Whether you realize it or not, you experience the effects of mental imagery every day, for good or ill. Perhaps on your way home today, you imagined what you will eat for dinner. Did the thought make your mouth water? Daydreaming about your next vacation may evoke a soothing picture of a sun-drenched day on the beach. In contrast, imagining the sights and sounds of a tension-packed meeting may make your head pound and your blood pressure skyrocket. These are just a few examples of the powerful effect that mental images can have on the body.

The images your mind creates are mental representations of physical sensations. Scientists cannot say exactly how imagery works to promote health. But a type of brain scan, called positron emission tomography (PET), reveals that imagery provokes activity in the very parts of the brain that would be active if you were actually having an experience instead of simply thinking about it.

PET-scan findings have led experts to suggest that the images you see, hear and feel arise from the part of your brain that is responsible for higher thinking. The brain activity set in motion by imagery prompts your brain's emotional center to send messages to your hormonal and nervous systems. These systems in turn influence a variety of functions, including breathing, heart rate and blood pressure.