The internal battle between the need to act and the need to suppress an action is something I have been through multiple times this summer: trying to suppress the urge to scratch itchy mosquito bites. Such a state is common in everyday life and also important to several clinical disorders but has yet to be studied under real-world conditions. In a new study, researchers sought to simulate real-world pain suppression to better understand urge suppression – finding a critical role of the motor system and a possible effect in mitigating discomfort.
In a study published in the Journal of Cognitive Neuroscience by Kelsey Sundby, Johanna Wagner, and Adam Aron from the University of California, San Diego, participants had to resist the impulse to press a button to gain relief from pain in the other arm. “Our goal was to test whether prolonged response suppression occurs during an urge to act,” Sundby explains. “Although prior discussions on urge identified response suppression as a potential strategy for withholding an action during an urge, specific evidence for this theory was lacking. Here we used two methods that allowed us to more directly measure changes in motor cortex that are indicative of response suppression.”
They used TMS (transcranial magnetic stimulation) to measure the motor excitability of the hand that could press for relief and then also EEG to further understand brain activity during response suppression. “The hope was to provide new insights into why suppressing could be an effective strategy and how individuals may differ,” she says.
Sundby spoke with CNS about the study, its unique design, and the potential significance of the findings to future work.
CNS: How did you become personally interested in this research area?
Sundby: Human behavior is fascinating to me on many levels, but I have always been interested in impulse control. I am particularly intrigued by why we sometimes perform actions that we know will frustrate our goals and lead to unwanted consequences. Broadly speaking, my interest in impulse control is rooted in the potential impact that this work can have on our daily lives. The more we understand the complex processes that underlie impulse control, the better equipped we will be in helping individuals adhere to their goals, reach their fullest potential, and avoid self-defeating behaviors.
CNS: What have we known previously about impulse control and response suppression?
Sundby: ‘Impulse control’ is a general term that applies to everyday behaviors, as well as psychiatric and neurological problems such as tics, compulsions and substance use disorders. In the lab, we model impulse control with simple tasks. For instance, we may measure the ability of subjects to rapidly stop a pre-potent button press. Most of the lab paradigms that study this type of control process only capture very punctate moments of stopping an action. In the case of urges, however, we are often required to withhold an action for an extended period of time, such as suppressing a cough until the end of a speech. An open question is whether response suppression can function in this more prolonged manner.
CNS: Why did you choose to study response suppression with respect to relieving pain?
Sundby: First, we have a natural tendency to want to respond to relieve pain. Thus, the pain paradigm allowed us to easily induce a strong impulse to act in our subjects, a key feature of an urge. Additionally, the heat pain paradigm had several useful features that were not available in more classic approaches to inducing an urge such as creating an itch or thirst. Specifically, the pain paradigm gave us tight control over the timing of the urge as we could quickly turn the heat on and off. Additionally, we were able to reinstate the urge repeatedly and over many trials. Together, these features made pain a useful vehicle for studying a realistic model of urge in a well-controlled manner.
CNS: Can you highlight any novel aspects of your study design?
Sundby: Others have investigated the topic of human urges with a wide range of lab paradigms including blink and cough suppression. But as far as we know, no-one has ever designed a study like this. Thus, many aspects were novel: the behavioral design, the use of heat pain, the measurements made for this paradigm with paired pulse TMS and sensorimotor beta in both left and right hemispheres to measure putative suppression over the hand and a proxy of the pain respectively. Further, because we applied heat to the subjects’ left arms while they withheld responding with their right hand, we could test interesting questions about the relationship between neural signatures of response suppression (contralateral to the right hand) and pain (contralateral to the left heat stimulation). This gave us a unique opportunity to try to tease apart several aspects of the urge.
CNS: What were your most excited to find?
Sundby: I was very excited to find evidence from two different methodologies that the urge to relieve heat pain included a motor suppressive component. An alternative to suppressing was that people might refrain from responding using a more cognitive strategy such as mental distraction. Our result gives us more insight into the types of strategies that we can use to exert control over an urge. Additionally, because urges are so challenging to operationalize and measure, these results may pave the road for more studies that can use the motor cortex to index urges.
I was also surprised to see the relationship between the neural signature we used as a proxy of pain and our measure of response suppression. Specifically, we found that a reduced pain response related to greater response suppression across subjects. Although highly speculative, this preliminary finding raises an intriguing idea that response suppression may somehow help to mitigate the discomfort or pain experienced during an urge.
CNS: What do you most want people to understand about this work?
Sundby: A key takeaway from this study is that understanding how we exercise control over an urge is a very complicated question. However, with neuroscience tools, we can begin to tease apart the components of urge that may not be visible from behavior alone. As this study indicates, our motor system may aid in resisting an urge to act by suppressing the prohibited motor response. We speculate that this strategy may somehow even help to mitigate our discomfort. This is an exciting idea that requires much more research and the proper tools to study it.
CNS: What’s next for this line of work?
Sundby: This work has motivated a new line of research in our lab that more directly tests how the state of our motor system relates to sensory processing and our subjective experience of pain.
-Lisa M.P. Munoz