Haemodynamic brain responses to acute pain in humans - Sensory and attentional networks

Turning attention towards or away from a painful heat stimulus is known to modify both the subjective intensity of pain and the cortical evoked potent ials to noxious stimuli. Using PET, we investigated in 12 volunteers whethe r pain-related regional cerebral blood flow (rCBF) changes were also modula ted by attention. High (mean 46.6 degrees C) or low (mean 39 degrees C) int ensity thermal stimuli were applied to the hand under three attentional con ditions: (i) attention directed towards the stimuli, (ii) attention diverte d from the stimuli, and (iii) no task. Only the insular/second somatosensor y cortices were found to respond whatever the attentional context and might , therefore, subserve the sensory-discriminative dimension of pain (intensi ty coding). In parallel, other rCBF changes previously described as 'pain-r elated' appeared to depend essentially on the attentional context. Attentio n to the thermal stimulus involved a large network which was primarily righ t-sided, including prefrontal, posterior parietal, anterior cingulate corti ces and thalamus. Anterior cingulate activity was not found to pertain to t he intensity coding network but rather to the attentional neural activity t riggered by pain. The attentional network disclosed in this study could be further subdivided into a non-specific arousal component, involving thalami c and upper brainstem regions, and a selective attention and orientating co mponent including prefrontal, posterior parietal and cingulate cortices. A further effect observed in response to high intensity stimuli was a rCBF de crease within the somatosensory cortex ipsilateral to stimulation, which wa s considered to reflect contrast enhancing and/or anticipation processes. A ttentional processes could possibly explain part of the variability observe d in previous PET reports and should therefore be considered in further stu dies on pain in both normal subjects and patients with chronic pain.