Abstract
BackgroundPerceptual illusions play an important role in untangling neural mechanisms underlying conscious phenomena. The thermal grill illusion (TGI) has been suggested as a promising model for exploring percepts involved in neuropathic pain, such as cold-allodynia (pain arising from contact with innocuous cold). The TGI is an unpleasant/painful sensation from touching juxtapositioned bars of cold and warm innocuous temperatures.AimTo develop an MRI-compatible TGI-unit and explore the supraspinal correlates of the illusion, using fMRI, in a group of healthy volunteers.MethodsWe constructed a TGI-thermode allowing the rapid presentation of warm(41°C), cold(18°C) and interleaved(41°C+18°C = TGI) temperatures in an fMRI-environment. Twenty volunteers were tested. The affective-motivational (“unpleasantness”) and sensory-disciminatory (“pain-intensity”) dimensions of each respective stimulus were rated. Functional images were analyzed at a corrected α-level <0.05.ResultsThe TGI was rated as significantly more unpleasant and painful than stimulation with each of its constituent temperatures. Also, the TGI was rated as significantly more unpleasant than painful. Thermal stimulation versus neutral baseline revealed bilateral activations of the anterior insulae and fronto-parietal regions. Unlike its constituent temperatures the TGI displayed a strong activation of the right (contralateral) thalamus. Exploratory contrasts at a slightly more liberal threshold-level also revealed a TGI-activation of the right mid/anterior insula, correlating with ratings of unpleasantness(rho = 0.31).Conclusion/SignificanceTo the best of our knowledge, this is the first fMRI-study of the TGI. The activation of the anterior insula is consistent with this region's putative role in processing of homeostatically relevant feeling-states. Our results constitute the first neurophysiologic evidence of thalamic involvement in the TGI. Similar thalamic activity has previously been observed during evoked cold-allodynia in patients with central neuropathic pain. Our results further the understanding of the supraspinal correlates of the TGI-phenomenon and pave the way for future inquiries into if and how it may relate to neuropathic pain.
Highlights
Illusions in the visual and somatosensory domain have contributed considerably to our understanding of the neural mechanisms involved in various conscious processes
Unlike its constituent temperatures the thermal grill illusion (TGI) displayed a strong activation of the right thalamus
The TGI has been studied with regard to pharmacologic [11,12], multi-sensory [13] and affective [14] manipulations and, recently, we reported of a putative genetic component relating to variation in thermal-pain sensitivity [15]
Summary
Illusions in the visual and somatosensory domain have contributed considerably to our understanding of the neural mechanisms involved in various conscious processes. Perceptual illusions allow the testing of models for conscious phenomena and – importantly – when coupled to neurophysiological measurements, inferences about the underlying neural substrates. One such potentially useful sensory illusion is the thermal grill illusion (TGI). The TGI was first described by Torsten Thunberg in 1898 [1] It is as an unpleasant, potentially painful, burning sensation that arises when touching an alternating pattern of innocuous cold and warm temperatures. The thermal grill illusion (TGI) has been suggested as a promising model for exploring percepts involved in neuropathic pain, such as cold-allodynia (pain arising from contact with innocuous cold). The TGI is an unpleasant/painful sensation from touching juxtapositioned bars of cold and warm innocuous temperatures
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