After-effects of cognitive control on pain

Cognitive fatigue weakens the effects of distraction on pain

ABSTRACT

Perceived pain can be viewed because of a competition between nociceptive inputs and other competing goals, such as performing a demanding cognitive task. Task performance, however, suffers when cognitively fatigued. We therefore predicted that cognitive fatigue would weaken the pain-reducing effects of performing a concurrent cognitive task, which would indicate a causal link between fatigue and heightened pain sensitivity. In this study, 2 groups of pain-free adults performed cognitive tasks while receiving painful heat stimuli. In 1 group, we induced cognitive fatigue before performing the tasks. We found that fatigue led to more pain and worse performance when the task was demanding, suggesting that fatigue weakens one's ability to distract from pain. These findings show that cognitive fatigue can impair performance on subsequent tasks and that this impairment can lower a person's ability to distract from and reduce their pain.

Cognitive exertion affects the appraisal of one's own and other people's pain

Correctly evaluating others' pain is a crucial prosocial ability. In both clinical and private settings, caregivers assess their other people's pain, sometimes under the effect of poor sleep and high workload and fatigue. However, the effect played by such cognitive strain in the appraisal of others' pain remains unclear. Fifty participants underwent one of two demanding tasks, involving either working memory (Experiment 1: N-Back task) or cognitive interference (Experiment 2: Stroop task). After each task, participants were exposed to painful laser stimulations at three intensity levels (low, medium, high), or video-clips of patients experiencing three intensity levels of pain (low, medium, high). Participants rated the intensity of each pain event on a visual analogue scale. We found that the two tasks influenced rating of both one's own and others' pain, by decreasing the sensitivity to medium and high events. This was observed either when comparing the demanding condition to a control (Stroop), or when modelling linearly the difficulty/performance of each depleting task (N-Back). We provide converging evidence that cognitive exertion affects the subsequent appraisal of one's own and likewise others' pain.

Distraction and cognitive control independently impact parietal and prefrontal response to pain

Previous studies have found that distracting someone through a challenging activity leads to hypoalgesia, an effect mediated by parietal and prefrontal processes. Other studies suggest that challenging activities affect the ability to regulate one's aching experiences, due to the partially common neural substrate between cognitive control and pain at the level of the medial prefrontal cortex. We investigated the effects of distraction and cognitive control on pain by delivering noxious stimulations during or after a Stroop paradigm (requiring high cognitive load) or a neutral condition. We found less-intense and unpleasant subjective pain ratings during (compared to after) task execution. This hypoalgesia was associated with enhanced activity at the level of the dorsolateral prefrontal cortex and the posterior parietal cortex, which also showed negative connectivity with the insula. Furthermore, multivariate pattern analysis revealed that distraction altered the neural response to pain, by making it more similar to that associated with previous Stroop tasks. All these effects were independent of the nature of the task, which, instead, led to a localized neural modulation around the anterior cingulate cortex. Overall, our study underscores the role played by two facets of human executive functions, which exert an independent influence on the neural response to pain.

The Impact of Pain on Subsequent Effort and Cognitive Performance

Abstract. Theoretical accounts of pain and empirical evidence indicate that pain and cognitive control share common neurocognitive processes. Numerous studies have examined the interactions between pain and cognitive performance when they occur simultaneously, typically showing analgesic effects on task performance and impaired performance due to pain. However, the sequential impact of pain on cognitive control and effort remains less clear. This study investigated the influence of a first task, including painful vs. non-painful thermal stimuli, on effort-related cardiac response and performance in a subsequent moderately difficult cognitive task. Drawing on the hypothesis that experiencing pain should recruit cognitive resources (such as attention and working memory) and reduce perceived ability, we predicted lower task performance and/or stronger compensatory effort in the subsequent cognitive task after the painful than after the non-painful first task. Results support our predictions regarding the effect of pain on subsequent cognitive performance, which was moderately lower after the painful task. However, such a decrease in task proficiency was not associated with a comparable decrease in perceived capacity or increase in effort-related cardiac reactivity. Nevertheless, further correlational analyses indicated that effort and perceived capacity were significantly related to pain ratings. Moderate pain was associated with stronger effort during the cognitive task, whereas high pain led to disengagement, that is, a low effort. Moreover, in line with our predictions, higher pain ratings were associated with a lower self-reported capacity to perform the cognitive task. We discuss these findings regarding the relationship between effort and performance, the impact of fatigue on motivation, and interindividual variability in these after-effects.

Does muscular or mental fatigue have an influence on the nociceptive flexion reflex? A randomized cross-over study in healthy people

BACKGROUND

The nociceptive flexion reflex (NFR) is a spinally-mediated withdrawal reflex occurring in response to noxious stimuli and is used as an electrophysiological marker of spinal nociception. Although it is well-documented that the NFR is subject to powerful modulation of several personal factors, the effects of experimentally induced fatigue on the NFR have not yet been examined. Hence, this study aimed to characterize if and how fatigue affects spinal nociception in healthy adults.

METHODS

The NFR of 58 healthy people was measured prior to and following rest and two fatiguing tasks performed in randomized order. The NFR was elicited by transcutaneous electrical stimulation of the sural nerve and objectified by electromyographic recordings from the biceps femoris muscle. An isokinetic fatiguing protocol was used to induce localized muscle fatigue of the hamstrings. The modified incongruent Stroop-word task was used to provoke mental fatigue. A linear mixed model analysis was performed to assess the influence of fatigue on the NFR.

RESULTS

Low-to-moderate levels experimentally induced localized muscle and mental fatigue did not affect the NFR in healthy adults. These results suggest that descending pain inhibitory processes to dampen spinal nociception are resistant to the effects of localized muscle and mental fatigue.

CONCLUSIONS

The relative robustness of the NFR to fatigue may be beneficial in both clinical and research settings where the influence of confounders complicates interpretation. Furthermore, the findings possibly help enhance our understanding on why even demanding cognitive/physical exercise-based treatment programs form effective treatment strategies for patients with chronic pain.

SIGNIFICANCE

The present study unraveled that low-to-moderate levels experimentally induced localized muscle and mental fatigue did not affect the NFR. These results suggest that descending pain inhibitory processes to dampen spinal nociception are resistant to the effects of localized muscle and mental fatigue. This relative robustness of the NFR may be beneficial in a clinical setting in which the evaluation of spinal nociception that is unaffected by clinical symptoms of fatigue may be useful (e.g. chronic fatigue syndrome, cancer-related fatigue, etc.).

Distraction by a cognitive task has a higher impact on electrophysiological measures compared with conditioned pain modulation

Background

Conditioned pain modulation (CPM) evaluates the effect of a painful conditioning stimulus (CS) on a painful test stimulus (TS). Using painful cutaneous electrical stimulation (PCES) as TS and painful cold water as CS, the pain relief was paralleled by a decrease in evoked potentials (PCES-EPs). We now aimed to compare the effect of CPM with cognitive distraction on PCES-induced pain and PCES-EP amplitudes.

Methods

PCES was performed using surface electrodes inducing a painful sensation of 60 (NRS 0–100) on one hand. In a crossover design healthy subjects (included: n = 38, analyzed: n = 23) immersed the contralateral hand into 10 °C cold water (CS) for CPM evaluation and performed the 1-back task for cognitive distraction. Before and during the CS and 1-back task, respectively, subjects rated the pain intensity of PCES and simultaneously cortical evoked potentials were recorded.

Results

Both CPM and cognitive distraction significantly reduced PCES-EP amplitudes (CPM: 27.6 ± 12.0 μV to 20.2 ± 9.5 μV, cognitive distraction: 30.3 ± 14.2 µV to 13.6 ± 5.2 μV, p < 0.001) and PCES-induced pain (on a 0–100 numerical rating scale: CPM: 58 ± 4 to 41.1 ± 12.3, cognitive distraction: 58.3 ± 4.4 to 38.0 ± 13.0, p < 0.001), though the changes in pain intensity and PCES-amplitude did not correlate. The changes of the PCES-EP amplitudes during cognitive distraction were more pronounced than during CPM (p = 0.001).

Conclusions

CPM and cognitive distraction reduced the PCES-induced pain to a similar extent. The more pronounced decrease of PCES-EP amplitudes after distraction by a cognitive task implies that both conditions might not represent the general pain modulatory capacity of individuals, but may underlie different neuronal mechanisms with the final common pathway of perceived pain reduction.

Effects of Conditioned Pain Modulation on the Nociceptive Flexion Reflex in Healthy People: A Systematic Review

OBJECTIVES

The nociceptive flexion reflex (NFR) is a spinal reflex induced by painful stimuli resulting in a withdrawal response. Research has shown that the NFR is inhibited through endogenous pain inhibitory mechanisms, which can be assessed by conditioned pain modulation (CPM) paradigms. Although accumulating research suggests that the NFR can be affected by CPM, no clear overview of the current evidence exists. Therefore, the present review aimed at providing such a synthesis of the literature. In addition, the influence of personal factors on the CPM of the NFR was investigated.

MATERIALS AND METHODS

A systematic review was performed and reported following the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) guidelines. Five electronic databases were searched to identify relevant articles. Retrieved articles were screened on eligibility using predefined inclusion criteria. Risk of bias was investigated according to the modified Newcastle-Ottawa Scale. Levels of evidence and strength of conclusion were assigned following the guidelines of the Dutch Institute for Healthcare Improvement.

RESULTS

Forty articles were included. There is some evidence that CPM produced by thermal or mechanical stimuli induces inhibitory effects on the NFR. However, inconclusive evidence exists with regard to the effect of electrical conditioning stimuli. While several personal factors do not affect CPM of the NFR, increased cognitive interference is associated with reduced NFR inhibition.

DISCUSSION

The present review demonstrates that certain types of nociceptive conditioning stimuli have the potential to depress, at the spinal level, nociceptive stimuli elicited from distant body regions. Although CPM of the NFR seems to be robust to the influence of several personal factors, it can be affected by cognitive influences.

The Perception of Available Resources Influences the After-Effect of Cognitive Control on Cognitive Performance and Pain

Abstract. In two experiments, we investigated the impact of perceived available cognitive resources using a sequential-task paradigm. First, participants worked on an easy or difficult cognitive task. Then, they received a cue suggesting that their cognitive resources were still optimal or they did not receive any information on their resources. Subsequently, they worked on a second difficult cognitive task (Experiment 1) or received painful electrical stimulations (Experiment 2). We predicted that the cue on optimal resources would neutralize the effect of the first difficult task on subsequent cognitive performance and pain. Overall, results supported our predictions. We interpret these findings as showing an important role of perceived available resources in the after-effect induced by the sequential-task paradigm.

Delayed effects of attention on pain sensitivity and conditioned pain modulation

BACKGROUND

Efficacy of pain modulation is assessed as the difference in pain sensitivity during a painful conditioning, compared to before (conditioning pain modulation, CPM). Attention can be assessed with the Stroop task, in which participants report the number of words on a screen; either congruent or incongruent with the value of the words. Attention away from painful stimuli during CPM enhances the CPM effect. However, it is unknown if attention influences CPM effects when the two are done in sequence.

METHODS

Healthy men (n = 25) underwent cuff algometry CPM-assessment where the pressure-pain detection and tolerance thresholds (PTT) were recorded on one leg with and without contralateral conditioning. Two identical sessions of four test stimuli equal to PTT (5 s, 1-min interval, scored on a visual analogue scale, VAS) with a painful conditioning from the second to the last test-stimulus were performed. Stroop sessions were followed by test stimuli with or without painful conditioning.

RESULTS

The VAS scores in the first two sessions showed excellent reliability (ICC = 0.92). VAS scores were lower in sessions with Stroop compared to sessions without Stroop (p = .05) indicating an analgesic effect of Stroop. Participants were subgrouped into CPM responders and CPM non-responders according to CPM effects in the first two sessions. CPM non-responders (n = 13) showed facilitation to repeated noxious stimuli in all sessions with no effect of conditioning or Stroop (p = .02).

CONCLUSION

Attention and CPM both modulate pain in healthy men. Attention-induced analgesia works in CPM non-responders. Results indicate that attention and CPM are not the same and that they do not demonstrate additive effects when applied in sequence.

SIGNIFICANCE

Pain sensitivity is reduced after an attention task in healthy men. The delayed effects from attention only have minor effects on Conditioned Pain Modulation (CPM), and results support that attention-driven analgesia works independently of CPM. Results indicate that individual strategies for pain inhibition exist and that an overlap between the mechanisms of CPM and selective attention is limited. Moreover, painful phasic stimuli may increase the number of healthy volunteers with negative CPM effects.

Beyond unpleasantness. Social exclusion affects the experience of pain, but not of equally-unpleasant disgust

Seminal theories posit that social and physical suffering underlie partly-common representational code. It is unclear, however, if this shared information reflects a modality-specific component of pain, or alternatively a supramodal code for properties common to many aversive experiences (unpleasantness, salience, etc.). To address this issue, we engaged participants in a gaming experience in which they were excluded or included by virtual players. After each game session, participants were subjected to comparably-unpleasant painful or disgusting stimuli. Subjective reports and cardiac responses revealed a reduced sensitivity to pain following exclusion relative to inclusion, an effect which was more pronounced in those participants who declared to feel more affected by the gaming manipulation. Such modulation was not observed for disgust. These findings indicate that the relationship between social and physical suffering does not generalize to disgust, thus suggesting a shared representational code at the level of modality-specific components of pain.

Is blood glucose associated with descending modulation of spinal nociception as measured by the nociceptive flexion reflex?

Objectives

Prior research has shown a relationship between blood glucose levels and some forms of self-regulation (eg, executive function), with low blood glucose levels associated with impaired self-regulation. Further, engagement in self-regulation tasks depletes blood glucose. Given these relationships, the present study examined whether blood glucose is associated with another form of self-regulation, ie, descending pain modulatory processes.

Methods

Forty-seven (32 female) pain-free participants were recruited and completed testing. Blood glucose was measured from finger sticks and a digital meter before and after experimental pain tests. Pain tests included the nociceptive flexion reflex (NFR) threshold to assess descending modulation of spinal nociception, but also electric pain threshold to assess perceptual pain detection. The Stroop color word naming test was also assessed before and after pain testing to examine changes in executive function.

Results

Results indicated that mean blood glucose levels decreased after pain testing, but Stroop performance did not significantly change. Importantly, changes in blood glucose were correlated with NFR threshold, such that decreases in blood glucose were associated with lower NFR thresholds (reduced descending inhibition). Changes in blood glucose were unrelated to pain threshold or executive function.

Conclusion

This study suggests that glucose depletion may impair performance of descending inhibitory processes, without impacting the perceptual detection of pain (pain threshold). Although findings need to be replicated, maintaining adequate glucose levels may be necessary to support inhibition of spinal nociception.