Imaging attentional modulation of pain in the periaqueductal gray in humans

Abstract

Pain is an unpleasant sensory and emotional experience usually triggered by stimulation of peripheral nerves and often associated with actual or potential tissue damage. It is well known that pain perception for patients and normal subjects can be modulated by psychological factors, such as attention, stress, and arousal. Our understanding of how this modulation occurs at a neuroanatomical level is poor. Here we neuroanatomically defined a key area in the network of brain regions active in response to pain that is modulated by attention to the painful stimulus. High-resolution functional magnetic resonance imaging was used to define brain activation to painful heat stimulation applied to the hand of nine normal subjects within the periaqueductal gray region. Subjects were asked to either focus on or distract themselves from the painful stimuli, which were cued using colored lights. During the distraction condition, subjects rated the pain intensity as significantly lower compared with when they attended to the stimulus. Activation in the periaqueductal gray was significantly increased during the distraction condition, and the total increase in activation was predictive of changes in perceived intensity. This provides direct evidence supporting the notion that the periaqueductal gray is a site for higher cortical control of pain modulation in humans.

Fig. 1.

A, B, Graphs showing the pain scores (mean ± SE) between conditions for intensity (A) and aversiveness (B) (*p < 0.05).

Fig. 2.

A, B, Figure showing representative activation within the periaqueductal gray for one subject (A). The corresponding time course of MR signal intensity change during warm (yellow boxes) and painful (red boxes) stimulation for the voxel with highest Z statistic is also shown (B).

Fig. 3.

A, B, Total signal intensity (arbitrary units) within the periaqueductal gray for the two attentional conditions (A) (mean ± SE; *p < 0.05). B, Correlation of total signal intensity change (arbitrary units) within the periaqueductal gray and total change in pain intensity (visual analog score) between the two conditions (p < 0.025).