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. 2021 Mar 25;11:6861. doi: 10.1038/s41598-021-85819-w

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© The Author(s) 2021

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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Study design. Subjects first underwent an intensity-matching protocol on the non-dominant forearm (upper box). Using a stair-case paradigm, CHEPs and LEPs were matched for stimulation intensity based on individual perception. Subsequently, subjects were randomized to the capsaicin (white background) or sham (gray background) group. Both experimental groups started with the assessment of pain sensation using pinprick stimulation within the confirmed secondary hyperalgesia area on the dominant volar forearm. After 30 min of topical application (sham or capsaicin), the pinprick protocol was repeated and then 20 laser or contact heat stimulations were applied. Brain responses to contact heat and laser stimulations applied in the secondary hyperalgesia area were recorded with a 32-channel EEG set up. Subjects were exposed to all four conditions in a randomized order. The volar forearm at the bottom is an illustration of the 1 ml capsaicin or sham cream application (gray perimeter with 1 cm width). The measured pinprick, laser and contact heat site is the inner square with the size of 6 × 8 cm (expected area of secondary hyperalgesia) and is centered to the forearm (wrist to elbow).