The sensation of pain not only involves the perception of basic stimulus parameters, such as intensity, but it also has an affective dimension — how unpleasant a painful stimulus feels. How does the brain distinguish between these components? Where are they processed? Three recent papers shed new light on these questions.

Johansen et al. injected formalin into the paws of rats and placed them in a specific environment to test whether they could learn to avoid it (conditioned place avoidance). The authors reasoned that the acquisition of place avoidance would reflect the affective component of pain, whereas the acute behavioural responses (lifting and licking the injected paw) would correspond to the sensory dimension of the stimulus. Using this model, they found that destruction of the anterior cingulate cortex (ACC), a structure implicated in the affective representation of pain in humans, prevented the rats from acquiring place avoidance but did not affect the acute behavioural responses.

If ACC is important for the affective side of pain, where is the sensory dimension represented? Hofbauer et al. addressed this question by exposing subjects under hypnosis to painful stimuli, and they used PET to measure cerebral activity. During hypnosis, the authors prompted the subjects to change their perception of pain intensity, but not of its affective properties. In contrast to changes in pain affect, which are known to engage the ACC, changes in pain intensity correlated with activity in the primary somatosensory cortex.

Zubieta et al. found a similar double dissociation while performing imaging experiments to evaluate the function of μ-opioid receptors during sustained pain. They found that the degree of activation of this system in the nucleus accumbens and amygdala correlated with sensory ratings of the pain experience, whereas opioid activation in the ACC correlated with affective ratings. So, it seems that two complementary brain systems process the two faces of pain but, clearly, we experience pain as a unitary phenomenon. A colossal challenge in the field is to understand how the two systems interact to generate this unity.