Crossing the line of pain: FMRI correlates of crossed-hands analgesia

Endogenous antinociceptive system and potential ways to influence It

The biological significance of pain is to protect the organism from possible injury. However, there exists a situation, where, in the interest of survival, it is more important not to perceive pain. Spontaneous suppression of pain or weakening of nociception is mediated by an endogenous antinociceptive (analgesic) system. Its anatomical substrate ranges from the periaqueductal gray matter of the midbrain, through the noradrenergic and serotonergic nuclei of the brain stem to the spinal neurons, which receive "pain" information from nociceptors. Moreover, the activity of this system is under significant control of emotional and cognitive circuits. Pain can be moderated primarily through stimulation of positive emotions, while negative emotions increase pain. Paradoxically, one pain can also suppress another pain. Analgesia can be induced by stress, physical exercise, orosensory stimulation via a sweet taste, listening to music, and after placebo, i.e. when relief from pain is expected. Since pain has sensory, affective, and cognitive components, it turns out that activation of these entire systems can, in specific ways, contribute to pain suppression.

Testing the exteroceptive function of nociception: The role of visual experience in shaping the spatial representations of nociceptive inputs

Adequately localizing pain is crucial to protect the body against physical damage and react to the stimulus in external space having caused such damage. Accordingly, it is hypothesized that nociceptive inputs are remapped from a somatotopic reference frame, representing the skin surface, towards a spatiotopic frame, representing the body parts in external space. This ability is thought to be developed and shaped by early visual experience. To test this hypothesis, normally sighted and early blind participants performed temporal order judgment tasks during which they judged which of two nociceptive stimuli applied on each hand's dorsum was perceived as first delivered. Crucially, tasks were performed with the hands either in an uncrossed posture or crossed over body midline. While early blinds were not affected by the posture, performances of the normally sighted participants decreased in the crossed condition relative to the uncrossed condition. This indicates that nociceptive stimuli were automatically remapped into a spatiotopic representation that interfered with somatotopy in normally sighted individuals, whereas early blinds seemed to mostly rely on a somatotopic representation to localize nociceptive inputs. Accordingly, the plasticity of the nociceptive system would not purely depend on bodily experiences but also on crossmodal interactions between nociception and vision during early sensory experience.

Influence of transient spatial attention on the P3 component and perception of painful and non-painful electric stimuli in crossed and uncrossed hands positions

Recent reports show that focusing attention on the location where pain is expected can enhance its perception. Moreover, crossing the hands over the body’s midline is known to impair the ability to localise stimuli and decrease tactile and pain sensations in healthy participants. The present study investigated the role of transient spatial attention on the perception of painful and non-painful electrical stimuli in conditions in which a match or a mismatch was induced between skin-based and external frames of reference (uncrossed and crossed hands positions, respectively). We measured the subjective experience (Numerical Rating Scale scores) and the electrophysiological response elicited by brief electric stimuli by analysing the P3 component of Event-Related Potentials (ERPs). Twenty-two participants underwent eight painful and eight non-painful stimulus blocks. The electrical stimuli were applied to either the left or the right hand, held in either a crossed or uncrossed position. Each stimulus was preceded by a direction cue (leftward or rightward arrow). In 80% of the trials, the arrow correctly pointed to the spatial regions where the stimulus would appear (congruent cueing). Our results indicated that congruent cues resulted in increased pain NRS scores compared to incongruent ones. For non-painful stimuli such an effect was observed only in the uncrossed hands position. For both non-painful and painful stimuli the P3 peak amplitudes were higher and occurred later for incongruently cued stimuli compared to congruent ones. However, we found that crossing the hands substantially reduced the cueing effect of the P3 peak amplitudes elicited by painful stimuli. Taken together, our results showed a strong influence of transient attention manipulations on the NRS ratings and on the brain activity. Our results also suggest that hand position may modulate the strength of the cueing effect, although differences between painful and non-painful stimuli exist.

Shaping visual space perception through bodily sensations: Testing the impact of nociceptive stimuli on visual perception in peripersonal space with temporal order judgments

Coordinating spatial perception between body space and its external surrounding space is essential to adapt behaviors to objects, especially when they are noxious. Such coherent multisensory representation of the body extended into external space is conceptualized by the notion of peripersonal reference frame, mapping the portion of space in which somatic and extra-somatic inputs interact closely. Studies on crossmodal interactions between nociception and vision have been scarce. Here we investigated how the perception of visual stimuli, especially those surrounding the body, can be impacted by a nociceptive and potentially harmful stimulus inflicted on a particular body part. In two temporal order judgment tasks, participants judged which of two lateralized visual stimuli, presented either near or far from the body, had been presented first. Visual stimuli were preceded by nociceptive stimuli, either applied unilaterally (on one single hand) or bilaterally (on both hands simultaneously). In Experiment 1 participants' hands were always placed next to the visual stimuli presented near the trunk, while in Experiment 2 they could also be placed next to the visual stimuli presented far from the trunk. In Experiment 1, the presence of unilateral nociceptive stimuli prioritized the perception of visual stimuli presented in the same side of space as the stimulated hand, with a significantly larger effect when visual stimuli were presented near the body than when presented farther away. Experiment 2 showed that these visuospatial biases were related to the spatial congruency between the hand on which nociceptive stimuli were applied and the visual stimuli, independently of the relative distance of both the stimulated hand and the visual stimuli from the trunk. Indeed, nociceptive stimuli mostly impacted the perception of the closest visual stimuli. It is hypothesized that these crossmodal interactions may rely on representations of the space directly surrounding specific body parts.

Mechanical pinprick pain in patients with unilateral spatial neglect: The influence of space representation on the perception of nociceptive stimuli

BACKGROUND

Crossing the hands over the midline can reduce the perceived intensity of nociceptive stimuli applied onto the hands. It remains unclear to what extent intact representation of peripersonal space influences this effect. Here we used the crossed-hands paradigm in patients with unilateral spatial neglect, a neuropsychological condition characterized by the inability to detect, attend and respond to contralesional (most often left) stimuli, and spared ability to process stimuli in the non-affected space.

METHODS

Sixteen post-stroke patients without unilateral neglect and 11 patients with unilateral spatial neglect received punctate mechanical pinprick stimuli onto their crossed or uncrossed hands. We tested: (i) whether deficits in space representation reduce the possibility of observing 'crossed-hands analgesia', and; (ii) whether placing the contralesional hand, normally lying in the affected space in the healthy space would increase the number of detected stimuli.

RESULTS

Our results showed that neglect patients did not exhibit 'crossed-hands' analgesia, but did not provide strong evidence for an improvement in the number of detected stimuli when the contralesional hand was in the healthy space.

CONCLUSION

These findings uphold the notion that the perception of nociceptive stimuli is modulated by the relative position of the hands in space, but raise questions about the conditions under which these effects may arise.

SIGNIFICANCE

We show that deficits in space representation can influence the processing of mechanical pinprick stimuli. Our results raise several questions on the mechanisms underlying these effects, which are relevant for the clinical practice.

Prism adaptation contrasts perceptual habituation for repetitive somatosensory stimuli

Prism adaptation (PA) is a non-invasive procedure that requires performing a visuo-motor pointing task while wearing prism goggles inducing a visual displacement of the pointed target. This procedure involves a reorganization of sensorimotor coordination, and induces long-lasting effects on numerous higher-order cognitive functions in healthy volunteers and neglect patients. Prismatic displacement (PD) of the visual field can be induced when prisms are worn but no sensorimotor task is required. In this case, it is unlikely that any subsequent reorganization takes place. The effects of PD are short-lived in the sense that they last as long as prisms are worn. In this study we aimed, to the best of our knowledge for the first time, at investigating whether PA and PD induce changes in the perception of intensity of nociceptive and non- nociceptive somatosensory stimuli. We induced, in healthy volunteers, PD (experiment 1), or PA (experiment 2) and asked participants to rate the intensity of the stimuli applied to the hand undergoing the visuo-proprioceptive conflict (experiment 1) or adaptation (experiment 2). Our results indicate that: 1) the visuo-proprioceptive conflict induced by PD does not reduce the perceived intensity of the stimuli, 2) PA prevents perceptual habituation for both nociceptive and non-nociceptive somatosensory stimuli. Moreover, to investigate the possible underlying mechanisms of the effects of PA we conducted a third experiment in which stimuli were applied both at the adapted and the non-adapted hand. In line with the results of experiment 2, we found that perceptual habituation was prevented for stimuli applied onto the adapted hand. Moreover, we observed the same finding for stimuli applied onto the non-adapted hand. This result suggests that the detention of habituation is not merely driven by changes in spatial attention allocation. Taken together, these data indicate that prisms can affect the perceived intensity of somatosensory stimuli, but only when PA is induced.

Psychosocial versus physiological stress - Meta-analyses on deactivations and activations of the neural correlates of stress reactions

Stress is present in everyday life in various forms and situations. Two stressors frequently investigated are physiological and psychosocial stress. Besides similar subjective and hormonal responses, it has been suggested that they also share common neural substrates. The current study used activation-likelihood-estimation meta-analysis to test this assumption by integrating results of previous neuroimaging studies on stress processing. Reported results are cluster-level FWE corrected. The inferior frontal gyrus (IFG) and the anterior insula (AI) were the only regions that demonstrated overlapping activation for both stressors. Analysis of physiological stress showed consistent activation of cognitive and affective components of pain processing such as the insula, striatum, or the middle cingulate cortex. Contrarily, analysis across psychosocial stress revealed consistent activation of the right superior temporal gyrus and deactivation of the striatum. Notably, parts of the striatum appeared to be functionally specified: the dorsal striatum was activated in physiological stress, whereas the ventral striatum was deactivated in psychosocial stress. Additional functional connectivity and decoding analyses further characterized this functional heterogeneity and revealed higher associations of the dorsal striatum with motor regions and of the ventral striatum with reward processing. Based on our meta-analytic approach, activation of the IFG and the AI seems to indicate a global neural stress reaction. While physiological stress activates a motoric fight-or-flight reaction, during psychosocial stress attention is shifted towards emotion regulation and goal-directed behavior, and reward processing is reduced. Our results show the significance of differentiating physiological and psychosocial stress in neural engagement. Furthermore, the assessment of deactivations in addition to activations in stress research is highly recommended.

Seeing One's Own Painful Hand Positioned in the Contralateral Space Reduces Subjective Reports of Pain and Modulates Laser Evoked Potentials

Studies report that viewing the body or keeping one's arms crossed while receiving painful stimuli may have an analgesic effect. Interestingly, changes in ratings of pain are accompanied by a reduction of brain metabolism or of laser evoked potentials amplitude. What remains unknown is the link between visual analgesia and crossed-arms related analgesia. Here, we investigated pain perception and laser evoked potentials in 3 visual contexts while participants kept their arms in a crossed or uncrossed position during vision of 1) one's own hand, 2) a neutral object in the same spatial location, and 3) a fixation cross placed in front of the participant. We found that having vision of the affected body part in the crossed-arms position was associated with a significant reduction in pain reports. However, no analgesic effect of having vision of the hand in an uncrossed position or of crossing the arms alone was found. The increase of the late vertex laser evoked potential P2 amplitude indexed a general effect of vision of the hand. Our results hint at a complex interaction between cross-modal input and body representation in different spatial frames of reference and at the same time question the effect of visual analgesia and crossed-arms analgesia alone.

PERSPECTIVE

We found that nociceptive stimuli delivered to the hand in a crossed-arms position evoke less pain than in a canonical anatomic position. Yet we report no significant analgesic effect of vision or crossing the arms on their own. These findings foster the integration of visuospatial and proprioceptive information in rehabilitation protocols.

Effects of arm crossing on spatial perspective-taking

Human social interactions often require people to take a different perspective than their own. Although much research has been done on egocentric spatial representation in a solo context, little is known about how space is mapped in relation to other bodies. Here we used a spatial perspective-taking paradigm to investigate whether observing a person holding his arms crossed over the body midline has an impact on the encoding of left/right and front/back spatial relations from that person's perspective. In three experiments, we compared performance in a task in which spatial judgments were made from the perspective of the participant or from that of a co-experimenter. Depending on the experimental condition, the participant's and the co-experimenter's arms were either crossed or not crossed over the midline. Our results showed that crossing the arms had a specific effect on spatial judgments based on a first-person perspective. More specifically, the responses corresponding to the dominant hand side were slower in the crossed than in the uncrossed arms condition. Crucially, a similar effect was also found when the participants adopted the perspective of a person holding his arms crossed, but not when the other person's arms were held in an unusual but uncrossed posture. Taken together these findings indicate that egocentric space and altercentric space are similarly coded in neurocognitive maps structured with respect to specific body segments.