BRAIN CORRELATES OF PAIN REALITY
Suggestion-induced
pain activates brain’s pain circuitry as well as a real pain experience. These
two mind states correspond to different degrees of conscious reality. Recently
Tuukka T. Raij et al. (Brain correlates of subjective reality
of physically and psychologically induced pain. Proc. Natl. Acad. Sci. USA
10.1073/pnas 0409542102 published on-line before print, January 31 2005)
investigated the neurological basis of such a difference.
Meaningful
behaviour requires successful differentiation of events surfacing one’s mind from those arising from
the external world. Such judgements may be especially demanding
during pain because of the strong contribution from psychological
factors to this experience. It is unknown how the subjective reality
of pain (SRP) is constructed in the human brain, and neuronal
mechanisms of the subjective reality are poorly understood in
general.
To address these
questions, 14 suggestion-prone healthy subjects rated reality of
pain that was induced either by laser pulses to the skin or by
hypnotic suggestion during functional MRI. Both pain states were
associated with activation of the brain’s pain circuitry. During
laser stimulation, the sensory parts of this circuitry were
activated more strongly, and their activation strengths correlated
positively with the SRP. During suggestion-induced pain, the reality
estimates were lower and correlated positively with activation
strengths in the rostral and perigenual anterior cingulate cortex
and in the pericingulate regions of the medial prefrontal cortex;
a similar trend was evident during laser-induced pain. These findings
support the view that information about sensory-discriminative characteristics
of pain contributes to the SRP.
Differences in
such information between physically and psychologically induced pain,
however, could be quantitative rather than qualitative and therefore
insufficient for judging the reality of pain without knowledge about
the source of this information. The medial prefrontal cortex is a likely
area to contribute to such source monitoring.