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Supraspinal characterization of the thermal grill illusion with fMRI

Albert Leung12*, Shivshil Shukla1, Eric Li3, Jeng-Ren Duann4 and Tony Yaksh1

Author Affiliations

1 Department of Anesthesiology, University of California, San Diego, School of Medicine, 9500 Gilman Drive, MC 0818, 92093 La Jolla, CA, USA

2 Veterans Administration San Diego Healthcare System, 9300 Campus Point Drive, MC 7651, 92037 La Jolla, CA, USA

3 University of California, San Diego, School of Medicine, 92037 La Jolla, CA, USA

4 Swartz Center for Computational Neuroscience, Institute for Neural Computation, University of California, San Diego, 9500 Gilman Drive # 0559, 92093-0559 La Jolla, CA, USA

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Molecular Pain 2014, 10:18  doi:10.1186/1744-8069-10-18

Published: 11 March 2014



Simultaneous presentation of non-noxious warm (40°C) and cold (20°C) stimuli in an interlacing fashion results in a transient hot burning noxious sensation (matched at 46°C) known as the thermal grill (TG) illusion. Functional magnetic resonance imaging and psychophysical assessments were utilized to compare the supraspinal events related to the spatial summation effect of three TG presentations: 20°C/20°C (G2020), 20°C/40°C (G2040) and 40°C/40°C (G4040) with corresponding matched thermode stimuli: 20°C (P20), 46°C (P46) and 40°C (P40) and hot pain (HP) stimuli.


For G2040, the hot burning sensation was only noted during the initial off-line assessment. In comparison to P40, G4040 resulted in an equally enhanced response from all supraspinal regions associated with both pain sensory/discriminatory and noxious modulatory response. In comparison to P20, G2020 presentation resulted in a much earlier diminished/sedative response leading to a statistically significantly (P < 0.01) higher degree of deactivation in modulatory supraspinal areas activated by G4040. Granger Causality Analysis showed that while thalamic activation in HP may cast activation inference in all hot pain related somatosensory, affective and modulatory areas, similar activation in G2040 and G2020 resulted in deactivation inference in the corresponding areas.


In short, the transient TG sensation is caused by a dissociated state derived from non-noxious warm and cold spatial summation interaction. The observed central dissociated state may share some parallels in certain chronic neuropathic pain states.

Thermal grill illusion; Neuropathic pain; Sensory integration; Spatial summation; fMRI