I participated in a few.
Two involved pain. One was with a heat lamp and white noise. The other was with an ice bath and neutral and pornographic images.
The white noise one was funny, because I fell asleep during it, the noise spiked, and I just about jumped out of my chair. What happened eventually was that I reacted to noise spikes by flinching away the finger that was near the heat lamp.
The ice bath one didn't work, because my tolerance was too high.