A technique in which a noise or sight stimulus follows on from a physiological response. Thus, a person's ELECTROCARDIOGRAM may be monitored, and a signal passed back to him or her indicating their heart rate: for example, a red light if the rate is between 50 and 60 beats a minute or a green light if it is between 60 and 70 a minute. Once the person has learned to discriminate between these two rates, he or she can learn to control the heart rate. How this is learned is not clear, but by utilising biofeedback some subjects can control heart rate and blood pressure, relax spastic muscles, bring migraine under control and even help relieve constipation.