A FMRI Study Of Deactivation And Default Mode Network Activity In Human Brain

Currently, functional Neuroimaging studies typically focus on the specific task-related increase in brain activity by subtracting a reference state from activated state and signified these changes by using the term "activation". However, some researchers have noted that when performing a reverse subtracting, generally, reference condition subtracting task condition, there is still fMRI signals can be found in certain brain regions. Which are commonly referred as "deactivation" in the context of functional brain imaging. Deactivations can roughly be classed into two sorts, one is task dependent, another is task-independent. The former had been shown to be task specific, and, therefore, to vary in location depending on the demands of the task. The later seemed to be largely task independent, varying little in their location across a wide range of cognitive tasks, for example, language processing, memory and attention task can activate the similar deactivation brain areas in the medial frontal, medial parietal, and posterior cingulate cortex. The consistent with which certain brain areas show deactivation across different tasks, stimuli, and imaging modalities and these areas have a high resting metabolism suggest there might be an organized mode of brain function, which is originally termed "default mode brain networks" activity by Raichle M.E. Numerous functional brain imaging studies have provided the evidences for supporting the this hypothesis. Though the function implication of this brain network is poorly understood, it has been widely presumed that the networks would be closely involved in monitoring external and internal environment, reviewing past knowledge, and episodic memory processing. In addition, it has been suggested the deactivation degree of this networks is strongly associated with the processing difficulty of the external cognitive task, with the more cognitive-demands of the task, the more great deactivation intensity is. This hypothesis has crucial implication for understanding the neural correlates of human consciousness and for deeply interpreting the fMRI results which commonly obtained by subtracting strategies.This study mainly to investigate:(1) the generality of default mode brain networks ( whether the anxiety patient or moderately to severely hypertension patient share the same network as the healthy subjects);(2) the relationship between deactivation amplitude and cognition demanding of the stimuli task.(3) the fMRI response trait of this network under some pathological condition.(4)the possible neuropsychological functions that are instantiated in the networks.All the manuscript consists of three parts. In the first part, we mainly focus onliterature and theory investigations. We reviewed the conception of deactivation, the probable physiological mechanism underling the deactivation and the relationship between deactivation and default mode brain networks, further, we emphasize the research background of default mode brain networks and its significant.In the second part, we are primary to examine whether there is a changed default mode networks in anxiety patient compared with healthy control subjects. In addition , we explored the specific activation patterns which induced by emotionally neutral or threat-related words stimuli. The subjects underwent two fMRI scans. In experimentl, subjects listened actively to emotionally neutral words alternating with no words. In experiment2, subjects listened to threat-related words alternating with emotionally neutral words. Our fMRI data revealed there is a remarkably similar pattern of regions showing deactivation in experiment 1 both for patient and healthy control. In experiment2, the similar deactivation regions can only be found for patient group and no significant deactivation brain regions have been found for health controls. The observed deactivation regions were nearly overlapping with the default mode brain networks. The related areas include medial prefrontal cortex (MPFC ,BA l(k BA24 / 32) , posterior cingulate (PC ,BA 31 / 30) and bilateral inferior parietal cortex (BA 39 / 40).The BOLD response time course was similar for both of patients group and health group, however, the mean t values in the MPFC area was significantly higher for control group than that of patients control, contrastingly, the mean t values in the PCC area was significantly higher for patient control than that of control group. Our findings suggest the anxiety patient may exist an altered default model network in brain during a eyes closed resting or passive task. Further more, the dysfunction of MPFC and PCC may play an important role in neural circuits of the psychopathology in anxiety patient.In the third part, we are mainly to explore default model network characteristic of the patient with moderately to severely hypertension but having no behavioral cognition impairment. All subjects performed two fMRI scans. The subjects be told to listen passively the noise sound from the MR scanners during resting state, which used as control task. In experiment 1,subjects were demanded tolisten actively to the no meaning words (pseudowords) from the ear phone. In experiment 2, subjects should listen actively to real noun words and make the valence(abstract or concrete) of the words in silence. Our fMRI demonstrated the activated patterns and deactivated patterns were nearly similarity for patients and controls in experiment 1 and experiment2, respectively. Both of activation or deactivation magnitude increased with the cognition growing demanding of the task. Though the activation brain regions were different for experiment 1 and experiment 2, the deactivated patterns were similarity for patients and controls both in experiment 1 and experiment 2.The deactivation regions included medial prefrontal cortex, posterior cingulate and bilateral inferior parietal cortex, which are also remarkably similar to the default model network proposed by Raichle M. E. However, the deactivation magnitude and extent of the brain regions was significantly greater for patients than that for normal controls both in experiment 1 and experiment 2.Our findings suggest the hypertension patient may exist a default model networks in brain which similar to healthy subjects during a passive task with altered activity trait. In addition , some brain dysfunctions may have existed in patients with moderately to severely hypertension disorders, though their behavior performance were within the normal range of age.Elementary Conclusions:l.The default model networks may exist generally in human brain. It exists not only in healthy adult subjects but also in curtain patient with brain dysfunction (for example, the anxiety patient) or patients with moderately to severely hypertension disorders.2.The deactivation magnitude of this network is closely related to the processing difficulty of the external cognitive task, with the more cognitive-demands of the task, the more great deactivation intensity is.3.Some of the psychiatric patient may have an altered default model networks activity.4.The default model networks may closely involve in memory and emotion processing.5. Investigation the default model networks can provide us a new alternative way to examine the mechanism for some brain dysfunction disorders.