1. Transforming the Thermal Grill Effect by Crossing the Fingers
Angela Marotta, Elisa Raffaella Ferrè, Patrick Haggard 
Current Biology 
Volume 25, Issue 8, Pages (April 2015)
DOI: /j.cub
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2. Figure 1 Experimental Conditions and Results
(A) General overview of the experimental apparatus.
(B) In the baseline session, cold stimuli (blue circles) were applied on a target finger (middle and index) while these two fingers were either uncrossed or crossed. The non-target fingers received neutral temperature stimulation (pink circles). In the thermal grill session, cold stimuli (blue circles) were applied as in the baseline session, but the non-target fingers received warm stimulation (red circles). The combination of warm and cold stimuli evokes the paradoxical, sometimes painful, burning heat sensation reported in the TGI.
(C) The thermal grill effect was measured as overestimation of the target temperature in thermal grill conditions, relative to the baseline. Note that crossing the fingers reduced the thermal grill effect when the middle finger was the target but increased it when the index finger was the target. Bars indicate SD across participants.
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3. Figure 2 Spatial Influences on Thermal Perception
(A) The classic unmasking model of TGI (redrawn from [18]). Colors and letters indicate stimulus and skin temperatures and the neural pathways corresponding to the temperature: blue/C, cold; red/W, warm.
(B) A modified model involving additional thalamocortical summation of multiple warm stimuli and interaction with cold pathway from the middle finger. The gray levels indicate levels of activation (firing rates) in the warm and cold pathways (with the dark-gray color indicating strong activation). Warm activation is assumed to spread gradually across a neural representation of external space, leading to inhibition of cold on the middle finger (with the light-gray color indicating weak activation). The putative inhibitory synaptic interaction between warm and cold pathways is omitted for clarity.
(C) Summation of warm inputs occurs after remapping of somatotopic inputs into external spatial coordinates. Crossing the fingers therefore reduces the inhibition of cold on the now-peripheral middle finger, because it is less affected by spreading warm activation. The stronger cold activation results in stronger inhibition of nociceptive afferents and a reduced nociceptively mediated TGI sensation.
(D and E) The thermal grill sensation depends on the central spatiotopic position of the cold finger relative to the warm fingers, and not on finger crossing per se. When cold stimulation is given to the index finger, the TGI sensation is reduced in the uncrossed posture (D) relative to the crossed posture (E).
Current Biology  , DOI: ( /j.cub )
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