A technique whereby an auditory or visual stimulus follows on from a physiological response. Thus, a subject’s ELECTROCARDIOGRAM (ECG) may be monitored, and a signal passed back to the subject indicating his or her heart rate: for example, a red light if the rate is between 50 and 60 beats a minute; a green light if it is between 60 and 70 a minute. Once the subject has learned to discriminate between these two rates, he or she can then 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 constipation.