Figure 1.

Experimental procedure. During the Conditioning session held outside of the scanner, a 3×3 grid was drawn on the right inner arm of each participant and 9 moderate heat pain sequences were applied. Subsequently 3 identical inert creams labeled Capsaicin, Lidocaine and Neutral were applied on each grid row. Following the 20-minute waiting period, the heat pain intensity was surreptitiously lowered in the Lidocaine labeled squares; raised in the Capsaicin labeled squares and left unchanged in the Neutral squares. During the test session held in the scanner, first 9 moderate heat pain sequences were applied on each square. Then 3 creams were applied in the same way as during conditioning. Subjects were informed that the two sessions were identical. However, decreased and increased levels of heat were applied only on the first of the 3 Lidocaine- and Capsaicin- labeled squares, for the other 2 the same medium pain level was applied as during the pre-cream phase and the Neutral cream squares. Throughout all sessions subjects rated pain intensity on the 0–20 Gracely Scale.