An intensity matched comparison of laser- and contact heat evoked potentials

Figure 1. 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).

Figure 2. Perceived pain matching. Illustrated is the correlation between the contact heat (x-axis) and laser (y-axis) perceived pain as well as the intraclass correlation coefficient (ICC).

Figure 3. CHEPs and LEPs baseline comparison. The measurement illustrates on the x-axis, CHEPs in white and LEPs in gray. The y-axis illustrates the LEPs and CHEPs parameters (latency or amplitude). Subjects’ responses of both methods are connected by a grey line.

Figure 4. Measure of Mechanical and Thermal Hyperalgesia. Comparison of the sham and capsaicin condition for (row a) mechanical and (row b) thermal pain rating for the CHEPs (white background) and LEPs session (grey background) separately. The condition (sham and capsaicin) is illustrated on the x-axis. In (row a) the y-axis illustrates the difference in overall pain rating, which is calculated by subtracting the pre-cream from the post-cream pain. In (row b) the y-axis illustrates average pain rating during the sessions. Subjects’ responses of both conditions are connected by a grey line.

Figure 5. CHEPs and LEPs Grand Average. Illustrated are the grand average potentials measured at (column a) the vertex (N2P2 waveform) and (column b) contralateral electrodes C3 or C4 (N1 waveform). CHEPs are indicated with a white background and LEPs with a grey background. The blue lines illustrate the grand average of the sham condition and the red line the grand average of the capsaicin condition. Included are the 95% confidence intervals, shown with a less dense blue and red area. N2 occurs around 377 ms for CHEPs and 205 ms for LEPs. P2 occurs around 507 ms for CHEPs and 321 ms for LEPs. N = 19. N1 occurs around 327 ms and 160 ms for CHEPs and LEPs, respectively.

Figure 6. CHEPs and LEPs Single Subject Average. Illustrated are the average potentials measured at (column a) the vertex (N2P2 waveform) and (column b) contralateral electrodes C3 (N1 waveform) of subject C14. CHEPs are indicated with a white background and LEPs with a grey background. The blue lines illustrate the grand average of the sham condition and the red line the grand average of the capsaicin condition.