http://heartmath.org/research/research-intuition/The_Resonant_Heart.pdf

What Is Meant By Psychophysiological Resonance?

Like the strings of a piano, if we strike one, then every eighth string will vibrate or resonate, producing a sound. In physiology, resonance describes a state in which two or more of the body's oscillatory systems, such as respiration and heart rhythm patterns, become synchronous and operate at the same frequency. Studies have found that sustained positive emotions such as appreciation are associated with a distinct mode of physiological function called resonance. Correlates of psychophysiological resonance include increased heart-brain synchronization, increased left brain-right brain synchronization and improved cognitive performance.

http://heartmath.org/research/research-abstracts/mri-during-coherence.html