Investigating Pain-Related Avoidance Behavior using a Robotic Arm-Reaching Paradigm

Avoidance is central to chronic pain disability, yet adequate paradigms for examining pain-related avoidance are lacking. Therefore, we developed a paradigm that allows investigating how pain-related avoidance behavior is learned (acquisition), spreads to other stimuli (generalization), can be mitigated (extinction), and how it may subsequently re-emerge (spontaneous recovery).

Pain-related avoidance behavior majorly contributes to chronic pain disability. It's existing paradigms often like ecological and construct validity by employing an instructed and low or no cost avoidance response. Our paradigm tackles these limitations by allowing investigation of the ways in which avoidance is naturally learned and reinforced and by incorporating a cost study avoidance response.

Our paradigm can be uniquely used to explain the processes underlying the learning of pain-related to avoidance behavior, how does behavior becomes disabling in chronic pain, and how it can be mitigated. Place a computer in one room or section for the researcher and place a large television in a separate room or section for the participant. To prepare for a test session, have everyone disinfect their hands upon arrival to the lab.

Have the participant sit in a chair with armrest approximately 2.5 meters from the television screen at a comfortable distance of approximately 15 centimeters from the sensor of the robotic arm. After obtaining written informed consent from participant, fill the center of each electrode with conductive electrolyte gel. And use a strap to attach the stimulation electrodes over the triceps tendon of the participant right arm.

After explaining the calibration procedure, ask the participant to rate each stimulus on a numerical scale from zero to 10, with zero representing I feel nothing, and 10 representing the worst pain imaginable. Turn on the stimulator. You set the intensity to one milliamp to start, and announce that a pain stimulus is about to be delivered.

When the participant acknowledges the announcement, press the orange trigger button on the constant current stimulator to deliver the stimulus. After obtaining the participant's pain rating, apply the stimulus at the next level of intensity as demonstrated gradually increasing the intensity of the pain stimulus in a stepwise manner in one, two, three, and 4 milliamp increments. When the participant reaches a pain intensity that they would describe as significantly painful and demanding some effort to tolerate, terminate the calibration procedure, and document the final pain intensity in milliamps and the pain intensity rating of the participant.

Before starting the robotic arm reaching pain-related avoidance task, provide the participant with on-screen standardized written instructions of the task. Program the task such that three arches are presented situated midway through the movement plane, ensure that the shortest arm movement T1 is paired with no deviation or resistance. The middle arm movement T2, is paired with moderate deviation and resistance and the furthest arms movement T3 is paired with the largest deviation and strongest resistance.

Instruct the participant to use the dominant hand to operate the robotic arm by moving it's sensor, which is represented by a green ball on the television screen. And to move the sensor from a starting point at the lower left corner of the movement plane, to a target at the upper left corner of the movement plane. Inform the participant that they can freely choose which of the available movement trajectories to perform on each trial.

And instruct the participant to provide self-report measures of pain expectancy and fear of movement related pain, on a continuous rating scale by scrolling to the left and right on the scale using two respective foot pedals on a triple foot switch. At the end of the practice phase, after answering any questions leave the room and dim the lights. Observe the participant from the researcher section or room.

To execute the acquisition protocol, have the participants press the confirm foot pedal to initiate the experiment. During the avoidance acquisition phase, if the participant performs the shortest movement trajectory T1, program the constant current stimulator to always deliver the pain stimulus, once two thirds of the movement has been completed. If the participant selects the middle movement trajectory T2, present the pain stimulus 50%of the time while ensuring that the participant will have to exert more effort.

If the participant performs the furthest most effortful movement trajectory T3, do not present the pain stimulus but ensure that the participant will have to exert the most effort to reach the target. A successfully completed trial will be indicated by the presentation of visual and auditory stop signals. The robotic arm should be programmed to automatically return to its starting position at the end of a trial.

After 3000 milliseconds, present the visual and auditory start signals, which indicate that the participant can start the next trial. When testing for generalization of avoidance the onscreen trajectory arches are separated during the acquisition phase to leave room for the generalization trajectory arches. To test for the generalization of avoidance after the acquisition phase, present the three novel generalization movement trajectories G1, G2 and G3 adjacent to the acquisition trajectories.

To investigate the extinction of avoidance with response prevention, after the acquisition phase inform the participant that in the upcoming phase, they can only perform T1.During the response prevention phase, visually and optically block T2 and T3 so that only T1 is available. Thus, it is ensured that during the extinction with response prevention phase, the participant only performs the shortest movement trajectory T1.Approximately 24 hours later, attach the stimulation electrodes and provide brief on-screen refresher instructions of the task without including any information regarding the pain stimulus. Then present the three acquisition trajectories in the absence of the pain stimulus.

Upon completion of the experiment, detach the stimulation electrodes and thoroughly clean the stimulation electrodes with a disinfectant solution. Then dry the electrodes with soft tissue paper and clean the sensor of the robotic arm with disinfectant wipes or spray. Acquisition of avoidance behavior is demonstrated by participants avoiding pain more at the end of an acquisition phase compared to the beginning of the acquisition phase, or as compared to control group.

The acquisition of fear and pain expectancy is evidenced by participants reporting lower fear for and expecting the pain stimulus less during T3 compared to T1 and T2.Generalization of fear and pain expectancy is indicated by participants in the experimental group reporting lower fear two and expecting the pain stimulus less during G3 compared to G1 and G2.The extinction of fear and pain expectancies is evident when participants report lower fear and expect the pain stimulus less during T1 at the end of the response prevention phase compared to the end of the acquisition phase. The spontaneous recovery of avoidance behavior is indicated by participants avoiding more at the beginning of the test of spontaneous recovery compared to the end of the response prevention phase. The spontaneous recovery of fear and pain expectancy is indicated by participants reporting higher fear and pain expectancy for T1, during the beginning of the test of spontaneous recovery compared to the end of the response prevention phase.

Moving away from a verse of stimuli is not a pain specific defensive response. This method could also be applied to investigate avoidance of disgust or embarrassment which are relevant for anxiety disorders. Our paradigm enables testing potential differences in avoidance learning, in chronic pain compared to healthy populations.

A deeper understanding of the underlying mechanisms of avoidance may optimize or offer new treatment options.