Abstract
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.
Highlights
The experienced level of pain strongly depends on the context in which nociceptive stimuli occur
We investigated the effect of anodal (Experiment 1) and cathodal (Experiment 2) transcranial direct current stimulation (tDCS) over the occipital cortex and the centro-parietal cortex, which contributes to sensory processing of nociceptive stimuli (Valentini et al, 2012; Liang, Mouraux, & Iannetti, 2011; Tracey, 2011)
In the pre-tDCS sessions, pain ratings were reduced in the hand-view condition in comparison with the object-view condition, t(11) = 2.88, p < .015, confirming visually induced analgesia for electrical nociceptive stimulation
Summary
The experienced level of pain strongly depends on the context in which nociceptive stimuli occur. Expectations, and motivation are well-known examples of contextual modulation (Wiech, Ploner, & Tracey, 2008). Looking at ones own body can be analgesic. Passive vision of the hand reduces pain ratings and the N2/P2 complex of laser-evoked potentials for the same stimuli (Longo, Betti, Aglioti, & Haggard, 2009). Viewing ones own hand increases contact heat pain thresholds by 3.2°C, relative to viewing a neutral object (Mancini, Longo, Kammers, & Haggard, 2011). This “visually induced analgesia” can be considered a form of multisensory modulation in which the visual representation of the body has effects on pain processing
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