title: Supraspinal characterization of the thermal grill illusion with fMRI.
abstract: BACKGROUND
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Anesthesiology, University of California, San Diego, School of Medicine, 9500 Gilman Drive, MC 0818, 92093 La Jolla, CA, USA. ayleung@ucsd.edu.
authors.forename: Albert
authors.initials: A
authors.lastname: Leung
authors.forename: Shivshil
authors.initials: S
authors.lastname: Shukla
authors.forename: Eric
authors.initials: E
authors.lastname: Li
authors.forename: Jeng-Ren
authors.initials: JR
authors.lastname: Duann
authors.forename: Tony
authors.initials: T
authors.lastname: Yaksh
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001921:Brain / Q000098:blood supply* / Q000503:physiopathology*; D001931:Brain Mapping; D005260:Female; D006358:Hot Temperature; D006801:Humans; D007088:Illusions / Q000523:psychology*; D007091:Image Processing, Computer-Assisted; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D010100:Oxygen / Q000097:blood; D010146:Pain / Q000503:physiopathology*; D010147:Pain Measurement; D017288:Pain Threshold / Q000502:physiology*; D011601:Psychophysics; D013697:Thermosensing / Q000502:physiology*; D055815:Young Adult
chemical_list: D010100:Oxygen