by the ongoing global brain state and are altered in several neurobiological disorders. In the present study, we induced a deep state of anesthesia in rats by means of a ketamine/medetomidine peritoneal injection, and analyzed the time course of the correlation between the brain activity in different areas while anesthesia spontaneously decreased over time. We compared results separately obtained from fMRI and local field potentials (LFPs) under the same anesthesia protocol, finding that while most profound phases of anesthesia can be described by overall sparse connectivity, stereotypical activity and poor functional integration, during lighter states different frequency-specific functional networks emerge, endowing the gradual restoration of structured large-scale activity seen during rest. Noteworthy, our in vivo results show that those areas belonging to the same functional network (the default-mode) exhibited sustained correlated oscillations around 10 Hz throughout the protocol, suggesting the presence of a specific functional backbone that is preserved even during deeper phases of anesthesia. Finally, the overall pattern of results obtained from both imaging and in vivo-recordings suggests that the progressive emergence from deep anesthesia is reflected by a corresponding gradual increase of organized correlated oscillations across the cortex.

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Rat brain activity was recorded while anesthesia spontaneously decreased over time.

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Fading of anesthesia modulates the overall pattern of brain structured co-activations.

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Areas of the same network show frequency-specific coupling even in deep anesthesia.

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Light anesthesia is characterized by the gradual emergence of large-scale connectivity.

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Correlated band-limited oscillations distinguish between states and network properties.

An integrated network model of psychotic symptoms

The full body of research on the nature of psychosis and its determinants indicates that a considerable number of factors are relevant to the development of hallucinations, delusions, and other positive symptoms, ranging from neurodevelopmental parameters and altered connectivity of brain regions to impaired cognitive functioning and social factors. We aimed to integrate these factors in a single mathematical model based on network theory. At the microscopic level this model explains positive symptoms of psychosis in terms of experiential equivalents of robust, high-frequency attractor states of neural networks. At the mesoscopic level it explains them in relation to global brain states, and at the macroscopic level in relation to social-network structures and dynamics. Due to the scale-free nature of biological networks, all three levels are governed by the same general laws, thereby allowing for an integrated model of biological, psychological, and social phenomena involved in the mediation of positive symptoms of psychosis. This integrated network model of psychotic symptoms (INMOPS) is described together with various possibilities for application in clinical practice.

Age-related differences in default-mode network connectivity in response to intermittent theta-burst stimulation and its relationships with maintained cognition and brain integrity in healthy aging

The default-mode network (DMN) is affected by advancing age, where particularly long-range connectivity has been consistently reported to be reduced as compared to young individuals. We examined whether there were any differences in the effects of intermittent theta-burst stimulation (iTBS) in DMN connectivity between younger and older adults, its associations with cognition and brain integrity, as well as with long-term cognitive status. Twenty-four younger and 27 cognitively normal older adults were randomly assigned to receive real or sham iTBS over the left inferior parietal lobule between two resting-state functional magnetic resonance imaging (rs-fMRI) acquisitions. Three years later, those older adults who had received real iTBS underwent a cognitive follow-up assessment. Among the younger adults, functional connectivity increased following iTBS in distal DMN areas from the stimulation site. In contrast, older adults exhibited increases in connectivity following iTBS in proximal DMN regions. Moreover, older adults with functional responses to iTBS resembling those of the younger participants exhibited greater brain integrity and higher cognitive performance at baseline and at the 3-year follow-up, along with less cognitive decline. Finally, we observed that 'young-like' functional responses to iTBS were also related to the educational background attained amongst older adults. The present study reveals that functional responses of the DMN to iTBS are modulated by age. Furthermore, combining iTBS and rs-fMRI in older adults may allow characterizing distinctive cognitive profiles in aging and its progression, probably reflecting network plasticity systems that may entail a neurobiological substrate of cognitive reserve.

Discrete alterations of brain network structural covariance in individuals at ultra-high risk for psychosis

BACKGROUND

Investigation of aberrant large-scale brain networks offers novel insight into the role these networks play in diverse psychiatric disorders such as schizophrenia. Although studies report altered functional brain connectivity in participants at ultra-high risk (UHR) for psychosis, it is unclear whether these alterations extend to structural brain networks.

METHODS

Whole-brain structural covariance patterns of 133 participants at UHR for psychosis (51 of whom subsequently developed psychosis) and 65 healthy control (HC) subjects were studied. Following data preprocessing (using VBM8 toolbox), the mean signal in seed regions relating to specific networks (visual, auditory, motor, speech, semantic, executive control, salience, and default-mode) were extracted, and voxel-wise analyses of covariance were conducted to compare the association between whole-brain signal and each seed region for UHR and HC individuals. The UHR participants who transitioned to psychosis were compared with the UHR participants who did not.

RESULTS

Significantly reduced structural covariance was observed in the UHR sample compared with the HC sample for the default-mode network, and increased covariance was observed for the motor and executive control networks. When the UHR participants who transitioned to psychosis were compared with the UHR participants who did not, aberrant structural covariance was observed in the salience, executive control, auditory, and motor networks.

CONCLUSIONS

Whole-brain structural covariance analyses revealed subtle changes of connectivity of the default-mode, executive control, salience, motor, and auditory networks in UHR individuals for psychosis. Although we found significant differences, these are small changes and tend to reflect largely intact structural networks.

Decreased functional connectivity within the default-mode network in acute brainstem ischemic stroke

PURPOSE

Ischemic stroke within the brainstem is associated with an increased risk of cognitive dysfunction. This study aimed to explore the integrity of a default-mode network (DMN) and its relationship with clinical variables in patients with acute ischemic brainstem stroke using an independent component analysis (ICA) approach.

MATERIALS AND METHODS

Twenty-one patients with acute ischemic brainstem stroke and 25 well-matched healthy subjects were enrolled in this study and underwent resting-state functional magnetic resonance imaging. The ICA was adopted to extract the DMN, including its anterior and posterior components. Pearson correlation analyses were performed to investigate the relationship between DMN connectivity and clinical variables.

RESULTS

Compared with healthy controls, patients with acute ischemic stroke showed significantly decreased functional connectivity in the right medial prefrontal cortex (mPFC) and right precuneus within the anterior and posterior DMN, respectively. After correcting for age, sex, and education, hypoconnectivity in the right mPFC and right precuneus was negatively correlated with higher homocysteine in patients with stroke (r = -0.592, p = 0.010 and r = -0.491, p = 0.039, respectively).

CONCLUSION

The finding of decreased functional connectivity within the DMN of patients with acute brainstem stroke provides novel insight into the neural mechanisms that underlie cognitive impairment following ischemic insult to this brain region.

Simultaneous EEG-fMRI reveals a temporal cascade of task-related and default-mode activations during a simple target detection task

Focused attention continuously and inevitably fluctuates, and to completely understand the mechanisms responsible for these modulations it is necessary to localize the brain regions involved. During a simple visual oddball task, neural responses measured by electroencephalography (EEG) modulate primarily with attention, but source localization of the correlates is a challenge. In this study we use single-trial analysis of simultaneously-acquired scalp EEG and functional magnetic resonance image (fMRI) data to investigate the blood oxygen level dependent (BOLD) correlates of modulations in task-related attention, and we unravel the temporal cascade of these transient activations. We hypothesize that activity in brain regions associated with various task-related cognitive processes modulates with attention, and that their involvements occur transiently in a specific order. We analyze the fMRI BOLD signal by first regressing out the variance linked to observed stimulus and behavioral events. We then correlate the residual variance with the trial-to-trial variation of EEG discriminating components for identical stimuli, estimated at a sequence of times during a trial. Post-stimulus and early in the trial, we find activations in right-lateralized frontal regions and lateral occipital cortex, areas that are often linked to task-dependent processes, such as attentional orienting, and decision certainty. After the behavioral response we see correlates in areas often associated with the default-mode network and introspective processing, including precuneus, angular gyri, and posterior cingulate cortex. Our results demonstrate that during simple tasks both task-dependent and default-mode networks are transiently engaged, with a distinct temporal ordering and millisecond timescale.

Olanzapine modulation of long- and short-range functional connectivity in the resting brain in a sample of patients with schizophrenia

Treatment effects of antipsychotic drugs on cerebral function are seldom examined. Exploring functional connectivity (FC) in drug-free schizophrenia patients before and after antipsychotic treatment can improve the understanding of antipsychotic drug mechanisms. A total of 17 drug-free patients with recurrent schizophrenia and 24 healthy controls underwent resting-state functional magnetic resonance imaging scans. Long- and short-range FC strengths (FCS) were calculated for each participant. Compared with the controls, the patients at baseline exhibited increased long-range positive FCS (lpFCS) in the bilateral inferior parietal lobule (IPL) and decreased lpFCS in the brain regions of the default-mode network (DMN) regions and sensorimotor circuits of the brain. By contrast, increased short-range positive FCS was observed in the right IPL of the patients at baseline compared with the controls. After treatment with olanzapine, increased FC in the DMN and sensorimotor circuits of the brain was noted, whereas decreased FC was observed in the left superior temporal gyrus (STG). Moreover, the alterations of the FCS values and the reductions in symptom severity among the patients after treatment were correlated. The present study provides evidence that olanzapine normalizes the abnormalities of long- and short-range FCs in schizophrenia. FC reductions in the right IPL may be associated with early treatment response, whereas those in the left STG may be related to poor treatment outcome.

Emotional detachment in psychopathy: Involvement of dorsal default-mode connections

Criminal psychopathy is defined by emotional detachment [Psychopathy Checklist - Revised (PCL-R) factor 1], and antisocial behaviour (PCL-R factor 2). Previous work has associated antisocial behaviour in psychopathy with abnormalities in a ventral temporo-amygdala-orbitofrontal network. However, little is known of the neural correlates of emotional detachment. Imaging studies have indicated that the 'default-mode network' (DMN), and in particular its dorsomedial (medial prefrontal - posterior cingulate) component, contributes to affective and social processing in healthy individuals. Furthermore, recent work suggests that this network may be implicated in psychopathy. However, no research has examined the relationship between psychopathy, emotional detachment, and the white matter underpinning the DMN. We therefore used diffusion tensor imaging (DTI) tractography in 13 offenders with psychopathy and 13 non-offenders to investigate the relationship between emotional detachment and the microstructure of white matter connections within the DMN. These included the dorsal cingulum (containing the medial prefrontal - posterior cingulate connections of the DMN), and the ventral cingulum (containing the posterior cingulate - medial temporal connections of the DMN). We found that fractional anisotropy (FA) was reduced in the left dorsal cingulum in the psychopathy group (p = .024). Moreover, within this group, emotional detachment was negatively correlated with FA in this tract portion bilaterally (left: r = -.61, p = .026; right: r = -.62, p = .023). These results suggest the importance of the dorsal DMN in the emotional detachment observed in individuals with psychopathy. We propose a 'dual-network' model of white matter abnormalities in the disorder, which incorporates these with previous findings.

Dissociable roles of default-mode regions during episodic encoding

We investigated the role of distinct regions of the default-mode network (DMN) during memory encoding with fMRI. Subjects encoded words using either a strategy that emphasized self-referential (pleasantness) processing, or one that emphasized semantic (man-made/natural) processing. During encoding subjects were intermittently presented with thought probes to evaluate if they were concentrated and on-task or exhibiting task-unrelated thoughts (TUT). After the scanning session subjects performed a source retrieval task to determine which of two judgments they performed for each word at encoding. Source retrieval accuracy was higher for words encoded with the pleasantness vs. the man-made/natural task and there was a trend for higher performance for words preceding on-task vs. TUT reports. fMRI results show that left anterior medial PFC and left angular gyrus activity was greater during successful vs. unsuccessful encoding during both encoding tasks. Greater activity in left anterior cingulate and bilateral lateral temporal cortex was related successful vs. unsuccessful encoding only in the pleasantness task. In contrast, posterior cingulate, right anterior cingulate and right temporoparietal junction were activated to a greater extent in unsuccessful vs. successful encoding across tasks. Finally, activation in posterior cingulate and bilateral dorsolateral prefrontal cortex was related to TUT across tasks; moreover, we observed a conjunction in posterior cingulate between encoding failure and TUT. We conclude that DMN regions play dissociable roles during memory formation, and that their association with subsequent memory may depend on the manner in which information is encoded and retrieved.

Cocaine addiction related reproducible brain regions of abnormal default-mode network functional connectivity: a group ICA study with different model orders

Model order selection in group independent component analysis (ICA) has a significant effect on the obtained components. This study investigated the reproducible brain regions of abnormal default-mode network (DMN) functional connectivity related with cocaine addiction through different model order settings in group ICA. Resting-state fMRI data from 24 cocaine addicts and 24 healthy controls were temporally concatenated and processed by group ICA using model orders of 10, 20, 30, 40, and 50, respectively. For each model order, the group ICA approach was repeated 100 times using the ICASSO toolbox and after clustering the obtained components, centrotype-based anterior and posterior DMN components were selected for further analysis. Individual DMN components were obtained through back-reconstruction and converted to z-score maps. A whole brain mixed effects factorial ANOVA was performed to explore the differences in resting-state DMN functional connectivity between cocaine addicts and healthy controls. The hippocampus, which showed decreased functional connectivity in cocaine addicts for all the tested model orders, might be considered as a reproducible abnormal region in DMN associated with cocaine addiction. This finding suggests that using group ICA to examine the functional connectivity of the hippocampus in the resting-state DMN may provide an additional insight potentially relevant for cocaine-related diagnoses and treatments.

Neurophysiological differences in the adolescent brain following a single night of restricted sleep - A 7T fMRI study

Sleep deprivation in youth has garnered international attention in recent years, as correlational studies have demonstrated significant relationships between lack of sleep and detrimental behavioral and academic outcomes. However, no study to date has systematically examined the neurophysiological consequences of a single night of sleep restriction (i.e., 4 h) in adolescents using ultra-high field functional neuroimaging. Much of what we know regarding the neural consequences of sleep deprivation has come from the adult literature, and among those studies, the majority use region of interest (ROI) approaches, thus disregarding the dynamic mechanisms that may subserve the behavioral effects of sleep restriction. Leveraging a crossover within-subjects design, we demonstrate that pivotal brain regions involved in the default mode and limbic regulatory centers have disrupted functioning following a night of restricted sleep compared to a night of "normal sleep". Specifically, a normal night (i.e., 8 h) of sleep led to increased global and local efficiency of bilateral amygdala, and less efficiency in the posterior cingulate, as measured by graph theory, compared to a night of sleep restriction. Furthermore, aberrant functional connectivity patterns were identified in key fronto-limbic circuitry, suggesting a potential pathophysiological mechanism underlying the widespread effects of sleep deprivation in youth.

Neurochemical Modulation in Posteromedial Default-mode Network Cortex Induced by Transcranial Magnetic Stimulation

BACKGROUND

The Default Mode Network (DMN) is severely compromised in several psychiatric and neurodegenerative disorders where plasticity alterations are observed. Glutamate and GABA are the major excitatory and inhibitory brain neurotransmitters respectively and are strongly related to plasticity responses and large-scale network expression.

OBJECTIVE

To investigate whether regional Glx (Glutamate + Glutamine) and GABA could be modulated within the DMN after experimentally-controlled induction of plasticity and to study the effect of intrinsic connectivity over brain responses to stimulation.

METHODS

We applied individually-guided neuronavigated Theta Burst Stimulation (TBS) to the left inferior parietal lobe (IPL) in-between two magnetic resonance spectroscopy (MRS) acquisitions to 36 young subjects. A resting-state fMRI sequence was also acquired before stimulation.

RESULTS

After intermittent TBS, distal GABA increases in posteromedial DMN areas were observed. Instead, no significant changes were detected locally, in left IPL areas. Neurotransmitter modulation in posteromedial areas was related to baseline fMRI connectivity between this region and the TBS-targeted area.

CONCLUSIONS

The prediction of neurotransmitter modulation by connectivity highlights the relevance of connectivity patterns to understand brain responses to plasticity-inducing protocols. The ability to modulate GABA in a key core of the DMN by means of TBS may open new avenues to evaluate plasticity mechanisms in a key area for major neurodegenerative and psychiatric conditions.

Cholinergic and serotonergic modulations differentially affect large-scale functional networks in the mouse brain

Resting-state functional MRI (rsfMRI) is a widely implemented technique used to investigate large-scale topology in the human brain during health and disease. Studies in mice provide additional advantages, including the possibility to flexibly modulate the brain by pharmacological or genetic manipulations in combination with high-throughput functional connectivity (FC) investigations. Pharmacological modulations that target specific neurotransmitter systems, partly mimicking the effect of pathological events, could allow discriminating the effect of specific systems on functional network disruptions. The current study investigated the effect of cholinergic and serotonergic antagonists on large-scale brain networks in mice. The cholinergic system is involved in cognitive functions and is impaired in, e.g., Alzheimer's disease, while the serotonergic system is involved in emotional and introspective functions and is impaired in, e.g., Alzheimer's disease, depression and autism. Specific interest goes to the default-mode-network (DMN), which is studied extensively in humans and is affected in many neurological disorders. The results show that both cholinergic and serotonergic antagonists impaired the mouse DMN-like network similarly, except that cholinergic modulation additionally affected the retrosplenial cortex. This suggests that both neurotransmitter systems are involved in maintaining integrity of FC within the DMN-like network in mice. Cholinergic and serotonergic modulations also affected other functional networks, however, serotonergic modulation impaired the frontal and thalamus networks more extensively. In conclusion, this study demonstrates the utility of pharmacological rsfMRI in animal models to provide insights into the role of specific neurotransmitter systems on functional networks in neurological disorders.

Anatomy of the dorsal default-mode network in conduct disorder: Association with callous-unemotional traits

We recently reported that emotional detachment in adult psychopathy was associated with structural abnormalities in the dorsal 'default-mode' network (DMN). However, it is unclear whether these differences are present in young people at risk of psychopathy. The most widely recognised group at risk for psychopathy are children/adolescents with conduct disorder (CD) and callous-unemotional (CU) traits. We therefore examined the microstructure of the dorsal DMN in 27 CD youths (14-with/13-without CU traits) compared to 16 typically developing controls using DTI tractography. Both CD groups had significantly (p < 0.025) reduced dorsal DMN radial diffusivity compared to controls. In those with diagnostically significant CU traits, exploratory analyses (uncorrected for multiple comparisons) suggested that radial diffusivity was negatively correlated with CU severity (Left: rho = -0.68, p = 0.015). These results suggest that CD youths have microstructural abnormalities in the same network as adults with psychopathy. Further, the association with childhood/adolescent measures of emotional detachment (CU traits) resembles the relationship between emotional detachment and network microstructure in adult psychopaths. However, these changes appear to occur in opposite directions - with increased myelination in adolescent CD but reduced integrity in adult psychopathy. Collectively, these findings suggest that developmental abnormalities in dorsal DMN may play a role in the emergence of psychopathy.

Functional connectivity increase in the default-mode network of patients with Alzheimer's disease after long-term treatment with Galantamine

Acetylcholinesterase inhibitors (AChEIs) are efficacious for the treatment of mild to moderate forms of Alzheimer's dementia (AD). Default-mode network (DMN) connectivity is considered to be early impaired in AD. Long-term effects of AChEIs on the DMN in AD have not yet been investigated. Twenty-eight AD patients and 11 age-matched healthy volunteers (HC) participated in the prospective study. AD patients were randomly assigned to either a pharmacotherapy arm (Galantamine, AD G) or to a placebo arm (AD P+G) for the period of 6 months followed by open-label Galantamine therapy from month 7-12. All subjects underwent neuropsychological testing, resting-state functional and structural MRI at baseline and after 12 months, AD patients additionally in between after 6 months. Thirteen AD patients completed the treatment trial and underwent all functional MRI follow-up sequences of good quality. Functional connectivity significantly increased within the AD G group in the posterior cingulate cortex and in the Precuneus between baseline and 12 months follow-up (pcorr<0.05). Between-group analyses demonstrated that functional connectivity in the AD G group significantly increased in the posterior cingulate cortex as well as in the Precuneus compared to the HC group and in the anteromedial aspect of the temporal lobes compared to the AD P+G group, respectively, at 12 months follow-up (pcorr<0.05). Cognitive performance remained stable within groups over time indicating that resting-state fMRI may be sensitive for the detection of pharmacologically induced effects on brain function of AD patients.

Multimodality magnetic resonance imaging in hepatic encephalopathy: An update

Hepatic encephalopathy (HE) is a neuropsychiatric complication of cirrhosis or acute liver failure. Currently, HE is regarded as a continuous cognitive impairment ranging from the mildest stage, minimal HE to overt HE. Hyperammonaemia and neuroinflammation are two main underlying factors which contribute to the neurological alterations in HE. Both structural and functional impairments are found in the white mater and grey mater involved in HE. Although the investigations into HE pathophysiological mechanism are enormous, the exact pathophysiological causes underlying HE remain controversial. Multimodality magnetic resonance imaging (MRI) plays an important role in helping to understand the pathological process of HE. This paper reviews the up-to-date multimodality MRI methods and predominant findings in HE patients with a highlight of the increasingly important role of blood oxygen level dependent functional MRI.

Increased anterior default-mode network homogeneity in first-episode, drug-naive major depressive disorder: A replication study

BACKGROUND

Abnormal default-mode network (DMN) homogeneity has been involved in the neurophysiology of major depressive disorder (MDD) with inconsistent findings. The inconsistency may be due to clinical and methodological variability, and the reproducibility of the findings is limited. The present study aimed to examine alterations of the DMN homogeneity in two independent samples of patients with first-episode, drug-naive MDD.

METHODS

The samples included 59 patients with MDD and 31 comparison subjects from Sample 1 and 29 patients with MDD and 24 comparison subjects from Sample 2. Network homogeneity (NH) was computed with an overlapping technique, which was employed to define brain regions with abnormal NH common to the MDD samples.

RESULTS

Compared with comparison subjects, patients with MDD exhibited increased NH in an overlapped brain region of the left superior medial prefrontal cortex (MPFC). No correlations were found between abnormal NH and HAMD total/subscale scores in the patients of each sample and in the combined patients from both samples.

CONCLUSIONS

This study is the first to examine alterations of DMN homogeneity in first-episode, drug-naive patients with MDD in two independent samples by using an overlapping technique. Patients with MDD exhibit increased NH in an overlapped region in the anterior DMN. The present study thus highlights the importance of the DMN in the neurophysiology of MDD.