Most of the resting-state functional magnetic resonance imaging (fMRI) studies demonstrated the correlations between spatially distinct brain areas from the perspective of functional connectivity or functional integration. The functional connectivity approaches do not directly provide information of the amplitude of brain activity of each brain region within a network. Alternatively, an index named amplitude of low-frequency fluctuation (ALFF) of the resting-state fMRI signal has been suggested to reflect the intensity of regional spontaneous brain activity. However, it has been indicated that the ALFF is also sensitive to the physiological noise. The current study proposed a fractional ALFF (fALFF) approach, i.e., the ratio of power spectrum of low-frequency (0.01-0.08 Hz) to that of the entire frequency range and this approach was tested in two groups of resting-state fMRI data. The results showed that the brain areas within the default mode network including posterior cingulate cortex, precuneus, medial prefrontal cortex and bilateral inferior parietal lobule had significantly higher fALFF than the other brain areas. This pattern was consistent with previous neuroimaging results. The non-specific signal components in the cistern areas in resting-state fMRI were significantly suppressed, indicating that the fALFF approach improved the sensitivity and specificity in detecting spontaneous brain activities. Its mechanism and sensitivity to abnormal brain activity should be evaluated in the future studies.

The default mode network (DMN) in humans has been suggested to support a variety of cognitive functions and has been implicated in an array of neuropsychological disorders. However, its function (s) remains poorly understood. We show that rats possess a DMN that is broadly similar to the DMNs of nonhuman primates and humans. Our data suggest that, despite the distinct evolutionary paths between rodent and primate brain, a well-organized, intrinsically coherent DMN appears to be a fundamental feature in the mammalian brain whose primary functions might be to integrate multimodal sensory and affective information to guide behavior in anticipation of changing environmental contingencies.functional MRI | resting state | intrinsic activity | connectivity | spontaneous fluctuation I n the absence of an immediate need for goal-directed attention to the surrounding environment, our minds wander from recollection of past happenings to imagination of future events. Neuroimaging studies have consistently identified a set of interconnected brain areas that becomes less active during attentiondemanding cognitive tasks (1). This so-called default mode network (DMN) is posited to play a fundamental role in brain organization and supports a variety of self-referential functions such as understanding others' mental state, recollection and imagination (2), conceptual processing (3), and even in the sustenance of conscious awareness (4). Many of these functions have been considered to be unique to humans. Intriguingly, similar coherent structures have been shown to exist in anesthetized macaque monkeys and chimpanzees (5, 6). Furthermore, the functions of the default network are disrupted in such neuropsychological disorders as schizophrenia, Alzheimer's disease, and autism (7-9), underscoring the clear and critical need for further investigating the neurobiological basis of DMN using animal models.The evolutionary clade of rodents is about 35 million years earlier than that of old world monkeys and about 60 million years earlier than humans (10). Although many of the structures and functions of subcortical nuclei are conserved across these three species, the neocortex, in particular the "association" cortex, has extensively expanded in the primate as a result of evolutionary pressure, which is considered to be crucial in the development of higher cognitive and behavioral functions (10, 11). On the other hand, such structures as cingulate cortex, prefrontal cortex, and hippocampal formation, all of which are critical elements of the DMN, are also present in rodents (11). Given the distant evolutionary paths between rodent and primate brain, an intriguing question arises: Does the rat possess a similar DMN? Such a network, once demonstrated, would not only suggest that an operational DMN is a common feature in the mammalian brain, perhaps induced via parallel evolution as a result of natural selection, it would also offer a novel platform to explore the physiological basis and behavioral significance of the DMN. Such a demonstratio...

Human brain functional networks contain a few densely connected hubs that play a vital role in transferring information across regions during resting and task states. However, the relationship of these functional hubs to measures of brain physiology, such as regional cerebral blood flow (rCBF), remains incompletely understood. Here, we used functional MRI data of blood-oxygenation-level-dependent and arterial-spin-labeling perfusion contrasts to investigate the relationship between functional connectivity strength (FCS) and rCBF during resting and an N-back working-memory task. During resting state, functional brain hubs with higher FCS were identified, primarily in the default-mode, insula, and visual regions. The FCS showed a striking spatial correlation with rCBF, and the correlation was stronger in the default-mode network (DMN; including medial frontal-parietal cortices) and executive control network (ECN; including lateral frontal-parietal cortices) compared with visual and sensorimotor networks. Moreover, the relationship was connection-distance dependent; i.e., rCBF correlated stronger with long-range hubs than short-range ones. It is notable that several DMN and ECN regions exhibited higher rCBF per unit connectivity strength (rCBF/FCS ratio); whereas, this index was lower in posterior visual areas. During the working-memory experiment, both FCS-rCBF coupling and rCBF/FCS ratio were modulated by task load in the ECN and/or DMN regions. Finally, task-induced changes of FCS and rCBF in the lateral-parietal lobe positively correlated with behavioral performance. Together, our results indicate a tight coupling between blood supply and brain functional topology during rest and its modulation in response to task demands, which may shed light on the physiological basis of human brain functional connectome.fMRI | connectomics | graph theory | modularity | metabolism T he human brain is a complex network that supports efficient communication through a collection of interconnected brain units, i.e., nodes (1, 2). Within the brain network, most nodes have few connections, but a few so-called hub nodes have a large number of connections (3-5). Graph-theory analysis of both human structural and functional connectivity data has revealed that these brain hubs are located predominantly in the posterior cingulate cortex/precuneus (PCC/PCu), medial-prefrontal cortex (mPFC), and lateral temporal and parietal cortices (4-8). Most of these brain regions constitute the putative default-mode network (DMN) that exhibits a high level of metabolism at rest (9). The spatial similarity between connectivity hubs and metabolism distribution suggests a relationship between intrinsic network connectivity and metabolic demands of the human brain.Brain metabolism includes oxidative phosphorylation, which consumes most of the glucose and produces most of the energy, and aerobic glycolysis, which accounts for a much smaller portion of the consumed glucose but is critical to a number of cellular functions (10). It has been shown that regio...