Changwei W. Wu

Static and dynamic characteristics of cerebral blood flow during the resting state.

In this study, the static and dynamic characteristics of cerebral blood flow (CBF) in the resting state were investigated using an arterial spin labeling (ASL) perfusion imaging technique. Consistent with previous PET results, static CBF measured by ASL was significantly higher in the posterior cingulate cortex (PCC), thalamus, insula/superior temporal gyrus (STG) and medial prefrontal cortex (MPFC) than the average CBF of the brain. The dynamic measurement of CBF fluctuations showed high correlation (functional connectivity) between components in the default mode network. These brain regions also had high local temporal synchrony and high fluctuation amplitude, as measured by regional homogeneity (ReHo) and amplitude of low-frequency fluctuation (ALFF) analyses. The spatial pattern of the static CBF correlated well with that of the dynamic indices. The high static and dynamic activities in the PCC, MPFC, insula/STG and thalamus suggest that these regions play a vital role in maintaining and facilitating fundamental brain functions.

Aging Effects on the Activation of the Auditory Cortex during Binaural Speech Listening in White Noise: An fMRI Study

The functional significance of age-related pathology of the auditory cortex is not well established. The purpose of this study was to elucidate the activation pattern of the auditory cortex in aged subjects in response to speech signals. Functional magnetic resonance imaging was performed on 12 elderly subjects with normal hearing acuity during selective listening with both ears to speech sounds in quiet and in white noise. Twelve young, normal-hearing subjects served as controls. Our results showed that activation of the auditory cortex during selective listening to speech decreased in elderly subjects compared to young subjects, especially in noise. Reduced activation occurred in the anterior and posterior regions of the bilateral superior temporal gyrus (STG), but mainly in the posterior part of the left STG. In addition, background noise had a greater masking effect on speech perception in the elderly subjects than in the young ones. These findings suggest that early functional changes associated with central presbycusis occur mainly in the posterior part of the left STG.

Impaired small-world network efficiency and dynamic functional distribution in patients with cirrhosis

Hepatic encephalopathy (HE) is a complex neuropsychiatric syndrome and a major complication of liver cirrhosis. Dysmetabolism of the brain, related to elevated ammonia levels, interferes with intercortical connectivity and cognitive function. For evaluation of network efficiency, a ‘small-world’ network model can quantify the effectiveness of information transfer within brain networks. This study aimed to use small-world topology to investigate abnormalities of neuronal connectivity among widely distributed brain regions in patients with liver cirrhosis using resting-state functional magnetic resonance imaging (rs-fMRI). Seventeen cirrhotic patients without HE, 9 with minimal HE, 9 with overt HE, and 35 healthy controls were compared. The interregional correlation matrix was obtained by averaging the rs-fMRI time series over all voxels in each of the 90 regions using the automated anatomical labeling model. Cost and correlation threshold values were then applied to construct the functional brain network. The absolute and relative network efficiencies were calculated; quantifying distinct aspects of the local and global topological network organization. Correlations between network topology parameters, ammonia levels, and the severity of HE were determined using linear regression and ANOVA. The local and global topological efficiencies of the functional connectivity network were significantly disrupted in HE patients; showing abnormal small-world properties. Alterations in regional characteristics, including nodal efficiency and nodal strength, occurred predominantly in the association, primary, and limbic/paralimbic regions. The degree of network organization disruption depended on the severity of HE. Ammonia levels were also significantly associated with the alterations in local network properties. Results indicated that alterations in the rs-fMRI network topology of the brain were associated with HE grade; and that focal or diffuse lesions disturbed the functional network to further alter the global topology and efficiency of the whole brain network. These findings provide insights into the functional changes in the human brain in HE.

Empirical evaluations of slice-timing, smoothing, and normalization effects in seed-based, resting-state functional magnetic resonance imaging analyses.

Recently, functional connectivity analyses using spontaneous functional magnetic resonance imaging (fMRI) fluctuations have been applied in the context of neurological and psychiatry diseases. In the analyses procedure, preprocessing steps are commonly utilized in exploring functional connectivity, the same strategy as what was conducted in the fMRI process. However, the effectiveness of these preprocessing steps on resting-state fMRI (rs-fMRI) was rarely investigated, and the significance of preprocessing steps on rs-fMRI needs to be studied. Therefore, the main purpose of the current study was to evaluate the effects of multiple preprocessing procedures, including slice-timing correction, smoothing, and spatial normalization, on rs-fMRI signal. Through a seed-based correlation analysis on the motor network, we empirically estimated three indices of spontaneous fMRI fluctuations induced: correlation coefficients (CC), amplitude of low-frequency fluctuations (ALFFs), and fractional ALFF (fALFF), with different strategies of three preprocessing steps. Multiple repetition times (TRs = 2, 3, 4 sec) were also compared to address the issue of temporal mismatch. In the temporal preprocess, we found that the use of slice-timing correction and different TRs had minimal effects on CC and fALFF. However, ALFF was significantly affected using different TR but not affected by slice-timing correction as well. In the spatial preprocess, fALFF was insensitive to both smoothing and normalization. Smoothing consistently increased spatial extents and CC, but suppressed ALFF values. Performing normalization before index calculations provided better spatial sensitivity with larger variability in ALFF, whereas performing normalization after index calculations might preserve the ALFF level as in the unnormalized data. Conclusively, the effects of choosing preprocessing parameters and strategies were presented in the current study, providing practical considerations when conducting rs-fMRI analyses.

Connectivity of Default-Mode Network Is Associated with Cerebral Edema in Hepatic Encephalopathy

Cerebral edema, a well-known feature of acute liver disease, can occur in cirrhotic patients regardless of hepatic encephalopathy (HE) and adversely affect prognosis. This study characterized and correlated functional HE abnormalities in the brain to cerebral edema using resting-state functional magnetic resonance imaging (rs-fMRI) and diffusion tensor imaging (DTI). Forty-one cirrhotic patients (16 without HE, 14 minimal HE, 11 overt HE) and 32 healthy controls were assessed. The HE grade in cirrhotic patients was evaluated by the West Haven criteria and neuro-psychological examinations. Functional connectivity correlation coefficient (fc-CC) of the default mode network (DMN) was determined by rs-fMRI, while the corresponding mean diffusivity (MD) was obtained from DTI. Correlations among inter-cortical fc-CC, DTI indices, Cognitive Ability Screening Instrument scores, and laboratory tests were also analyzed. Results showed that gradual reductions of HE-related consciousness levels, from “without HE” or “minimal HE” to “overt HE”, correlated with decreased anterior-posterior fc-CC in DMN [F(4.415), p = 0.000)]. The MD values from regions with anterior-posterior fc-CC differences in DMN revealed significant differences between the overt HE group and other groups. Increased MD in this network was inversely associated with decreased fc-CC in DMN and linearly correlated with poor cognitive performance. In conclusion, cerebral edema can be linked to altered cerebral temporal architecture that modifies both within- and between-network connectivity in HE. Reduced fc-CC in DMN is associated with behavior and consciousness deterioration. Through appropriate targets, rs-fMRI technology may provide relevant supplemental information for monitoring HE and serve as a new biomarker for clinical diagnosis.

Variations in Connectivity in the Sensorimotor and Default-Mode Networks During the First Nocturnal Sleep Cycle

The function of sleep in humans has been investigated using simultaneous electroencephalography (EEG) and functional magnetic resonance imaging recordings to provide accurate sleep scores with spatial precision. Recent studies have demonstrated that spontaneous brain oscillations and functional connectivity dissociate during nonrapid eye movement (NREM) sleep; this leads to spontaneous cognitive processes, such as memory consolidation and emotional modulation. However, variations in network connectivity across the sleep stages or between sleep/wake transitions require further elucidation. We observed changes in the connectivity of the sensorimotor and default-mode networks (DMN) mediated by midnight sleep among 18 healthy participants. The results indicated that (1) functional connectivity in both networks showed increasing dissociation as NREM sleep deepened, whereas hyperconnectivity occurred during rapid eye movement (REM) sleep; and (2) compared with connectivity before sleep, the DMN presented a comparable connectivity pattern immediately after awakening, whereas the connectivity of the sensorimotor network remained disrupted. These findings showed that connectivity patterns dissociate and reconnect coherently in both cortical networks during NREM and REM sleep, respectively. After the person awakened, the DMN connectivity was re-established before the sensorimotor reconnection. These dynamic sleep-related dissociations and reconnections between sleep/wake conditions might provide the key to understanding cognitive modulations in sleep. If so, connectivity changes might serve as an alternative indicator beyond the EEG signature to unveil the spontaneous processes that occur during sleep.

Vascular space occupancy-dependent functional MRI by tissue suppression†

To measure the cerebral blood volume (CBV) dynamics during neural activation, a novel technique named vascular space occupancy (VASO)‐based functional MRI (fMRI) was recently introduced for noninvasive CBV detection. However, its application is limited because of its low contrast‐to‐noise ratio (CNR) due to small signal change from the inverted blood.

Hemispheric Difference in Activation Patterns of Human Auditory-Associated Cortex: An fMRI Study

The purpose of this study was to demonstrate interhemispheric differences in activation patterns of the auditory-associated cortex elicited by a series of sounds. Functional magnetic resonance imaging was performed while different sounds were presented binaurally to 10 healthy subjects with normal hearing. Characteristic activation patterns were elicited although variability was shown between subjects. The activation number was significantly higher on the right than on the left side for non-speech (but not for speech) stimulations. As stimulation increased in complexity (from a pure tone to white noise to ocean wave sounds to classical music), the activation pattern of the superior temporal lobe became more pronounced in both hemispheres while that of the auditory cortex tended to become more sustained and concentrated on the right rather than on the left side. No hemispheric differences in activation pattern were seen in response to speech.

The effects of masking on the activation of auditory-associated cortex during speech listening in white noise

Conclusions. Noise-induced masking has different effects on the two hemispheres during speech listening. Auditory-associated cortices in the left hemisphere were more affected by masking than the right side. However, activation of primary and secondary auditory cortices was not affected in both sides under the masking with high signal to noise ratio. Objectives. The purpose of this study was to investigate the effects of masking on the central auditory system during speech listening in white noise. Materials and methods. Twelve healthy young subjects with normal hearing participated in this study. Functional magnetic resonance imaging (fMRI) was performed while subjects were listening to speech sounds alone and speech plus white noise binaurally. Results. In humans, the activation of several regions including the middle parts of the superior and middle temporal gyri, parahippocampal gyrus, cuneus and thalamus of the left hemisphere was significantly reduced under the masking paradigm with +5 dB signal to noise ratio. In addition, reduced activation was also found at the lingual gyrus, anterior and middle parts of the superior temporal gyrus (STG), uncus, fusiform gyrus, and inferior frontal gyrus of the right hemisphere during masking.

Resting-state functional magnetic resonance imaging analysis with seed definition constrained by regional homogeneity

Researchers have recently focused their attention on the intrinsic functional connectivity (FC) in the brain using resting-state functional magnetic resonance imaging. Seed-based correlation analysis (SCAC), which correlates a predefined seed region with other voxels in the brain, is a common index for FC. However, definition of seed sizes and locations was ambiguous in previous studies and this may lead to spurious results for people with a unique functional anatomy. To address this issue, this study proposes a novel method (SCAReHo) that provides a data-driven seed selection (including sizes and locations) method by incorporating regional homogeneity (ReHo) in the SCAC method. The disparities between SCAC and SCAReHo methods among 12 healthy participants were evaluated in the FC of default mode network (DMN), task-positive network (TPN), and amygdala network. The SCAReHo method bypasses the seed-selection ambiguity and enhances the sensitivity in detecting FC of the DMN, TPN, and amygdala network. This study suggests that the SCAReHo method improves the sensitivity of FC analysis and reduces the uncertainty of seed selection. Thus, this method may be particularly useful for psychiatric and neurological investigations.