These studies measure physiological parameters, such as skin resistance, heart rate, pupil size, EEG, fMRI results, and other variables to look at unconscious precognition. Typically, they involve the randomized application of stimuli, which may involve sounds or images while an individual is hooked up to monitoring equipment. Although neither the experimenter nor the participant knows which stimulus is coming up next (whether neutral, erotic, violent, or something else), their physiological parameters show a change 1-3 seconds before the actual stimulus occurs, and, perhaps even more interestingly, before it has even been randomly chosen by the computer. This finding has been independently replicated by a variety of experimenters and laboratories around the world, whether one looks at reactions in the autonomic nervous system or central nervous system.