The overall goal of the following experiment is to study the MRI signals related to the attentional responses to visual stimuli. This is achieved by scanning subjects while they attend to emotional stroop stimuli. At the same time button, press responses to the stimuli are recorded, which are the measure of behavior that will be correlated to the signals in the brain.
Additionally, the subject's eye movements are recorded within an eye tracker within the scanner in order to obtain a measure of attentional selection. Results are obtained that show inferior frontal gyrus activation based on analysis of the correlations of behavior and FMRI signal during the stimulus response task. The main advantage of this technique over other ones such as positron emission tomography, is that subjects can be scanned for up to two hours.
This system helps us understand a little bit more about frontal cortex inhibition. Dysfunction of this system is implicated in various neuropathology, such as a DHD and Tourette syndrome. Before beginning this experiment, potential participants should complete a study consent form as well as a standard MRI screening questionnaire subjects.
With any contraindications to MRI must be excluded. Once paperwork is complete, provide the subject with a training session on the antis task. Here, a green fixation indicates a pro secod trial.
Instruct the participant to look at the target appearing in the periphery of the screen at a visual angle of eight to 10 degrees. A red fixation indicates an antis COD trial instruct the participant to look into the mirror opposite of the target appearing in the periphery of the screen again at a visual eight to 10 degrees. For example, if a right target appears, the subject should look to the left.
Next, provide training for the emotional stroop task. This is a pseudo event related paradigm in which descriptive words indicating the expressions are superimposed over pictures of individual faces. The words are either congruent or incongruent with the emotion depicted by the face.
In the picture, instruct participants as to which buttons are pressed for reporting a happy expression or word, a neutral expression or word or a sad expression or word. The task should begin with a one second written instruction screen reminding the participants to either report the face expression or the written word by pressing the corresponding button as quickly as possible. This is followed by one second of a fixation cross, then by the face stimuli for 250 milliseconds, and then a response image for two seconds.
The response image serves to give the participants time to report their responses. The next visual presentation of the fixation cross begins after the end of the response image. Include 15 practice trials with different combinations of face word expressions.
To begin set up for scanning. First turn on a digital projector to display the computer stimulus as a focused image on a screen in the MRI bore. Then bring the participant into the scanner room and provide earplugs and headphones to provide hearing protection.
Have the subject lie supine and position the head in the center of the head.Coil.Cradle. Stabilize participant's body and head position with pillows or foam inserts to provide comfort while restricting movement of the head. Now, place the head coil.
Then have the subject tilt his or her head comfortably while looking straight ahead to view the mirror, reflecting the projected image, the eyes should be as close to primary position as possible in order to maintain the participant's comfort over the scanning session, which can last up to two hours. Ask the participant if the projected image is in focus. If it is not sharp, readjust the lens to improve the image clarity.
Then calibrate the eye tracker to ensure that the infrared camera is in the correct location. If the reflection on the cornea is not ideal, the infrared source should be adjusted or the mirror reflecting the infrared source near the participant's head position should be realigned. Also, ask the subject if more foam padding or pillows are needed to maintain this position.
Once equipment set up is complete, place an emergency contact squeeze ball in the participant's right hand and the response box in the left hand. Also place a vitamin E capsule on the right side of the headphones as this will be visible in anatomical scans and can help assure images are not flipped from left to right. Finally, raise and slide the bed into the center of the MRI.
Once in the control room, use the intercom to confirm that the participant is prepared to begin the scanning session and is as comfortable. Also, remind the participant that the scanner noises will be loud. Begin by acquiring a localizer scan.
Then prescribe a series of anatomical slices that cover the whole brain. Typically axial oblique slices should be used. Let the participant know about how long the anatomical scan will take.
This can be around six to 10 minutes depending on the prescribed number of slices. Alternatively, the anatomical scan can be acquired after the functional scans. Once anatomical scans are completed, prescribe a functional sequence and remind the participant of the task instructions for the stroop paradigm.
Then begin the scanner and paradigm synchronously recording all button presses. Be sure to monitor and record the participant's horizontal and vertical eye positions using the infrared eye tracker so that these data can be correlated with the behavioral paradigm. Next, instruct the subject that the anti Scho paradigm will be performed during the next scan.
Then start the scan with the synced paradigm. Depending on the imaging parameters chosen, the scan will be close to six minutes long. Avoid longer scans as subjects may fall asleep.
This entire imaging session can take 60 to 120 minutes depending on the time needed for setup and subject specific adjustments and the number of functional scans required for the analysis. The data obtained from this experiment can be analyzed using any FMRI analysis package, such as brain voyage or qx. Seen here.
A acne or SPM are other commonly used FMRI analysis programs. Statistical maps derived from the functional data should be superimposed onto anatomical brain images. To do this, first, use the general linear model or GLM to functionally define the brain regions of interest with separate predictors for each of the conditions in the task.
During the two types of scans, use the following predictors, congruent and incongruent face instruction and word instruction, antis, secod, and pros secod. Next, examine the signal intensity in all the activated frontal regions from the GLM contrasts. Then compute the standardized bold signal across all participants and compare the incongruent word face expression with the congruent word faces.
Expressions for both conditions finally correlate the reaction times collected on the trials that were used for. The GMs then correlate brain activity across each individual with their own reaction times for the specific trials. The image seen here shows left inferior frontal gyrus or IFG that was functionally localized using the contrast for the incongruent emotional stroop versus the congruent conditioning.
During the attend to word instruction set. Here we see that when the expression of the face was incongruent to the superimposed emotional word, the incongruency produced from reporting the written word produced higher bold signal intensity. In the left, IFG incongruent expressions showed significantly larger bold signal change compared to congruent expressions.
During the attend to word instruction, incongruent congruent contrast showed a positive correlation between the reaction times and bold signal intensity. This graph is an average of 10 subjects reaction times and bold signal during each of the six conditions. Here the top row is a schematic illustration of a trial sequence from one block of trials.
Bottom section is a depiction of the two gamma hemodynamic response function used to discover brain regions involved in the emotional face expressions. While attempting this procedure, it's important to remember to check on the subject in between scans and remind them to remain as still as possible to reduce motion artifact Following this procedure. Other techniques such as magne ence biography can be used to examine the timing effects within the inferior frontal gyrus.
