The importance of stimulus parameters for the experience of the thermal grill illusion

This study aimed to investigate the quality of the thermal grill illusion (TGI) and the importance of stimulus parameters (distance between, and number of stimulation bars).

Twenty-one different stimuli were applied to a group of 19 healthy subjects on the glabrous skin over the palm and fingers.

The TGI was found to be painful (19.42% on the palm; 17.98% on the fingers), mechanical (25.24% on the palm; 5.62% on the fingers), emotional (13.59% on the palm; 14.61% on the fingers) or unusual (42.72% on the palm; 61.8% on the fingers) sensations. A total of 89.5% (palm) and 94.4% (fingers) of the subjects reported TGI. Between 45% (fingers) and 50% (palm) of the stimuli elicited TGI. Neither the distance (2 approximately 10 mm) between adjacent warm (40 +/- 1 degrees C) and cold (20 +/- 1 degrees C) bars nor the number of the stimulation bars (2 approximately 6) significantly affected the occurrence of the TGI (N.S.). The average reaction time was 2.4 +/- 0.1 seconds to the TGI sensation. Females showed longer reaction time than males (P

The distance and number of stimulation bars were not important to the sensation of TGI, of which the responses varied. These results are useful for future TGI studies with respect to experimental design. The variability of the TGI needs to be considered in future experimental and clinical studies.