Much of our stress is driven by internal verbal processes. These worry about the future, rehash the past, and mutter about the present. In most people, the neural basis for language is on the left side of the brain while the right side handles imagery and other forms of holistic processing. (This is reversed for many left-handed people.) The two sides inhibit each other, so if one becomes more active, that quiets the other. Consequently, focusing on imagery will reduce verbal activity—and help you relax.
