Figure 4.

Heterotopic effects of conditioning electrical stimuli on pin prick-evoked pain. A, B, Conditioning HFS induced a significant enhancement of pin prick-evoked pain adjacent to the conditioning electrode (•) but not adjacent to the control electrode (○). This secondary hyperalgesia occurred after conditioning stimulus intensities of 10 × T and 20 × T. Pain perception was significantly increased already at 5 min and remained potentiated throughout the 60 min observation period (10 × T) or even increased further (20 × T). C, Conditioning LFS at 10 × T induced no changes in pin prick-evoked pain at any test site. D, For conditioning LFS at 20 × T, a small but significant increase in pin prick-evoked pain was found near the conditioning electrode but not the control electrode. Mean ± SEM values across eight subjects are shown. Dotted lines indicate mean level of baseline period. Each circle represents the normalized average of pain ratings across all seven stimulus intensities over a 5 min time window. Asterisks indicate post hoc paired t tests; conditioned versus control site; p < 0.05.