Objectives: Tactile dysfunction in chronic pain is explained as disruption in somatotopically based processing of stimuli. We hypothesized that people with chronic back pain also demonstrate a spatially defined disruption of tactile processing.
Methods: In 3 cross-sectional experiments, 26 patients with unilateral low back pain and 12 healthy controls made temporal order judgments of pairs of tactile stimuli. We analyzed the stimulus onset asynchrony at which participants perceived them to be simultaneous (PSS). Stimuli were delivered to either side of the back or to both index fingers. For hand stimuli, the position of the hands were 1) one either side of the back or 2) in front of the body, 3) one behind the back and one in front on the affected side or 4) on the unaffected side.
Results: In patients, mean ± SD PSS for stimuli to either side of the lower back occurred when the affected side received the stimulus 25 ± 25 msec before the unaffected side. PSS for stimuli to the hands with one hand held near the affected area was similar when the other hand was behind the back on the opposite side of the midline (17 ± 17 msec) or in front of the body on the affected side (31 ± 21 msec). These PSS values were greater than that for all other conditions and in healthy controls (p < 0.01), which approached zero.
Conclusions: Spatial representation of vibrotactile stimuli is disrupted in chronic unilateral back pain.