This study examined processes that contribute to the changing painfulness of a repeatedly presented thermal (heat) stimulus. The 3-second pulses were presented to the side of the hand at a rate of 4/min, too slow to engage wind-up. Over the course of 32 trials, pain intensity (measured by verbal report on a 0–100 scale) first declined and then (in most cases) rose again, indicating adaptation and sensitization, respectively. The magnitude of adaptation grew across a series of 3 runs, indicating that adaptation has a slow as well as a fast component. The rate of sensitization depended on stimulus temperature, but not on subjective pain intensity; this result implies that sensitization takes place at an early processing stage. Adaptation and sensitization were comparable in participants with fibromyalgia, temporomandibular disorders, and in healthy controls, indicating that these processes occur before the perceptual amplification that characterizes fibromyalgia and temporomandibular disorders. The ability of vibration to reduce pain has previously been shown to involve segmental inhibition; the finding in the present study that vibratory gating of pain is significantly (inversely) related to the rate of sensitization suggests that the latter also reflects segmental processes. Several lines of evidence thus point to the conclusion that adaptation and sensitization occur at early stages of sensory information processing.
Adaptation and sensitization occur during early stages of pain processing, in both chronic pain patients and controls, before pain is modulated by top-down mechanisms.
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aDepartment of Psychology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
bDepartment of Endodontics, School of Dentistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
cCenter for Neurosensory Disorders, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
*Corresponding author. Address: Department of Psychology, University of North Carolina at Chapel Hill, Davie Hall, Chapel Hill, NC 27599, USA. Tel.: +1 919 962 2441; fax: +1 919 962 2537.
Article history: Received 17 February 2010; Received in revised form 19 January 2011; Accepted 24 February 2011.