It is a lot harder to see pain than anyone guessed.
One of the most stunning imaging articles to come out lately is by 1: Christmann et al in Mannheim, writing in Neuroimage. 2007 Feb 15;34(4):1428-37.
This is bound to shake the imaging world and if these findings are correct, it is high time that they be shaken. These authors identified areas of hyperstimulation on EEG-functional MRI, but THEN, found that with increasing the pain, these same areas actually underperfused. then, to make things even more interesting, when the pain was really crnked up, the same areas could be seen to be hyperactive, but it was in the MOTOR areas not the sensory areas. Try reasoning that one out.
It certainly cautions us to be aware that we must correlate pain levels with what we see on imaging.
With painful electrical stimulus, “stimulus-correlated BOLD effects were found in the primary and secondary somatosensory areas (SI and SII), the operculum, the insula, the supplementary motor area (SMA proper), the cerebellum, and posterior parts of the anterior cingulate gyrus”. Yawn. These are for the most part tried and true areas where we EXPECT to see pain.
Wait a minute. When the pain was increased, “Higher unpleasantness rating was associated with SUPPRESSION of activity in areas known to be involved in stimulus categorization and representation (ventral premotor cortex, PCC, parietal operculum, insula) and enhanced activation in areas initiating, propagating, and executing motor reactions (ACC, SMA proper, cerebellum, primary motor cortex)
Furthermore, time was a factor in that, the “BOLD (blood oxgyen level dependent) signal change decreased in SI over time. Dipole strength of the ACC source decreased over the experiment and increased during the stimulation block suggesting sensitization and habituation effects in these areas.”
This article is consistent with an article by Coen et al in Am J Physiol Gastrointest Liver Physiol. 2007 Mar 29. This imaging study also found habituation resulting in decreased functional imaging signal over time, due to habituation to painful stimulation of the esophagus.
This makes is all harder. There are stages of pain response, and with time it all decreases due to the brain’s tendency to accomodate to habituation.
Future investigators will have to get the time factors down when claiming to image pain. Perhaps as pain increases, the motor areas which might give a response to the pain become more active, and later, with acommodation to the pain, the signal may lessen. This is a much more complex interpretation scheme than has been followed in the past.
We would love to see a similar study in central pain. These two studies were designed around nociceptive pain. Pain is very elusive. It is about as definable as consciousness and even harder to pin down with EEG.
