Pavlovian conditioning of muscular responses in chronic pain patients: central and peripheral correlates

A differential conditioning design using an aversive slide as conditioned stimulus (CS+) that was followed by an intracutaneous electric stimulus to the left index finger as unconditioned stimulus (US) and a pleasant slide that was not followed by shock (CS-) were used to test the hypothesis of enhanced aversive conditioning of muscular responses in chronic back pain patients (CBP). Heart rate, skin conductance levels, and integrated surface electromyograms (EMG) from the left and right m. flexor digitorum, the right m. trapezius, and bilaterally from the m. orbicularis oculi were recorded. The electroencephalogram (EEG) was measured from nine electrode positions. The CBP patients showed an enhanced muscular response of the left forearm (where the US was applied) to the CS+ already in the preconditioning phase. During acquisition both the left forearm and the right trapezius of the patients but not the controls displayed enhanced muscular responding to the CS+. During extinction the CBP patients' muscular responses to both CS+ and CS- were elevated. The contingent negative variation of the EEG differentiated between CS+ and CS- in the healthy controls but not the chronic pain patients. These data confirm the hypothesis of enhanced muscular responding in chronic pain patients and suggest a dissociation of muscular and central processes during aversive conditioning in the patients that might contribute to the chronicity problem.