Cortical responses to thermal pain depend on stimulus size: A functional MRI study

Cortical activity patterns to thermal painful stimuli of two different size s were examined in normal volunteers using functional magnetic resonance im aging (fMRI). Seven right-handed subjects were studied when the painful sti mulus applied to the right hand fingers covered either 1,074-mm(2)-area lar ge stimulator or 21-mm(2)-area small stimulator. Stimulus temperatures were adjusted to give rise to equivalent moderately painful ratings. fMRI signa l increases and decreases were determined for the contralateral parietal an d motor areas. When the overall activity in these regions was compared acro ss subjects, increased fMRI activity was observed over more brain volume wi th the larger stimulator, whereas decreased fMRI activity was seen in more brain volume for the smaller stimulator. The individual subject and group-a veraged activity patterns indicated regional specific differences in increa sed and decreased fMRI activity. The small stimulator resulted in decreased fMRI responses throughout the upper body representation in both primary so matosensory and motor cortices. In contrast, no decreased fMRI signals were seen in the secondary somatosensory cortex and in the insula. In another s even volunteers, the effects of the size of the thermal painful stimulus on vibrotactile thresholds were examined psychophysically. Painful stimuli we re delivered to the fingers and vibrotactile thresholds were measured on th e arm just distal to the elbow. Consistent with the fMRI results in the pri mary somatosensory cortex, painful thermal stimuli using the small stimulat or increased vibrotactile thresholds on the forearm, whereas similarly pain ful stimuli using the large stimulator had no effect on forearm vibrotactil e thresholds. These results are discussed in relation to the cortical dynam ics fur pain perception and in relation to the center-surround organization of cortical neurons.