The overall goal of this procedure is to disentangle physiological reactivity to low probability or uncertain threat, conceptualized as anxiety from physiological reactivity to high probability or otherwise certain threat conceptualized as fear. This is accomplished by first preparing the participant's skin and attaching electromyography sensors under the participant's eye and forehead. The second step is to assess the participant's sensitivity to electric shock in order to determine the level of shock to be used in the main task.
Next, the participant is provided with general task information and specific cue to shock contingencies Before beginning the threat probability task, the final step is to process and analyze startle potentiation during the uncertain low probability threat conditions and compare it to startle potentiation during the certain high probability threat conditions. Ultimately, the threat probability task shows that startle potentiation in the low probability uncertain threat condition versus the high probability certain threat condition is dampened by administration of anxiolytic drugs such as alcohol. The main advantages of this technique over existing methods such as self-report measurement and negative affect is that startle potentiation in the threat probability task is less disruptive to the participant's main task and less susceptible to demand characteristics and other forms of measurement bias.
This method, it can address key questions about the affect of mechanisms involved in the etiology, maintenance, and relapse of various psychiatric disorders, including alcohol and drug addiction. Demonstrating the procedure will be Katherine Magruder and Rachel Hummel. Two technicians from my laboratory Begin this protocol by first obtaining informed consent, and then ask the participant to wash their face thoroughly with soap, paying particular attention to the target sensor locations, which are located below one eye and in the middle of the participant's forehead.
Next, prepare the participant's skin for electromyography by cleaning the target sensor locations with an alcohol pad. Then also clean the same locations with a gritty exfoliate gel using a small gauze pad to further remove dirt or dead skin cells that can impede measurement. Prepare silver, silver chloride EMG sensors by filling the sensor cups with conductive gel using a syringe and blunt needle.
Then attach a large silver, silver chloride sensor to the center of the participant's forehead using an adhesive collar. Now attach to additional small sensors below the participant's eye using adhesive collars. Place the first of these small sensors in line with the pupil at forward gaze and the second sensor lateral and adjacent to the first.
Do not allow the adhesive collars to overlap as this may increase. Movement artifact also prevent gel overflow to avoid forming a gel bridge between the two sensors below the eye as this will cause current to flow via the bridge and impair measurement of EMG activity. Once the sensors are positioned, start the physiology acquisition software and ask the participant to blink a few times to verify that the EMG response is being recorded properly and that eye blinks can be observed on the display of the data collection software.
Finally, check the impedance for each sensor before shock tolerance assessment. First, perform a baseline measurement of general startle reactivity, then a fixed two shock electrodes with standard medical tape to the distal phalanges of the index and ring fingers of the participant's hand. Next, explain to the participant that they will receive a series of increasingly intense electric shocks and that after each shock is administered, they should rate how aversive they found the shock on a 100 point scale.
Instruct the participant that it is important to accurately report the highest shock they can tolerate. The participant should not be informed that their report will impact the actual shocks they receive as this may lead to bias in their report. Now begin the shock tolerance assessment.
A rating of zero should be used if they cannot feel a shock at all. A rating of 50 for the first level of shock that they consider to be uncomfortable, and a rating of 100 for the highest level of shock that they can tolerate. Stop the shock tolerance assessment once the participant rates a shock as 100.
Record the shock level and administer shocks at this level in the threat probability task to control for individual differences in shock sensitivity. Begin by providing the participant with a cover story that encourages attention throughout the task. Then provide the participant with general task information and specific cue shock contingencies for each condition.
Instruct the participant that the task lasts about 20 minutes and includes cues that last five seconds each separated by 15 to 20 seconds. The cues are organized into sets with each set lasting two to three minutes each. Also instruct the participant that there are three types of sets, 20%shock sets, 100%shock sets and a no shock set.
They will receive shocks at the end of approximately one out of every five cues in 20%shock sets, and five out of every five cues in 100%shock sets assure the participant that they will receive no shocks at any time during no shock sets or during the time between the presentations of the cues in any of the sets. After instruction, allow the participant to ask questions about the task. Then quiz the participant to make sure they completely understand the shock contingencies.
Also remind the participant that they can discontinue their participation at any point during the experiment. Begin recording the EMG signal on a physiology computer with acquisition software installed and then start the stimulus presentation software on a separate computer that will control task stimuli. Now begin the threat probability task.
Be sure to carefully monitor the participant for voluntary movements closing of eyes, or any sign of excessive discomfort during the task. To begin data processing, first apply a fourth order 28 hertz Butterworth high pass filter to the raw continuous EMG. Then rectify the filtered continuous EMG, smooth the rectified EMG signal using a fourth order 30 hertz Butterworth Lowpass filter.
Next epic, the smooth continuous signal retaining data from 50 milliseconds before to 250 milliseconds after the acoustic startle probe onset. Also baseline, correct the epic signal by subtracting the mean of the pre-pro baseline from the entire epic signal Score the startle response from each epic as maximum response between 20 and 100 millisecond post probe onset. Reject any trials with excessive artifact such as excessive deflections in the pre-pro baseline.
Next, average the startle response for epics within each condition. Then calculate startle potentiation for uncertain shock as the difference between mean startle response to startle probes during 20%shock cues versus no shock cues. Calculate startle potentiation for certain shock as the difference between mean startle response to startle probes during 100%shock cues versus no shock cues.
Finally, analyze startle potentiation using a general linear model with repeated measures. Here it can be seen that acute administration of a moderate dose of alcohol produces selectively greater reduction in startle potentiation during 20%threat versus 100%threat in humans. Similarly, short-term deprivation of marijuana among heavy daily marijuana users produces selective greater increase in startle potentiation during 20%threat versus 100%threat in humans.
When using this procedure, it's important to remember to carefully monitor the participant's comfort level and allow them to stop at any time they wish to do so if they become too distressed Following this procedure, methods involving administration of individual difference questionnaires can be administered to answer additional questions like how personality traits interact with negative affect during certain and uncertain threat in this task.
