Continuous Theta Burst Stimulation of the Posterior Medial Frontal Cortex to Experimentally Reduce Ideological Threat Responses

Threats reliably evoke shifts in high-level ideological investment, but little work to date has explored the neural mechanisms underlying these dynamics. This paper describes how continuous theta burst transcranial magnetic stimulation may be employed to test the contribution of the posterior medial frontal cortex (and/or other regions) to threat-related ideological shifts.

Transcript

This method can help answer key questions in social neuroscience, such as how the brain supports shifts in nationalistic or religious beliefs. The main advantage of this technique is that it allows us to actually go in and change the level of activity in a given brain region. This allows us to infer causal contributions of that given brain region in a particular behavior.

Begin by seating the participant in the testing room, and go over in depth with them how TMS works and all potential side effects. Have the participant fill out the TMS safety screen to ensure that they have no pre-existing factors that cause adverse effects. Make sure to remove all metal jewelry or accessories.

Next, fit a grid-marked swim cap to the participant's head. Clean the skin over the tibialis anterior muscle thoroughly. Then attach two differential EMG recording electrodes to this area.

Attach a third ground electrode to the skin over a bone somewhere else on the hand or arm. Next, on the swim cap, measure and mark the center location, CZ, on the participant's head. Have the participant sit comfortably and begin recording the EMG electrode output to software that will filter and display the signal.

Next, to perform thresholding on the primary motor cortex, place the center of a double-cone coil over the motor cortex, held over to the scalp surface. Apply single-pulse TMS at 50%of the maximum stimulus output, or MSO, and observe whether a motor-evoked potential, or MEP, was present in the EMG signal following the stimulation. If no MEP is seen following the stimulation, reposition the coil one centimeter away in any direction, and try the stimulation again.

Continue moving the coil once one centimeter at a time, marking on the cap the stimulation sites that result in an MEP of 50 millivolts or greater. Next, have the participant contract the target muscle. Stimulate the located region for 10 repetitions separated by about seven seconds, at decreasing intensities, until a corresponding observable twitch in the leg muscle no longer occurs for 50%of the stimulations.

Navigate the coil to 3.5 centimeters anterior to the motor cortex, where the posterior medial frontal cortex is located. Finally, apply cTBS as follows:Three pulses at 50 hertz, repeated at 200 millisecond intervals for 40 seconds, totalling at 600 pulses, and ensure that the coil is perfectly still throughout the stimulation. Begin by seating a participant at a desktop computer to perform the computer-mediated survey tasks.

Remind the participant that their responses will be anonymous, confidential and inaccessible to the research assistant, particularly to the extent that the target judgments are likely to raise self-presentation concerns, which might obscure the effect of the cTBS manipulation. Then, present the participant with instructions for the first filler task for 10 minutes before starting the main task, in order to obtain the maximal effective cTBS and minimize demand effects. First, administer the multiple-source interference task, or MSIT, followed by an unrelated survey task.

Challenge the participant to estimate the number of colors present in images of jelly beans and sea shells that have been converted to grayscale. Following the unrelated survey task, instantiate the threat context. If the ideological shifts of theoretical interest pertain to responses to death, ask the participant to write two brief passages on the subject of their mortality.

Following the threat induction task, administer the positive and negative effects schedule-expanded form, to allow self-reporting of potential effects of the cTBS intervention on emotional responses to the threat induction, as well as to distract the participants from the death prime encountered earlier in the protocol. Next, administer the ideological judgment tasks by presenting the participant with two essays, one pro-US and the other anti-US, written by immigrants to the United States from Latin America, and ask participants to evaluate the authors and their arguments. After presenting each essay, ask participants to rate their agreement with six statements using an eight-point Likert scale, with one meaning strongly disagree and eight meaning strongly agree.

Finally, measure religious conviction by presenting the participant with the supernatural beliefs scale on the screen, in which two distinct sub-scales tap into positive and negative aspects of Western religious belief, narrowing the positive and negative valence of the two essays in the group bias measure. As predicted, cTBS of the PMFC increased positive evaluations of the critical immigrant author, who was rated 28.5%more positively in the TMS condition than in the sham condition. Further, participants who received cTBS reported an average of 32.8%less positive religious conviction in God, angels and heaven, relative to the sham participants.

While attempting this procedure, it's very important that your participant feel comfortable at all times, and that you hold the coil very steady during stimulation. In addition to TMS, neuroimaging methods should be used to clarify the precise mechanisms involved in threat-related ideological responses. This technique allows researchers to explore the neural basis of high-level shifts in social cognition.

Don't forget that working with transcranial-magnetic stimulation comes with serious potential risks. So all participants must be thoroughly screened and provide informed consent before stimulation.

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Journal (Behavior)

Journal (Behavior)

Continuous Theta Burst Stimulation of the Posterior Medial Frontal Cortex to Experimentally Reduce Ideological Threat Responses

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