Other biological rhythms can also affect the heart rate. It said that Rife's microscopes had been demonstrated to doctors and they were impressed by their clarity and high magnification. A 2013 Frontiers in Psychology journal study which demonstrated the connection between breathing rate and heart beats showed that choir singers are capable of not only harmonizing their voices but can also sync their heartbeats." A second review showed that certain types of electromagnetic waves could stop tumor growth. According to the paper, sound waves can cause vibrations that break glass. Headaches are one the most studied biofeedback uses.