Figure 1.

Experimental procedure for testing the roles of LTP and LTD in human pain perception. A, Conditioning electrical HFS or LFS was applied to the proximal volar forearm via a circular array of electrodes. Homotopic effects of HFS and LFS on the conditioned pathway were tested by pain ratings to single pulses through the conditioning electrode. Heterotopic effects outside of the conditioned pathway were tested by pain ratings to mechanical stimuli at a distance of 15 mm from the electrode arrays. B, Testing of mechanical sensitivity consisted of stimulus–response functions for pricking pain with punctate probes and soft stroking. Testing was arranged in runs comprising all 10 mechanical stimuli in a randomized order (small black bars) followed by three electrical pulses at 10× the detection threshold (•) with 5–10 sec interstimulus intervals. Runs were alternated between the conditioned skin site and the control site for 40 min before and 60 min after HFS or LFS. At the end of the experiment, the area of hyperalgesia to mechanical stimuli was mapped using a 200 mN von Frey hair. C, Laser Doppler imaging revealed large areas of increased skin perfusion (flare) at sites of HFS and LFS (20 × T).