tDCS modulation of visually induced analgesia.

Multisensory interactions can produce analgesic effects. In particular, viewing one's own body reduces pain levels, perhaps because of changes in connectivity between visual areas specialized for body representation, and sensory areas underlying pain perception. We tested the causal role of the extrastriate visual cortex in triggering visually induced analgesia by modulating the excitability of this region with transcranial direct current stimulation (tDCS). Anodal, cathodal, or sham tDCS (2 mA, 10 min) was administered to 24 healthy participants over the right occipital or over the centro-parietal areas thought to be involved in the sensory processing of pain. Participants were required to rate the intensity of painful electrical stimuli while viewing either their left hand or an object occluding the left hand, both before and immediately after tDCS. We found that the analgesic effect of viewing the body was enhanced selectively by anodal stimulation of the occipital cortex. The effect was specific for the polarity and the site of stimulation. The present results indicate that visually induced analgesia may depend on neural signals from the extrastriate visual cortex

Visually induced analgesia during face or limb stimulation in healthy and migraine subjects

Background: Visually induced analgesia (VIA) defines a phenomenon in which viewing one’s own body part during its painful stimulation decreases the perception of pain. VIA occurs during direct vision of the stimulated body part and also when seeing it reflected in a mirror. To the best of our knowledge, VIA has not been studied in the trigeminal area, where it could be relevant for the control of headache. Subjects and methods: We used heat stimuli (53°C) to induce pain in the right forehead or wrist in 11 healthy subjects (HSs) and 14 female migraine without aura (MO) patients between attacks. The subjects rated pain on a visual analog scale (VAS) and underwent contact heat-evoked potential (CHEP) recordings (five sequential blocks of four responses) with or without observation of their face/wrist in a mirror. Results: During wrist stimulation, amplitude of the first block of P1–P2 components of CHEPs decreased compared to that in the control recording when HSs were seeing their wrist reflected in the mirror (p = 0.036; Z = 2.08); however, this was not found in MO patients. In the latter, the VAS pain score increased viewing the reflected wrist (p = 0.049; Z = 1.96). Seeing their forehead reflected in the mirror induced a significant increase in N2 latency of CHEPs in HSs, as well as an amplitude reduction in the first block of P1–P2 components of CHEPs both in HSs (p = 0.007; Z = 2.69) and MO patients (p = 0.035; Z = 2.10). Visualizing the body part did not modify habituation of CHEP amplitudes over the five blocks of averaged responses, neither during wrist nor during forehead stimulation. Conclusion: This study adds to the available knowledge on VIA and demonstrates this phenomenon for painful stimuli in the trigeminal area, as long as CHEPs are used as indices of central pain processing. In migraine patients during interictal periods, VIA assessed with CHEPs is within normal limits in the face but absent at the wrist, possibly reflecting dysfunctioning of extracephalic pain control. © 2018 Sava et al

Cathodal transcranial direct current stimulation of the extrastriate visual cortex modulates implicit anti-fat bias in male, but not female, participants.

Explicit negative attitudes towards obese individuals are well documented and seem to modulate the activity of perceptual areas, such as the Extrastriate Body Area (EBA) in the lateral occipito-temporal cortex, which is critical for body-shape perception. Nevertheless, it is still unclear whether EBA serves a role in implicit weight-stereotypical bias, thus reflecting stereotypical trait attribution on the basis of perceptual cues. Here, we used an Implicit Association Test (IAT) to investigate whether applying transcranial direct current stimulation (tDCS) over bilateral extrastriate visual cortex reduces pre-existing implicit weight stereotypical associations (i.e. "Bad" with Fat and "Good" with Slim, valence-IAT). Furthermore, an aesthetic-IAT, which focused on body-concepts related to aesthetic dimensions (i.e. "Ugly" and "Beauty"), was developed as a control condition. Anodal, cathodal, or sham tDCS (2 mA, 10min) over the right and left lateral occipito-temporal (extrastriate visual) cortex was administered to 13 female and 12 male participants, before performing the IATs. Results showed that cathodal stimulation over the left extrastriate visual cortex reduced weight-bias for the evaluative dimensions (Bad vs. Good) as compared to sham stimulation over the same hemisphere. Furthermore, the effect was specific for the polarity and hemisphere of stimulation. Importantly, tDCS affected the responses only in male participants, who presented a reliable weight-bias during sham condition, but not in female participants, who did not show reliable weight-bias at sham condition. The present results suggest that negative attitudes towards obese individuals may reflect neural signals from the extrastriate visual cortex

Viewing the body modulates both pain sensations and pain responses

Viewing the body can influence pain perception, even when vision is non-informative about the noxious stimulus. Prior studies used either continuous pain rating scales or pain detection thresholds, which cannot distinguish whether viewing the body changes the discriminability of noxious heat intensities or merely shifts reported pain levels. In Experiment 1, participants discriminated two intensities of heat-pain stimulation. Noxious stimuli were delivered to the hand in darkness immediately after participants viewed either their own hand or a non-body object appearing in the same location. The visual condition varied randomly between trials. Discriminability of the noxious heat intensities (d?) was lower after viewing the hand than after viewing the object, indicating that viewing the hand reduced the information about stimulus intensity available within the nociceptive system. In Experiment 2, the hand and the object were presented in separate blocks of trials. Viewing the hand shifted perceived pain levels irrespective of actual stimulus intensity, biasing responses toward ‘high pain’ judgments. In Experiment 3, participants saw the noxious stimulus as it approached and touched their hand or the object. Seeing the pain-inducing event counteracted the reduction in discriminability found when viewing the hand alone. These findings show that viewing the body can affect both perceptual processing of pain and responses to pain, depending on the visual context. Many factors modulate pain; our study highlights the importance of distinguishing modulations of perceptual processing from modulations of response bias

Influence of body position, emotions, placebo and cognitive modulation on pain experience and pain-related somatosensory ERPs

The present work contributed to our understanding of the neurocognitive mechanisms underlying pain modulation through sensory, attentional, emotional and cognitive processes. We used subjective, behavioral, and electrophysiological indexes to reveal the effects of body position, emotions, placebo expectations and cognitive reappraisal on subjective pain experience and pain-related somatosensory potentials. Four studies were conducted to investigate different forms of pain modulation. Study 1 tested the hypothesis that the horizontal body position reduces pain perception and cortical pain processing. We demonstrated that the supine vs. sitting body position was associated with dampened perception of non-painful stimuli and inhibited cortical late processing (300-600 ms) of non-painful and painful stimuli, related to neural activity within frontal right regions (anterior cingulate cortex and superior frontal gyrus). Study 2 investigated gender differences in the emotional modulation of pain. Although males and females did not differ at the behavioral level and reported reduced pain ratings only during the visual perception of erotic pictures, striking gender differences emerged in the N2 and P2 potentials, elicited by painful stimuli. Males showed inhibited cortical processing of pain stimuli when viewing erotic pictures only, whereas females showed a differentiated cortical pain modulation for each emotional content took into consideration (erotic vs. sport/adventure vs. neutral vs. fear/threat vs. mutilation pictures), in particular for N2 potentials. In Study 3, we examine the role of individual beliefs on the effectiveness of a traditional and a homeopathic analgesic treatment. We utilized a deceptive paradigm, i.e., neither the participants nor the experimenters were aware that the administered treatment was an inert substance. We found that only the participants who took a treatment that was coherent with their beliefs showed a reduced cortical pain processing, indicated by dampened P2 amplitudes. Finally, Study 4 demonstrated that healthy participants are able to modify their pain experience using an imaginary-based reappraisal strategy. Perceived pain intensity and unpleasantness were either reduced or enhanced with respect to a neutral condition, and an effective pain inhibition was associated with increased N2 and decreased P2 amplitude

Visual-somatosensory interactions in mental representations of the body and the face

The body is represented in the brain at levels that incorporate multisensory information. This thesis focused on interactions between vision and cutaneous sensations (i.e., touch and pain). Experiment 1 revealed that there are partially dissociable pathways for visual enhancement of touch (VET) depending upon whether one sees one’s own body or the body of another person. This indicates that VET, a seeming low-level effect on spatial tactile acuity, is actually sensitive to body identity. Experiments 2-4 explored the effect of viewing one’s own body on pain perception. They demonstrated that viewing the body biases pain intensity judgments irrespective of actual stimulus intensity, and, more importantly, reduces the discriminative capacities of the nociceptive pathway encoding noxious stimulus intensity. The latter effect only occurs if the pain-inducing event itself is not visible, suggesting that viewing the body alone and viewing a stimulus event on the body have distinct effects on cutaneous sensations. Experiment 5 replicated an enhancement of visual remapping of touch (VRT) when viewing fearful human faces being touched, and further demonstrated that VRT does not occur for observed touch on non-human faces, even fearful ones. This suggests that the facial expressions of non-human animals may not be simulated within the somatosensory system of the human observer in the same way that the facial expressions of other humans are. Finally, Experiment 6 examined the enfacement illusion, in which synchronous visuo-tactile inputs cause another’s face to be assimilated into the mental self-face representation. The strength of enfacement was not affected by the other’s facial expression, supporting an asymmetric relationship between processing of facial identity and facial expressions. Together, these studies indicate that multisensory representations of the body in the brain link low-level perceptual processes with the perception of emotional cues and body/face identity, and interact in complex ways depending upon contextual factors

Interrelations between the cerebral cortex and the trigeminal system: studies in healthy subjects and in migraine patients therapeutic perspectives

Understanding migraine pathophysiology is probably the most challenging point in migrainemanagement, since an efficient acute and preventive treatment should rely on clearpathophysiological bases. Migraine is characterized interictally by a lack of habituation ofevoked responses, possibly due to a decreased preactivation level of sensory cortices. Bycontrast, during an attack and in chronic migraine, the preactivation level increases andhabituation normalizes. New neurostimulation techniques could be useful to durably modifythe activation of the underlying cortex, decreasing the repetition of attacks, giving alsoinsight on the pathophysiology of migraine.The visual cortex plays a pivotal role in migraine pathophysiology, but its effect on thetrigeminal nociceptive system remains poorly understood. On the other hand migraine attackis often associated to photophobia, but the pathophysiological relation between headache andthe discomfort to the light, during the ictal but also the interictal phase, is unclear.This thesis puts a new insight into the relation between the visual cortex modulation and theresponse of the trigeminal nociceptive system, showing a possible inhibitory functionalinterrelation between these structures, via thalamic modulation.The hypothesis is based on our first finding investigating the modulation of the visual cortexby the repetitive transcranial magnetic stimulation on the nociceptive blink reflex in healthysubjects and migraine patients.This role of the activation of the visual cortex is also better understood by using the flash lightstimulation, and thanks to the conception of a new device of flash light stimulation, weperformed several protocols in healthy subjects and migraine patient with the final result aproof-of-concept trial using the flash light stimulation in migraine patients