Name: developing neuroimaging phenotypes of the default mode network in ptsd: integrating the resting state, working memory, and structural connectivity
Uploaded: 2014-07-01T15:00:00
Description: Watch this Scientific Journal Video about Developing Neuroimaging Phenotypes of the Default Mode Network in PTSD: Integrating the Resting State, Working Memory, and Structural Connectivity at JoVE.com

Generation of representative data was supported by NIH Grant R01HL084178, 5R01MH068767-08, and grants from the Brown MRI Research Facility and Rhode Island Foundation. VA CSR&#38;D Grant 1 IK2 CX000724-01A2 supported protocol development and further work. We thank all of our participants.

Eligible participants sign written, informed consent to participate in the research project. Research is performed in compliance with institutional, national and international guidelines for human welfare.
1. Participant Screening and Diagnostic Interviews
<ol>
	<li>After informed consent, perform diagnostic interviews to verify the diagnosis of PTSD and illness severity. NOTE: These measures include the Structured Clinical Interview for DSM-IV-TR (SCID)24 and the Clinician Admin...

The two most critical steps for successful implementation of the neuroimaging protocol are accurately capturing resting state and working memory effects.
Conceptually, the acquisition of resting state images is straightforward. Since there is no task to perform, experimenters often describe brain activity during these epochs as &#8220;rest.&#8221; However, as this field is relatively new compared to other areas of the neuroimaging1, there is no explicit consensus of how to precisely...

Neuroimaging represents a tool with unprecedented potential to examine diagnostic validity, severity of illness, prognostics and treatment response in neuropsychiatry. A wide range of complementary neuroimaging techniques is now available to characterize the structure and function of key brain systems, and to aid in the identification of neuroimaging phenotypes in psychiatric populations. Of these systems, the Default Mode Network (DMN) has received a great deal of attention in the cognitive and clinical neuroscience literature over the last decade.
The DMN is a so-called &#8220;resting state network&#8221; that includes the medial prefrontal cortex (MPFC) as the main anterior node, posterior cingulate cortex/precuneus (PCC) as the principle posterior node, along with the inferior-lateral parietal cortices and medial temporal regions. They key feature of this network is that it exhibits its highest activity during periods of rest, which occurs while subjects are awake and alert but not involved in a specific task; this resting state activity was coined the &#8220;Default Mode&#8221; of brain function1. Resting state activity in the DMN is also highly synchronized, which is described as resting state functional connectivity. The other key feature of the DMN is that it demonstrates diminished activity during periods of increased external cognitive demands, which is observed as task-induced deactivation during functional neuroimaging paradigms2,3. It is hypothesized that the balance between the internal (i.e. the resting state) and external (i.e. task-related activity) demands are needed to maintain healthy brain functioning3-5.
The following sections provide a brief overview of three methods to study the DMN: functional connectivity and task-associated deactivation, followed by structural connectivity. These three methods are described as complementary ways to characterize this network in clinical samples, such as patients with post-traumatic stress disorder and related psychiatric conditions.
Resting State DMN Functional Connectivity
Resting state functional connectivity has recently become a common approach used to evaluate patterns of baseline brain function in the absence of task demands. Functional connectivity is an analytic method that quantifies coherence, or the degree of synchrony in blood oxygen level dependent (BOLD) signal over time, across different brain regions. A growing body of research literature suggests that the typical patterns of DMN connectivity may be altered in clinical and at-risk populations, and particularly those with previous exposure to significant stress or trauma. The most common finding has been decreased DMN resting state functional connectivity associated with PTSD6. This diminished connectivity may have direct clinical applications, as decreased DMN connectivity may be predictive of those who may develop PTSD after an acute stressor7. Diminished DMN functional connectivity can be interpreted in several ways, most commonly that it reflects poor communication between crucial brain regions involved in self-processing, which may lead to an inability to reallocate internal resources from baseline DMN processing to external demands. This network disruption may explain core clinical symptoms of psychiatric disorders such as PTSD and other stress-related psychiatric conditions8. Further investigation into the etiology of these disruptions is an important area for future research.
From a more general perspective, advantages of examining the functional connectivity of the DMN include relatively easy implementation and a robust pattern of resting state functional connectivity in healthy controls that allows for a reliable comparison9,10. Therefore, this method has the potential to be developed into an easily implemented and robust neuroimaging biomarker of stress related psychiatric disorders that informs how the brain functions in the absence of specific task demands in individuals with PTSD and other stress-related psychiatric conditions.
Task-Associated DMN Deactivations
Examining DMN response during working memory (WM) offers another approach to investigate the function and dysfunction of this network beyond resting state synchrony. This approach, which reflects a more standard functional magnetic resonance imaging (FMRI) method, provides different information about response to task demands that may have clinical significance11. Previous research has documented that participants with PTSD demonstrate impaired WM functioning and a greater degree of DMN deactivations during WM tasks, perhaps reflecting increased cognitive effort12-15. Using WM as an FMRI challenge has several advantages. For instance, it reliably disengages several key DMN regions, from resting to an active state. Most relevant to PTSD and other stress related psychiatric conditions, WM tasks reliably disengage the MPFC, the major anterior DMN node that is involved in critical pathways dysregulated in PTSD. It has been well established that the MPFC modulates ascending amygdala activity, and likely plays a crucial role in fear conditioning16. Assessments of MPFC activity may also be a useful metric in future clinical care. For instance, in one previous study of traumatized police officers, exposure psychotherapy increased MPFC activity and decreased amygdala activity during traumatic memory retrieval. These neuroimaging changes were associated with decreased PSTD symptoms17. This instance of WM-induced MPFC deactivations is but one example of how neuroimaging metrics can be applied to clinical populations, and further exploration of other DMN components is likely to be a fruitful area of future research.
In this protocol, the n-back task of verbal working memory is used. The n-back task is widely used in FMRI research, and provides reliable activation of executive activation and default mode network deactivation regions18,19. This task includes three components, a 0-back letter vigilance task, the 2-back task of working memory and resting baseline for comparison. During the 0-back vigilance task, Participants respond &#8220;yes&#8221; when a predetermined target consonant (&#8220;H&#8221; or &#8220;h&#8221;) appeared and &#8220;no&#8221; for other consonants using a two-button response box while inside the scanner. Six 0-back control blocks of 9 consonants are presented during this task. During the 2-back, a series of consonants are presented visually for 500 msec each, with an interstimulus interval of 2,500 msec. Participants make a &#8220;yes&#8221; or &#8220;no&#8221; response, after each consonant presented, to indicate whether it is the same or different from the consonant presented two previously in a series (e.g., w, N, r, N, R, Q, r, q, N, W etc., with correct answers indicated in bold). During the 2-back, six 45 sec series of 15 consonants are presented. To perform successfully the participant must maintain a demanding cognitive set that includes constant phonemic buffering (i.e. holding consonants in short term memory), subvocal phonemic rehearsal (i.e. repeating consonants without articulating out loud), and executive coordination. In both 0- and 2-back blocks, the rate of presentation is the same, 33% of targets are presented in random locations, and capitalization is randomized to encourage verbal encoding. A 30 sec resting baseline with a crosshair fixation point is presented prior to each 0-back block; this baseline is used for subsequent comparisons of task-associated activity compared to baseline during data analyses.
Taken together, the extant data suggests that characterization of task-associated DMN activity during a variety of tasks may play an important role in the clinical use of functional DMN analysis. There are other advantages to using WM as an FMRI challenge in stress related psychiatric conditions. Similar to resting state connectivity, there is a clear pattern of DMN deactivations during WM in healthy individuals, which facilitates comparisons with clinical samples. WM is also trauma neutral, which may avoid triggering clinical PTSD symptoms during scanning. Therefore this method also has the potential to be developed into a neuroimaging biomarker that reflects how the brain responds to external demands in stress related psychiatric disorders.
DMN Structural Connectivity
While functional imaging is able to describe changes in brain connectivity or activity associated with stress exposure, functional approaches do not describe the etiology behind observed brain changes. Structural imaging methods, such as diffusion tensor imaging (DTI), are able to measure and quantify the integrity of the white matter tracts connecting brain regions. DTI is the most common structural neuroimaging approach and measures white matter integrity based on the anisotropic (i.e. directional) flow of water molecules along white matter tracts, as water flows predominantly along white matter tracts (compared to across them). This difference in directional flow is expressed as fractional anisotropy (FA). Lower degrees of FA are thought to reflect microstructural changes in white matter tracts, which may be manifestations of neuronal injury from a variety of causes, including the consequences of stress exposure4. From a network perspective, coordinated brain activity (i.e. resting state activity or coordinated task-related activity) must rely upon structural connections. In the case of previous DMN findings, structural injury impairs the communication between DMN nodes, leading to decreased DMN functional connectivity. Similarly, increased patterns of deactivation may reflect microstructural damage that necessitates the recruitment of greater areas of cortex during task response. Relevant to PTSD and the DMN, several studies have shown decreased FA in the cingulum bundle20,21, which is the white matter tract that connects major limbic structures of the brain22. It is likely that more precise measures utilizing tractography (i.e. that directly trace white matter tracts at the neuronal level) will be able to elucidate specifically which white matter fibers are involved in network disruption. The advantages to DTI imaging is that it is relatively easy to acquire as there are no required tasks to perform in the scanner.
In the following protocol, the functional approaches of resting state functional connectivity and quantification of task-induced deactivations are combined with an examination of structural connectivity using DTI, in order to map DMN structure and function and relate these findings to illness severity and relevant clinical factors in PTSD. We have previously implemented this approach in trauma-exposed healthy adults18,23 and found that this protocol provides a cogent method to characterize the DMN that lends itself to adaptation to the study of PTSD and other stress related psychiatric illnesses.

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developing neuroimaging phenotypes of the default mode network in ptsd: integrating the resting state, working memory, and structural connectivity

Complementary structural and functional neuroimaging techniques used to examine the Default Mode Network (DMN) could potentially improve assessments of psychiatric illness severity and provide added validity to the clinical diagnostic process. Recent neuroimaging research suggests that DMN processes may be disrupted in a number of stress-related psychiatric illnesses, such as posttraumatic stress disorder (PTSD).
Although specific DMN functions remain under investigation, it is generally thought to be involved in introspection and self-processing. In healthy individuals it exhibits greatest activity during periods of rest, with less activity, observed as deactivation, during cognitive tasks, e.g., working memory. This network consists of the medial prefrontal cortex, posterior cingulate cortex/precuneus, lateral parietal cortices and medial temporal regions.
Multiple functional and structural imaging approaches have been developed to study the DMN. These have unprecedented potential to further the understanding of the function and dysfunction of this network. Functional approaches, such as the evaluation of resting state connectivity and task-induced deactivation, have excellent potential to identify targeted neurocognitive and neuroaffective (functional) diagnostic markers and may indicate illness severity and prognosis with increased accuracy or specificity. Structural approaches, such as evaluation of morphometry and connectivity, may provide unique markers of etiology and long-term outcomes. Combined, functional and structural methods provide strong multimodal, complementary and synergistic approaches to develop valid DMN-based imaging phenotypes in stress-related psychiatric conditions. This protocol aims to integrate these methods to investigate DMN structure and function in PTSD, relating findings to illness severity and relevant clinical factors.

Representative results are based on data collected using the same imaging approach in two different samples of individuals with a history of childhood trauma and maltreatment, but without PTSD21,22. Results from resting state functional connectivity analyses revealed a spatial pattern consistent with major nodes of the DMN (Figure 1)1-3,8 including the MPFC, PCC, angular gyrus/inferior parietal lobule and middle temporal regions. Confirmation of this spatial distribution serves as a...

Watch this Scientific Journal Video about Developing Neuroimaging Phenotypes of the Default Mode Network in PTSD: Integrating the Resting State, Working Memory, and Structural Connectivity at JoVE.com

Developing Neuroimaging Phenotypes of the Default Mode Network in PTSD: Integrating the Resting State, Working Memory, and Structural Connectivity

This protocol describes the complementary neuroimaging techniques of resting state structural connectivity, task-induced deactivation, and structural connectivity analyses to examine the default network in post-traumatic stress disorder. The use of synergistic methods could potentially lead to improved diagnostics and assessments of severity, outcome, and other relevant clinical factors.

Complementary structural and functional neuroimaging techniques used to examine the Default Mode Network (DMN) could potentially improve assessments of ...

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