In general, I don't think it should be included. Your participation level is usually small enough that you're not exercising a great deal of responsibility, or even know that much about the study parameters.
But if for some reason it had a huge impact on your outlook (e.g., seeing the experience from the research subject's side confirmed your desire to do research, etc.), then you might want to mention it in your essays. This is probably unlikely (hopefully your motivation would be more substantial), but might be true in some cases.
Good luck!