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

The overall goal of the following experiment is to examine the default mode network in post-traumatic stress disorder using the complementary neuroimaging techniques of resting state functional connectivity, task induced deactivation and structural connectivity. This is achieved by first training participants to perform the N back working memory task outside the scanner in order to not overwhelm participants who may be anxious about study procedures, yet still provide a sufficient challenge to their working memory for the imaging of task induced deactivations. As a second step, have participants lie down inside the gurney of a three Tesla MRI scanner and place cushions around the head to minimize head motion.

Next, while acquiring MR Images, have participants perform the end back task followed by a four minute presentation on a fixation cross. This acquires images of task associated activity and resting state. Repeat these steps then tell participants to close their eyes while acquiring diffusion tensor images.

Results obtained show a spatial pattern consistent with major nodes of the default mode network for resting state functional connectivity analysis, increased executive network activation and default mode network deactivation during the two back working memory task and white matter tracks connecting default mode network regions obtained during DTI. This technique integrates functional and structural neuroimaging approaches that someday could assist with the diagnosis of PTSD and related psychiatric illnesses. Traditional task associated FMRI approaches are combined with acquisition of resting state and structural connectivity.

Demonstrating the study procedures will be Louisa Carpenter Vs.Who is a research assistant in my laboratory. This method can provide insight into understanding the function and dysfunction of the default mode network. These methods can also be easily extended to concurrently characterize multiple brain networks to evaluate how psychiatric illnesses affect network integration Before beginning the steps as described in this protocol.

First, obtain written and informed consent and be sure to thoroughly screen the participant for MRI safety. Then train the participant to perform the N back task outside the scanner. Begin the first training, run with the zero back letter vigilance test.

Instruct the subject to indicate yes to a target consonant such as an uppercase or lowercase H via a two button response box and no to all other consonants. The nine consonants should be displayed for 500 milliseconds each with an inter stimulus time of 2, 500 milliseconds for a total of 27 seconds. The target consonants will be shown four times within each zero back block.

Next, have the participant practice the two back test. Instruct them to make a yes or no response on the response box. After each consonant is presented to indicate whether it is the same or different from the consonant that was presented previously in the series.

Show the participant a series of 15 consonants for 500 milliseconds each with an interra stimulus interval of 2, 500 milliseconds for a total of 45 seconds. The target stimulus should be shown five times. Continue to train the participant until their performance reaches at least 75%correct on the two back component Prior to scanning.

First, have the participant remove all metal items, change into MRI compatible clothing, and then bring them inside the three Tesla MRI scanner room. Provide earplugs for hearing protection and have them lie down on the scanner bed place cushions around the head to minimize head motion. Provide them with the MRI compatible response box for the end back working memory task, as well as a squeeze bulb to stop the scan in case of an emergency.

Also place a pulse oximeter on a finger for physiologic monitoring and recording. Then place a 32 channel head coil and the presentation screen over the participant's head before moving them into the scanner. Ensure the participant is comfortable and can see the screen.

And then begin the MRI scanning session. Start by acquiring a high resolution one millimeter isotropic anatomic brain image. Use the high resolution MRI parameters as seen here.

Then start the MRI acquisition. Next, set the FMRI bold image acquisition parameters on the scanner console. Using the parameters seen here before starting the functional scan.

First project the instructions to the patient for three seconds prior to each zero or two back task. Using the stimulus presentation software. Then acquire FMRI images on working memory using the NAC test.

A 32nd baseline fixation cross should be presented to the patient prior to each of the zero back blocks. This will provide baseline for comparison for the other zero and two back blocks. During data analysis in total include three zero back and three two back portions along with two baseline blocks.

This should be presented in two separate imaging runs in a counterbalanced order. After completion of the N back paradigms, ensure that the participant is still comfortable and ready to continue the scan. Then instruct them that the rest block is next and tell them not to fall asleep.

Use the stimulus presentation software to display a fixation cross on the screen. Acquire resting state images for the next four minutes using the same FMRI settings as were used to acquire NAC images. Then repeat the acquisition for working memory again using the NAC test.

Next set, DTI image acquisition parameters in the scanner console to double spin echo planner diffusion weighted images with diffusion gradients applied in 64 nonlinear directions, partial echoes and interpolations should be turned on. Be sure to tell the participant that the scanner may be shaking during the subsequent sequences and that this is normal. Instruct them to close their eyes and relax as best they can in the scanner.

Then acquire the DTI sequence. Once all scanning is complete. Remove the participant from the scanner and inquire about how the session went.

Answer any questions they may have and thank them for their participation. Be sure to securely transfer all data or have the MRI scanner computer write a DBD with participant images and physiologic recording for subsequent data analysis. Finally, perform resting state connectivity analysis with seed region connectivity analysis to evaluate the relationship between apriori defined regions to evaluate functional connectivity.

Also use FMRI processing software to pre-process the working memory data and voxel based general linear modeling to quantify task specific activity in each brain voxel of individual data sets. Results seen here 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 PTSD results from resting state. Functional connectivity analysis revealed a spatial pattern consistent with major nodes of the default mode network, including the medial prefrontal cortex, posterior cingulate cortex, angular gyrus, inferior parietal lole, and middle temporal regions.

These sagittal sections illustrate patterns associated with the two back working memory task. Activation patterns within the executive network are illustrated in orange and red, while the default mode network deactivation is displayed in blue. Here we see zero back activity, which is typically combined with working memory to control for attention.

Activation patterns are in orange and red and deactivation in blue. Evident here is some default mode network deactivation with little executive activation last. The extent of the clum bundle as revealed by probabilistic tractography is displayed here.

The three dimensional shape and pattern of these fibers can be seen with cross-sections of the brain included for visual reference. This image illustrates how these fibers travel through the medial prefrontal cortex and the posterior cingulate cortex. And finally here we can see how these fibers travel through the medial temporal component of the default mode network.

After watching this video, you should have a good understanding of how to combine multimodal neuro imaging approaches, such as acquisition of task associated activity with resting state and structural connectivity. While attempting this procedure, it is important to remember that participant motion should be minimized at all costs. This is accomplished by placing pillows against the participant's head in the scanner and frequent assessments of their comfort during scanning procedures.