Advertisement
Browse Subject Areas
?

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Open Access

Peer-reviewed

Research Article

Altered Effective Connectivity Network of the Basal Ganglia in Low-Grade Hepatic Encephalopathy: A Resting-State fMRI Study with Granger Causality Analysis

  • Rongfeng Qi ,

    Contributed equally to this work with: Rongfeng Qi, Long Jiang Zhang

    Affiliation Department of Medical Imaging, Jinling Hospital, Clinical School of Medical College, Nanjing University, Nanjing, Jiangsu Province, China

    ⨯
  • Long Jiang Zhang ,

    Contributed equally to this work with: Rongfeng Qi, Long Jiang Zhang

    kevinzhanglongjiang@yahoo.com.cn (LJZ); cjr.luguangming@vip.163.com (GL)

    Affiliation Department of Medical Imaging, Jinling Hospital, Clinical School of Medical College, Nanjing University, Nanjing, Jiangsu Province, China

    ⨯
  • Jianhui Zhong,

    Affiliation Department of Biomedical Engineering, Zhejiang University, Hangzhou, Zhejiang Province, China

    ⨯
  • Zhiqiang Zhang,

    Affiliation Department of Medical Imaging, Jinling Hospital, Clinical School of Medical College, Nanjing University, Nanjing, Jiangsu Province, China

    ⨯
  • Ling Ni,

    Affiliation Department of Medical Imaging, Jinling Hospital, Clinical School of Medical College, Nanjing University, Nanjing, Jiangsu Province, China

    ⨯
  • Qing Jiao,

    Affiliation Department of Medical Imaging, Jinling Hospital, Clinical School of Medical College, Nanjing University, Nanjing, Jiangsu Province, China

    ⨯
  • Wei Liao,

    Affiliation Center for Cognition and Brain Disorders and the Affiliated Hospital, Hangzhou Normal University, Hangzhou, Zhejiang Province, China

    ⨯
  • Gang Zheng,

    Affiliation Department of Medical Imaging, Jinling Hospital, Clinical School of Medical College, Nanjing University, Nanjing, Jiangsu Province, China

    ⨯
  • Guangming Lu

    kevinzhanglongjiang@yahoo.com.cn (LJZ); cjr.luguangming@vip.163.com (GL)

    Affiliation Department of Medical Imaging, Jinling Hospital, Clinical School of Medical College, Nanjing University, Nanjing, Jiangsu Province, China

    ⨯

Abstract

Background

The basal ganglia often show abnormal metabolism and intracranial hemodynamics in cirrhotic patients with hepatic encephalopathy (HE). Little is known about how the basal ganglia affect other brain system and is affected by other brain regions in HE. The purpose of this study was to investigate whether the effective connectivity network associated with the basal ganglia is disturbed in HE patients by using resting-state functional magnetic resonance imaging (rs-fMRI).

Methodology/Principal Findings

Thirty five low-grade HE patients and thirty five age- and gender- matched healthy controls participated in the rs-fMRI scans. The effective connectivity networks associated with the globus pallidus, the primarily affected region within basal ganglia in HE, were characterized by using the Granger causality analysis and compared between HE patients and healthy controls. Pearson correlation analysis was performed between the abnormal effective connectivity and venous blood ammonia levels and neuropsychological performances of all HE patients. Compared with the healthy controls, patients with low-grade HE demonstrated mutually decreased influence between the globus pallidus and the anterior cingulate cortex (ACC), cuneus, bi-directionally increased influence between the globus pallidus and the precuneus, and either decreased or increased influence from and to the globus pallidus in many other frontal, temporal, parietal gyri, and cerebellum. Pearson correlation analyses revealed that the blood ammonia levels in HE patients negatively correlated with effective connectivity from the globus pallidus to ACC, and positively correlated with that from the globus pallidus to precuneus; and the number connectivity test scores in patients negatively correlated with the effective connectivity from the globus pallidus to ACC, and from superior frontal gyrus to globus pallidus.

Conclusions/Significance

Low-grade HE patients had disrupted effective connectivity network of basal ganglia. Our findings may help to understand the neurophysiological mechanisms underlying the HE.

Citation: Qi R, Zhang LJ, Zhong J, Zhang Z, Ni L, Jiao Q, et al. (2013) Altered Effective Connectivity Network of the Basal Ganglia in Low-Grade Hepatic Encephalopathy: A Resting-State fMRI Study with Granger Causality Analysis. PLoS ONE 8(1): e53677. https://doi.org/10.1371/journal.pone.0053677

Editor: Yong He, Beijing Normal University, China

Received: July 28, 2012; Accepted: December 3, 2012; Published: January 11, 2013

Copyright: © 2013 Qi et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This work was supported by the grants from the Natural Scientific Foundation of China [Grant No. 30700194 and No. 81230032 for Long Jiang Zhang, Grant No. 30800264 for Zhiqiang Zhang, and Grant No. 30971019 for Qing Jiao, Grant No. 81101039 for Gang Zheng], and Chinese Key Program (Grant No. BWS11J063 and No. 10z026 for Guangming Lu). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Hepatic encephalopathy (HE) is a common neuropsychiatric complication which caused disturbance of central nervous system function in patients with acute and chronic liver disease [1]. It encompasses a broad spectrum of neurological symptom of varying severity and is classified from low-grade to high-grade HE. Even the low-grade HE is associated with poor quality of life and increased work disability [2], [3], [4], both improve after liver transplantation or reasonable medical treatment with lactulose [5] and rifaximin [6]. Therefore, it is important to diagnose and treat HE before major neurological destroy occurs. Although the exact pathophysiological mechanisms of HE remain unclear, investigators have extensively investigated this disease with the aim of developing effective therapies and monitoring the effectiveness of treatment.

Accumulating evidences from neuroimaging studies suggest that an alteration of the cortico-striato-thalamic pathway might play an important role in the HE [7], [8]. Within this model, the common radiological findings of HE are hyperintensity in the basic ganglia (especially the globus pallidus) in conventional T1-weighted MR images [9], and redistribution of cerebral blood flow and metabolic rate of glucose and ammonia from various cortical regions (e.g., the frontal and parietal cortices) to subcortical grey matter regions (the basal ganglia and thalamus) in position emission tomography (PET) and single photon emission tomography (SPET) [8], [10].

Resting-state functional magnetic resonance imaging (rs-fMRI) which measures spontaneous low-frequency blood oxygenation level-dependent (BOLD) fluctuations [11] may help delineate the human neural functional architecture, and has been widely used to investigate the pathophysiology of many brain diseases, such as Alzheimer's disease [12] and attention deficit hyperactivity disorder [13]. In a very recent rs-fMRI study, Zhang et al. [14] reported a widespread disrupted functional connectivity between the basal ganglia and many other brain regions in minimal HE patients. Basal ganglia are involved in many neuronal pathways related to psychomotor behavior, emotional and cognitive functions [15], and is considered to play an important role in the pathophysiology of HE [16]. Even though the basal ganglia showed disrupted functional connectivity with many other brain regions [14] and abnormal metabolism [8], [10] in HE in previous studies, the question remains how the basal ganglia affect other brain system and is affected by other brain regions in this disease. To address this problem, in this rs-fMRI study, we aimed to evaluate altered directional connectivity patterns from and to the basal ganglia in the low-grade HE by using Granger causality analysis (GCA). GCA origins from the field of economics and has been widely used for time-directed prediction between BOLD-fMRI time series, and revealing the causal effects among brain regions [17], [18], [19]. Taking into account that the globus pallidus are the mainly affected regions within the basal ganglia in HE, we chosen bilateral globus pallidus as seed regions and hypothesized that effective connectivity networks of them were disrupted in low-grade HE patients. To the best of our knowledge, the present study is the first to examine the time-directed dynamic relations in HE with GCA.

Materials and Methods

Subjects

This study was approved by the Medical Research Ethics Committee of Jinling Hospital and Clinical School of Medical College at Nanjing University. Thirty five low-grade HE patients (28 male, 7 women, mean age: 53.86±7.82 years) and thirty five age-and gender-matched healthy controls (28 male, 7 women, mean age: 50.40±9.19 years) were included in this study, after giving the written informed consents. All participants: 1) were right-handed, with age 18 years or older. 2) had no reported history of brain injury and any psychiatric disorder; 3) had no reported substance abuse; and 4) had no physical limitations that prohibited them from finishing the MR exam, and no translation more than 1.0 mm or rotation than 1.0° during MR scanning.

The patients were recruited from hospitalized patients in Jingling hospital with criterion as follows: the patients with clinical proven hepatic cirrhosis, without comatose, had abnormal neuropsychological tests scores. The overt HE was graded with the West Haven Criteria [20]. Of these patients, 28 were defined as minimal HE (MHE) and 7 as grade I HE. We combined these two entities into one group according to the recent classification by the International Society for Hepatic Encephalopathy and Nitrogen Metabolism (ISHEN) [21], [22], in which both the MHE and grade 1 HE are named as covert HE for simplifying in practical use. The healthy controls were recruited from the local area by means of poster advertisement. They had no diseases of the liver and other systems, with no abnormal findings in abdominal ultrasound scans and conventional brain MR imaging. All controls underwent neuropsychological tests that also performed on HE patients before the MR scanning. No laboratory tests were performed thus unavailable for them.

Neuropsychological tests

The definition of HE was based on the West Haven criteria [20]. MHE was diagnosed according to the recommendation by the working party of 11th World Congress of Gastroenterology in Vienna in 1998 [20]. The test battery includes number connecting-A (NCT-A) and digit symbol test (DST), which are recommended by the working party. When the scores of at least one test were beyond 2SD (standard deviation) of mean value of age-matched healthy controls, the cirrhotic patients without previous and current overt HE could be regarding having MHE [2], [23].

Laboratory Examinations

Laboratory parameters including prothrombin time, protein metabolism tests, venous blood ammonia were obtained from all patients to assess the severity of liver disease, within one week before MR scanning. The grade of hepatic function was determined according to the Child-Pugh score [24], [25]. The score system considered five variables, i.e., ascites, encephalopathy, prothrombin time, and serum levels of bilirubin and albumin, and assigned a score ranging from 1 to 3 to each variable. Of these 35 low-grade HE patients, 12 patients had Child-Pugh grade A, 19 patients had Child-Pugh grade B, and 4 had Child-Pugh grade C.

MRI data acquisition

MRI data were acquired on a 3 Tesla MR scanner (TIM Trio, Siemens Medical Solutions, Erlangen, Germany). All the patients and healthy controls were instructed to close their eyes but be awake during the resting-state functional MR imaging examination. Foam pad was used to minimize the head motion of all subjects. Axial anatomical images were acquired using a T1-FLASH sequence (TR/TE = 350 ms/2.46 ms, matrix = 320×256, field of view (FOV) = 240×240 mm2, slice thickness/gap = 4.0 mm/0.4 mm, 30 slices covered the whole brain). Functional images were then obtained aligned along the anterior commissure-posterior commissure line with a single-shot, gradient-recalled echo planar imaging sequence (TR/TE = 2000 ms/30 ms, FOV = 240×240 mm2, flip angle = 90°, matrix = 64×64, voxel size = 3.75×3.75×4 mm3). A total of 250 brain volumes were collected, resulting in a total scan time of 500 s.

Data preprocessing

Data were pre-processed using SPM8 software package (http://www.fil.ion.ucl.ac.uk/spm). The first 10 images were excluded for magnetization to reach equilibrium. The remaining 240 consecutive volumes were used for data analysis. Slice-timing adjustment and realignment for head-motion correction were performed. No translation or rotation parameters in any given data set exceeded 1.0 mm or 1.0°. We also evaluated the group differences in translation and rotation of head motion according to the following formula [26]: where L is the length of the time series (L = 240 in this study), xi, yi and zi are translations/rotations at the ith time point in the x, y and z directions, respectively. The results showed that the two groups had no significant differences in image quality (two sample t test, t = 1.489, P = 0.141 for translational motion, and t = 1.391, P = 0.169 for rotational motion).

The functional images were then spatially normalized to standard stereotaxic coordinates of the standard Montreal Neurological Institute (MNI) and resampled into voxel size of 3×3×3 mm3, and then smoothed by convolution with an isotropic Gaussian kernel of 8 mm FWHW to decrease spatial noise. To further reduce the effects of confounding factors unlikely to be involved in specific regional correlation, we also removed several sources of spurious variance by linear regression, including six head motion parameters, and average signals from cerebrospinal fluid, white matter according to previous fMRI studies [27], [28]. Then, the residual time series were band filtered (0.01–0.08 Hz) using the Resting State fMRI Data Analysis Toolkit (REST) Version 1.6 [29] (http://www.restfmri.net).

Definition of seed regions

In individual rs-fMRI data analysis, we used the bilateral globus pallidus as seed regions. These seed regions were selected using the WFU PickAtlas Tool Version 3.0 (http://fmri.wfubmc.edu/software/PickAtlas) [30], which was used to define the reference time series and have been applied in previous rs-fMRI studies [31].

Effective connectivity analysis

In the current study, we used Granger causality to describe the effective connectivity between the reference time series of the seed regions (left and right globus pallidus, respectively) and the time series of each voxel within the whole brain. Voxel-wise GCA on the residual-based F was performed using the REST-GCA [32] in the REST toolbox. GCA is a technique that originally developed to evaluate causal relation between two time series in the field of economics [33]. It is based on the idea that, given two time series x and y, if knowing the past of y is useful for predicting the future of x then y must have a causal influence on x. In this study, the time series of the globus pallidus was defined as the seed time series x, and the time course of voxels within the whole brain are defined as y. The linear direct influence of x on y (Fx→y), and the linear direct influence of y on x (Fy→x) were calculated voxel by voxel across the brain. The residual-based F was transformed to normal distributed F′, then F′ of each voxel was standardized to Z score (Zx→y and Zy→x, subtracting the global mean F′ values, then being divided by standard deviation) [32].

Statistical analysis

Group analysis of functional connectivity of globus pallidus.

Within each group, mean values of the Zx→y and Zy→x maps were calculated. All eight Granger causality maps were obtained, with four for each direction and four for each group (the left globus pallidus with Zx→y and Zy→x and the right globus pallidus with Zx→y and Zy→x for both the patient and healthy control groups). Then a random effect two-sample t-test in a voxel-wise manner was then performed to determine the differences of effective connectivity of globus pallidus between two groups, with age and sex importing as covariates. Significant thresholds were set at a corrected P<0.05 [multiple correction using false discovery rate (FDR) criterion].

Pearson correlation analysis of effective connectivity of globus pallidus

To investigate the association between the clinical index and the effective connectivity of globus pallidus in the patients, the regions showing significantly different (increased or decreased) Granger influences between patient and healthy control groups were extracted as regions of interest (ROIs). Mean Granger causality values within these ROIs were correlated against the venous blood ammonia levels, and the neuropsychological performances (scores of NCT and DST) of all patients using the Pearson correlation analysis. Statistical threshold was set at P<0.05, uncorrected.

Results

Demographics and clinical data

Table 1 showed the demographics and clinical data of all the 70 participants in our study. All subjects were right-handed. There were no significant differences in gender, age between the low-grade HE and healthy control groups. However, HE patients had worse neuropsychological performances than healthy controls.

thumbnail
Download:
Table 1. Demographics and clinical data of low-grade HE patients and healthy controls.

https://doi.org/10.1371/journal.pone.0053677.t001

Effective connectivity from the left globus pallidus

Compared with healthy controls, patients with low-grade HE demonstrated significantly decreased effective connectivity from the left globus pallidus to seveal brain regions, including the left anterior cingulate cortex (ACC), bilateral cuneus, right inferior temporal gyrus (ITG), left superior temporal gyrus (STG), and increased effective connectivity from the left globus pallidus to the left precuneus, left middle frontal gyrus (MFG) and bilateral parahippocampal gyri (P<0.05, FDR corrected) (Figure 1A, Table 2).

thumbnail
Download:
Figure 1. Altered effective connectivity from the globus pallidus to other brain regions in low-grade HE patients (P<0.05, FDR corrected).

Compared with healthy controls, HE patients show decreased effective connectivity from the left globus pallidus to the left ACC, bilateral cuneus, right ITG, left STG, and increased effective connectivity to the left precuneus, left MFG and bilateral parahippocampal gyri (1A). Patients exhibit decreased effective connectivity from the right globus pallidus to the right ACC, left cuneus, bilateral STG and MFG, and increased effective connectivity to the right precuneus, right MFG, and right parahippocampal gyrus (1B). The hot and cold colors indicate the brain regions that show significantly increased and decreased effective connectivity, respectively. HE = hepatic encephalopathy; FDR = false discovery rate; ACC = anterior cingulate cortex; ITG = inferior temporal gyrus; STG = superior temporal gyrus; MFG = middle frontal gyrus.

https://doi.org/10.1371/journal.pone.0053677.g001

thumbnail
Download:
Table 2. Altered effective connectivity from the left globus pallidus to the other brain regions in low-grade HE patients.

https://doi.org/10.1371/journal.pone.0053677.t002

Effective connectivity from the right globus pallidus

When compared with the healthy controls, low-grade HE patients exhibited significantly decreased effective connectivity from the right globus pallidus to seveal brain regions that included the right ACC, left cuneus, bilateral STG and MFG, and increased effective connectivity to the right precuneus, right MFG, and right parahippocampal gyrus (P<0.05, FDR corrected) (Figure 1B, Table 3).

thumbnail
Download:
Table 3. Altered effective connectivity from the right globus pallidus to the other brain regions in low-grade HE patients.

https://doi.org/10.1371/journal.pone.0053677.t003

Effective connectivity to the left globus pallidus

In patients, several brain regions showed decreased effective connectivity to the left globus pallidus, including the left ACC, right cuneus, middle cingulate cortex, left SFG, left STG and putamen. Moreover, the left supplementary motor area (SMA), left inferior parietal lobule, right MFG, bilateral ITG and cerebellum displayed increased effective connectivity to the left globus pallidus (P<0.05, FDR corrected) (Figure 2A, Table 4).

thumbnail
Download:
Figure 2. Altered effective connectivity from the other brain regions to the globus pallidus in low-grade HE patients (P<0.05, FDR corrected).

The left ACC, right cuneus, MCC, left SFG, left STG and putamen show decreased effective connectivity to the left globus pallidus in patients when compared to healthy controls. Moreover, the left SMA, left IPL, right MFG, bilateral ITG and cerebellum display increased effective connectivity to the left globus pallidus (2A). In patients, the left SFG, bilateral STG, right MTG, and left putamen exhibit decreased effective connectivity to the right globus pallidus. Moreover, the right precuneus, right SMA, bilateral MFG, left MTG, right ITG and bilateral cerebellum demonstrate increased effective connectivity to the right globus pallidus (2B). HE = hepatic encephalopathy; FDR = false discovery rate; ACC = anterior cingulate cortex; MCC = middle cingulate cortex; SFG = superior frontal gyrus; STG = superior temporal gyrus; SMA = supplementary motor area; IPL = inferior parietal lobule; MFG = middle frontal gyrus; ITG = inferior temporal gyrus; MTG = middle temporal gyrus.

https://doi.org/10.1371/journal.pone.0053677.g002

thumbnail
Download:
Table 4. Altered effective connectivity from the other brain regions to the left globus pallidus in low-grade HE patients.

https://doi.org/10.1371/journal.pone.0053677.t004

Effective connectivity to the right globus pallidus

In patients, the left SFG, bilateral STG, right MTG, and left putamen showed decreased effective connectivity to the right globus pallidus. Moreover, the right precuneus, right SMA, bilateral MFG, left MTG, right ITG and bilateral cerebullum exhibited increased effective connectivity to the right globus pallidus (P<0.05, FDR corrected) (Figure 2B, Table 5).

thumbnail
Download:
Table 5. Altered effective connectivity from the other brain regions to the right globus pallidus in low-grade HE patients.

https://doi.org/10.1371/journal.pone.0053677.t005

Correlations results

Pearson correlation analyses revealed that the blood venous ammonia levels of low-grade HE patients negatively correlated with the decreased effective connectivity from the bilateral globus pallidus to the iso-lateral ACC (left side: R = −0.415, P = 0.013; right side: R = −0.392, P = 0.020), and positively correlated with the influence from the right globus pallidus to the right precuneus (R = 0.409, P = 0.015) (Figure 3). In addition, the number connectivity test scores in patients negatively correlated with the effective connectivity from the left globus pallidus to left ACC (R = −0.504, P = 0.003), and from left superior frontal gyrus to left globus pallidus (R = −0.507, P = 0.005). The other regions with aberrant effective connectivity showed no significant correlation with blood ammonia and neuropsychological performances.

thumbnail
Download:
Figure 3. Correlations results between the altered effective connectivity of the globus pallidus and blood venous ammonia and neuropsychological performances in low-grade HE patients (P<0.05, uncorrected).

Pearson correlation analyses reveals that the blood venous ammonia levels of low-grade HE patients negatively correlate with the decreased effective connectivity from the bilateral globus pallidus to the iso-lateral ACC (3A/B), and positively correlate with the influence from the right globus pallidus to the right precuneus (3C). The other regions with aberrant effective connectivity show no correlation with venous blood ammonia levels. In addition, the number connectivity test scores in patients negatively correlated with the effective connectivity from the left globus pallidus to left ACC, and from left superior frontal gyrus to left globus pallidus. HE = hepatic encephalopathy; ACC = anterior cingulate cortex.

https://doi.org/10.1371/journal.pone.0053677.g003

Discussion

The present resting-state fMRI study with Granger causality analysis demonstrated that low-grade HE patients had abnormal directionality of influence both from and to the globus pallidus; moreover, the disturbed connectivity between the globus pallidus and the ACC, as well as the precuneus was associated with the clinical index of HE (blood ammonia, neuropsychological performance). To the best of our knowledge, this is the first study to examine the causality interactions of basal ganglia in HE patients by using fMRI.

Effective connectivity revealed by Granger causality analysis

GCA used in the present study is a method based on multiple linear regression for investigating whether the past value of one time series could correctly predict the current value of another [18]. In the past few years, GCA has been applied on neuroimaging studies to reveal the causal effects among brain regions, first on electroencephalography (EEG) and magnetoencephalography (MEG) data [18], [34], [35], and later on fMRI data [17], [27], [36]. Moreover, there have been only a few reports of GCA in patients including social anxiety disorder [27], Alzheimer's disease [37], and major depressive disorder [36]. Convergent neuroimaging evidence has indicated that many brain regions show abnormal metabolism or activity in HE patients, suggesting there is an alteration of the cortico-striato-thalamic pathway in the HE patients [7], [8]. In addition, a very recent fMRI study demonstrated disrupted functional connectivity between the basic ganglia and many other brain regions in HE patients [14]. However, the causality interactions (effective connectivity) between the basic ganglia and other affected brain regions could not be inferred from that study. This resting-state fMRI study with GCA extends our understanding of the influences of the basic ganglia in the neuropathophysiology of HE.

Disrupted effective connectivity from and to the globus pallidus

This study revealed mutually decreased influence between the globus pallidus and the ACC, cuneus, which was partically supported by the negative correlation between the influence from the globus pallidus to the ACC. ACC is regarded as a core region to be involved in attention which severs to regular both cognitive and emotional processing [38]. Interestingly, attention defect is a fundamental aspect of HE [3], [4], and abnormal ACC function in cirrhotic patients with or without HE had been widespreadly reported by studies using PET [39], fMRI [40], and MRS [41]. In particular, a recent resting-state fMRI exhibited a decreased functional connectivity between the basal ganglia and the ACC [14]. That study focused on the temporal synchrony or correlation between the basal ganlia and the ACC, our study emphasized the effective influence between them. In addition, one recent study [42] showed that the transjugular intrahepatic portosystemic shunt (TIPS) had different short-term and long-term effects on cirrhotic patients' ACC activity, and the changes of ACC activity might have a potential role in predicting the development of postsurgical HE. Taken together, these findings of abnormal ACC function may account for the attentional and cognitive deregulation in HE patients.

Cuneus is regarded as a core region to visual and spatial attention, and inhibitory control [43]. Zafiris et al. reported an impaired neural interaction in early HE patients between the cuneus and many other brain regions in a task-driven fMRI study by using critical flicker frequency (CFF) test [44]. Ni et al. found that the cuneus exhibited decreased brain regional homogeneity (ReHo) in MHE patients, and the decreased ReHo values were associated with their neuropsychological performances [45]. A recent rs-fMRI study by Zhang et al. also showed a decreased functional connectivity between the pallidum and the cuneus [14]. So our finding of decreased influence between the globus pallidus and the cuneus is complementary to previous studies, the findings in all these studies indicate that the disturbance of basal ganglia-cuneus may partially mediate the attention and control dysfunction in HE patients [14].

We also found mutually increased influence between the globus pallidus and the precuneus. Precuneus is thought to be engaged in visuo-spatial imagery, episodic memory retrieval and self-processing operations [46]. Chen et al. reported an increased regional activity in precunecus in MHE patients in a rs-fMRI study with regional homogeneity measurement [47]. In addition, Qi et al. recently demonstrated an increased functional conectivity of the precunecus with other brain regions within the brian default mode network in a fMRI study in 14 MHE patients [40], and speculated it as a compensatory phenomenon in the early stage of HE. The increased influence from pallidum to precunecus in this study may also be interpreted as a compensatory mechanism, which needs to be confirmed in further studies. Our finding of positive correlation between the effective connection from pallidum to the precunecus and blood ammonia levels also supported this compensatory hypothesis.

In this study, we also found decreased influence from the globus pallidus to the right inferior temporal gyrus (ITG), and left superior temporal gyrus (STG). The ITG is involved in visual perception [48], and the STG subserves language processes [49]. A decreased interaction between globus pallidus and these regions may account for the impaired visual function which is often reported in the early stage of HE [4], [44]. Furthermore, increased influences from pallidum to bilateral middle frontal gyri and parahippocampal gyri were found in our study. Parahippocampal gyrus is a part of the limbic system, for which one previous PET study has showed an increased regional cerebral blood flow (rCBF) in early HE patients [50]. This perfusion increase in the limbic circuit was interepted as a compensatory response to premptor, motor, and complex attentional deficts in patients [50]. Validity of our findings and whether the increased influence indicated a compensatory mechanism need to be confirmed in further studies.

As for the influence from other brain regions to the pallidum in the present study, many frontal and temporal gyri, and cerebellum were found to show either decreased or increased influence to the globus pallidus, similar to the findings in prior neuroimaging studies of HE [9], [51]. We speculated that in the early phase of HE, there co-exist both impairment and compensatory mechanisms, which was also suggested in a previous fMRI study [40]. Our findings added important insights into understanding the brain functional impairment in early HE patients.

Our study has some limitations. First, the meaning of the GCA in the resting-state fMRI is not fully understood, and there are some shortages of GCA, e.g., during the process of GCA, spurious influences might be obtained because of the variability in the shape and latency of hemodynamic response functions (HRFs) in different brain regions and different subjects [52], and the slow dynamics of the BOLD signal (2 s used here) may induce some missing of rapid causal influences [53]. However, many scholars believe it reflects time-directed influence between a seed region and the rest of the brain [18], [36], [53]. To elucidate the association between effective connectivity and neuronal activity, further study combing fMRI and electrophysiology is needed to perform. Secondly, potential effects of medication such as diuretics for controlling ascites in some patients, might have an affect on the statistical analysis and results of this study. Further studies in a larger population with better treatment controls are needed to verify these findings. In addition, we only select the globus pallidus as seed region within the basal ganglia, the results could not extended to other basal ganglia regions, e.g., putamen, caudate, and substantial nigra. Thirdly, the study include only low-grade HE patients, the classification from low-grade to high-grade HE should be clarified in further studies. Finally, in this preliminary study investigating the influence between regions in HE patients, a lenient significnat threshold was used in the correlation analysis between effectivity and clinical index. This correlation analysis was exploratory in nature, and a stricter threshold should be used in the further studis.

Conclusions

In conclusion, we detected disrupted effective connectivity network of basal ganglia in low-grade HE patients, with abnormal directionality of influence both from and to the globus pallidus. Our findings may help to understand the neurophysiological mechanisms underlying the HE.

Author Contributions

Conceived and designed the experiments: RQ LJZ GL. Performed the experiments: RQ LN. Analyzed the data: RQ LN. Contributed reagents/materials/analysis tools: JZ ZZ QJ WL GZ. Wrote the paper: RQ.

References

  1. 1. Munoz SJ (2008) Hepatic encephalopathy. Med Clin North Am 92: 795–812, viii.
  2. 2. Groeneweg M, Quero JC, De Bruijn I, Hartmann IJ, Essink-bot ML, et al. (1998) Subclinical hepatic encephalopathy impairs daily functioning. Hepatology 28: 45–49.
  3. 3. Bajaj JS, Hafeezullah M, Hoffmann RG, Varma RR, Franco J, et al. (2008) Navigation skill impairment: Another dimension of the driving difficulties in minimal hepatic encephalopathy. Hepatology 47: 596–604.
  4. 4. Bajaj JS (2008) Minimal hepatic encephalopathy matters in daily life. World J Gastroenterol 14: 3609–3615.
  5. 5. Prasad S, Dhiman RK, Duseja A, Chawla YK, Sharma A, et al. (2007) Lactulose improves cognitive functions and health-related quality of life in patients with cirrhosis who have minimal hepatic encephalopathy. Hepatology 45: 549–559.
  6. 6. Sidhu SS, Goyal O, Mishra BP, Sood A, Chhina RS, et al. (2011) Rifaximin improves psychometric performance and health-related quality of life in patients with minimal hepatic encephalopathy (the RIME Trial). Am J Gastroenterol 106: 307–316.
  7. 7. Giewekemeyer K, Berding G, Ahl B, Ennen JC, Weissenborn K (2007) Bradykinesia in cirrhotic patients with early hepatic encephalopathy is related to a decreased glucose uptake of frontomesial cortical areas relevant for movement initiation. J Hepatol 46: 1034–1039.
  8. 8. Lockwood AH, Yap EW, Rhoades HM, Wong WH (1991) Altered cerebral blood flow and glucose metabolism in patients with liver disease and minimal encephalopathy. J Cereb Blood Flow Metab 11: 331–336.
  9. 9. McPhail MJ, Patel NR, Taylor-Robinson SD (2012) Brain imaging and hepatic encephalopathy. Clin Liver Dis 16: 57–72.
  10. 10. Keiding S, Sorensen M, Bender D, Munk OL, Ott P, et al. (2006) Brain metabolism of 13N-ammonia during acute hepatic encephalopathy in cirrhosis measured by positron emission tomography. Hepatology 43: 42–50.
  11. 11. Fox MD, Raichle ME (2007) Spontaneous fluctuations in brain activity observed with functional magnetic resonance imaging. Nat Rev Neurosci 8: 700–711.
  12. 12. Zhang HY, Wang SJ, Liu B, Ma ZL, Yang M, et al. (2010) Resting brain connectivity: changes during the progress of Alzheimer disease. Radiology 256: 598–606.
  13. 13. Zang YF, He Y, Zhu CZ, Cao QJ, Sui MQ, et al. (2007) Altered baseline brain activity in children with ADHD revealed by resting-state functional MRI. Brain Dev 29: 83–91.
  14. 14. Zhang LJ, Zheng G, Zhang L, Zhong J, Wu S, et al. (2012) Altered Brain Functional Connectivity in Patients with Cirrhosis and Minimal Hepatic Encephalopathy: A Functional MR Imaging Study. Radiology 265: 528–536.
  15. 15. Parent A, Hazrati LN (1995) Functional anatomy of the basal ganglia. I. The cortico-basal ganglia-thalamo-cortical loop. Brain Res Brain Res Rev 20: 91–127.
  16. 16. Spahr L, Burkhard PR, Grotzsch H, Hadengue A (2002) Clinical significance of basal ganglia alterations at brain MRI and 1H MRS in cirrhosis and role in the pathogenesis of hepatic encephalopathy. Metab Brain Dis 17: 399–413.
  17. 17. Jiao Q, Lu G, Zhang Z, Zhong Y, Wang Z, et al. (2011) Granger causal influence predicts BOLD activity levels in the default mode network. Hum Brain Mapp 32: 154–161.
  18. 18. Stephan KE, Roebroeck A (2012) A short history of causal modeling of fMRI data. Neuroimage 62: 856–863.
  19. 19. Ding L, Worrell GA, Lagerlund TD, He B (2006) Spatio-temporal source localization and Granger causality in ictal source analysis. Conf Proc IEEE Eng Med Biol Soc 1: 3670–3671.
  20. 20. Ferenci P, Lockwood A, Mullen K, Tarter R, Weissenborn K, et al. (2002) Hepatic encephalopathy–definition, nomenclature, diagnosis, and quantification: final report of the working party at the 11th World Congresses of Gastroenterology, Vienna, 1998. Hepatology 35: 716–721.
  21. 21. Kappus MR, Bajaj JS (2012) Covert hepatic encephalopathy: not as minimal as you might think. Clin Gastroenterol Hepatol 10: 1208–1219.
  22. 22. Bajaj JS, Cordoba J, Mullen KD, Amodio P, Shawcross DL, et al. (2011) Review article: the design of clinical trials in hepatic encephalopathy–an International Society for Hepatic Encephalopathy and Nitrogen Metabolism (ISHEN) consensus statement. Aliment Pharmacol Ther 33: 739–747.
  23. 23. Qi R, Zhang L, Wu S, Zhong J, Zhang Z, et al. (2012) Altered Resting-State Brain Activity at Functional MR Imaging during the Progression of Hepatic Encephalopathy. Radiology 264: 187–195.
  24. 24. Pugh RN, Murray-Lyon IM, Dawson JL, Pietroni MC, Williams R (1973) Transection of the oesophagus for bleeding oesophageal varices. Br J Surg 60: 646–649.
  25. 25. Weissenborn K, Ennen JC, Schomerus H, Ruckert N, Hecker H (2001) Neuropsychological characterization of hepatic encephalopathy. J Hepatol 34: 768–773.
  26. 26. Liao W, Chen H, Feng Y, Mantini D, Gentili C, et al. (2010) Selective aberrant functional connectivity of resting state networks in social anxiety disorder. Neuroimage 52: 1549–1558.
  27. 27. Liao W, Qiu C, Gentili C, Walter M, Pan Z, et al. (2010) Altered effective connectivity network of the amygdala in social anxiety disorder: a resting-state FMRI study. PLoS One 5: e15238.
  28. 28. Fox MD, Snyder AZ, Vincent JL, Corbetta M, Van Essen DC, et al. (2005) The human brain is intrinsically organized into dynamic, anticorrelated functional networks. Proc Natl Acad Sci U S A 102: 9673–9678.
  29. 29. Song XW, Dong ZY, Long XY, Li SF, Zuo XN, et al. (2011) REST: a toolkit for resting-state functional magnetic resonance imaging data processing. PLoS One 6: e25031.
  30. 30. Maldjian JA, Laurienti PJ, Kraft RA, Burdette JH (2003) An automated method for neuroanatomic and cytoarchitectonic atlas-based interrogation of fMRI data sets. Neuroimage 19: 1233–1239.
  31. 31. Tang L, Ge Y, Sodickson DK, Miles L, Zhou Y, et al. (2011) Thalamic resting-state functional networks: disruption in patients with mild traumatic brain injury. Radiology 260: 831–840.
  32. 32. Zang ZX, Yan CG, Dong ZY, Huang J, Zang YF (2012) Granger causality analysis implementation on MATLAB: a graphic user interface toolkit for fMRI data processing. J Neurosci Methods 203: 418–426.
  33. 33. Granger CWJ (1969) Investigating causal relations by econometric models and cross-spectral methods. Econometrica 37: 424–438.
  34. 34. Brovelli A, Ding M, Ledberg A, Chen Y, Nakamura R, et al. (2004) Beta oscillations in a large-scale sensorimotor cortical network: directional influences revealed by Granger causality. Proc Natl Acad Sci U S A 101: 9849–9854.
  35. 35. Hesse W, Moller E, Arnold M, Schack B (2003) The use of time-variant EEG Granger causality for inspecting directed interdependencies of neural assemblies. J Neurosci Methods 124: 27–44.
  36. 36. Hamilton JP, Chen G, Thomason ME, Schwartz ME, Gotlib IH (2011) Investigating neural primacy in Major Depressive Disorder: multivariate Granger causality analysis of resting-state fMRI time-series data. Mol Psychiatry 16: 763–772.
  37. 37. Miao X, Wu X, Li R, Chen K, Yao L (2011) Altered connectivity pattern of hubs in default-mode network with Alzheimer's disease: an Granger causality modeling approach. PLoS One 6: e25546.
  38. 38. Bush G, Luu P, Posner MI (2000) Cognitive and emotional influences in anterior cingulate cortex. Trends Cogn Sci 4: 215–222.
  39. 39. Lockwood AH, Murphy BW, Donnelly KZ, Mahl TC, Perini S (1993) Positron-emission tomographic localization of abnormalities of brain metabolism in patients with minimal hepatic encephalopathy. Hepatology 18: 1061–1068.
  40. 40. Qi R, Zhang LJ, Xu Q, Zhong J, Wu S, et al. (2012) Selective impairments of resting-state networks in minimal hepatic encephalopathy. PLoS One 7: e37400.
  41. 41. Zhang LJ, Lu GM, Yin JZ, Qi J (2010) Metabolic changes of anterior cingulate cortex in patients with hepatic cirrhosis: A magnetic resonance spectroscopy study. Hepatol Res 40: 777–785.
  42. 42. Qi R, Zhang LJ, Zhong J, Wu S, Zhang Z, et al. (2012) Dynamic changes of intrinsic brain activity in cirrhotic patients after transjugular intrahepatic portosystemic shunt: a resting-state FMRI study. PLoS One 7: e46681.
  43. 43. Qiu YW, Han LJ, Lv XF, Jiang GH, Tian JZ, et al. (2011) Regional homogeneity changes in heroin-dependent individuals: resting-state functional MR imaging study. Radiology 261: 551–559.
  44. 44. Zafiris O, Kircheis G, Rood HA, Boers F, Haussinger D, et al. (2004) Neural mechanism underlying impaired visual judgement in the dysmetabolic brain: an fMRI study. Neuroimage 22: 541–552.
  45. 45. Ni L, Qi R, Zhang LJ, Zhong J, Zheng G, et al. (2012) Altered regional homogeneity in the development of minimal hepatic encephalopathy: a resting-state functional MRI study. PLoS One 7: e42016.
  46. 46. Cavanna AE, Trimble MR (2006) The precuneus: a review of its functional anatomy and behavioural correlates. Brain 129: 564–583.
  47. 47. Chen HJ, Zhu XQ, Yang M, Liu B, Zhang Y, et al. (2012) Changes in the regional homogeneity of resting-state brain activity in minimal hepatic encephalopathy. Neurosci Lett 507: 5–9.
  48. 48. Scheff SW, Price DA, Schmitt FA, Scheff MA, Mufson EJ (2011) Synaptic loss in the inferior temporal gyrus in mild cognitive impairment and Alzheimer's disease. J Alzheimers Dis 24: 547–557.
  49. 49. Bigler ED, Mortensen S, Neeley ES, Ozonoff S, Krasny L, et al. (2007) Superior temporal gyrus, language function, and autism. Dev Neuropsychol 31: 217–238.
  50. 50. Catafau AM, Kulisevsky J, Bernà L (1999) Relationship between cerebral perfusion in frontal-limbic-basal ganglia circuits and neuropsychologic impairment in patients with subclinical hepatic encephalopathy. J Nucl Med 41: 405–410.
  51. 51. Zhang LJ, Zhong J, Lu GM (2012) Multimodality MR Imaging Findings of Low-Grade Brain Edema in Hepatic Encephalopathy. AJNR Am J Neuroradiol
  52. 52. Roebroeck A, Formisano E, Goebel R (2005) Mapping directed influence over the brain using Granger causality and fMRI. Neuroimage 25: 230–242.
  53. 53. Bressler SL, Seth AK (2011) Wiener-Granger causality: a well established methodology. Neuroimage 58: 323–329.