Man undergoing biofeedback therapy, showing him seated with electrodes attached to his head, viewing squares of coloured light. Biofeedback is a technique whereby an auditory or visual stimulus follows on from a physiological response. For example the electrodes might be used to measure the man's pulse rate, and the value of different rates indicated by means of coloured lights; this may allow him to control his heart rate by distinguishing between the different colours. How the technique works is unclear, but it is claimed that it can control pulse rate & blood pressure, relax spastic muscles and also control migraine (severe headache) attacks.