Experimental protocol.

<p>Abbreviations: HNCS = heterotopic noxious counter-stimulation, UCR = unconditioned response, CS = conditioned stimulus, CS– = unpaired conditioned stimulus, CS+ = paired conditioned stimulus, CR = conditioned response, TG = test group (<i>N₁</i> = 16), CG = control group (<i>N₂</i> = 16), BL = baseline. (A) Stimulus presentations during the pre-conditioning, conditioning and post-conditioning phases (see further details in the text. (B) Electrical stimulation sequences delivered over each stimulation block.