Temporal summation of ¡°secon
d pain?(TSSP) or ¡°win
dup?results from the summation of?C-fiber-evoke
d responses of
dorsal-horn neurons. This phenomenon is
depen
dent on stimulus frequency (?33 Hz) an
d relevant to central sensitization an
d chronic pain. Our previous neuroimaging stu
dies characterize
d brain regions associate
d with TSSP in normal control (NC) an
d fibromyalgia (FM) groups. During an fMRI scan, subjects receive
d sensitivity-a
djuste
d repetitive heat pulses at .33?on the right foot. FM subjects require
d significantly lower stimulus intensities than NC to achieve similar TSSP an
d no significant group
differences in the pain-relate
d brain activity were
detecte
d. In our current stu
dy, we aske
d whether the effective connectivity among a set of TSSP-relate
d brain regions i
dentifie
d in our previous work
differs amongst FM an
d NC groups. Structural equation mo
deling was use
d to characterize the effective connectivity amongst a priori selecte
d brain areas, inclu
ding the thalamus, S1, S2, posterior insula, an
d the anterior mi
dcingulate cortex (aMCC) within the left an
d right hemispheres. This analysis confirme
d our a priori mo
dels of effective connectivity among these regions mainly confirme
d those hypothesize
d, yet some unpre
dicte
d connections were a
dditionally i
dentifie
d (thalamus to aMCC an
d aMCC to S1). While the mo
dels of effective connectivity were not i
dentical in the FM an
d NC groups, they were very similar. A
dditionally, the TSSP relate
d effective connectivity of right an
d left hemisphere regions was very similar. These results provi
de evi
dence for significant overlap of the fun
damental brain mechanisms that process sensory an
d affective information relate
d to TSSP in NC an
d FM groups.
Perspective
Models of effective connectivity involving pain-related processes were estimated with fMRI data from chronic pain and healthy populations. Models were estimated in both hemispheres, and although similar, fibromyalgia was associated with unique models of pain-related processes. Group differences involved the left hemisphere and S1, S2, and posterior insula.