Herbert Benson, M.D., a professor at Harvard Medical School, describes the meditation experience as the "relaxation response." He discovered by studying various yogis and longtime meditators that the meditation process counteracted the effects of the sympathetic nervous system, the one that wants to fight or flee. Whereas the sympathetic system dilates the pupils and gets the heart rate, respiration, and blood pressure up, the parasympathetic system, activated when we meditate, does just the opposite. Muscle tension decreases, blood pressure drops, and for some extraordinary practitioners, even temperature and basal metabolism rates drop during a prolonged meditation. Oxygen needs of the body are reduced when you are in a highly relaxed state, and brain waves change from the busy beta-waves to the blissful alpha waves.
Functional magnetic resonance imaging (FMRI) has been used to identify and characterize the brain regions that are active during meditation. This research suggests that various parts of the brain known to be involved in attention and in the control of the autonomic nervous system are activated, providing a neurochemical and anatomical basis for the effects of meditation on various physiological activities. Recent studies involving imaging are advancing the understanding of mind-body mechanisms. For example, meditation has been shown in one study to produce significant increases in left-sided anterior brain activity, which is associated with positive emotional states. Moreover, in this same study, meditation was associated with increases in antibody titers to influenza vaccine, suggesting potential linkages among meditation, positive emotional states, localized brain responses, and improved immune function.