what is bilateral stimulation?

In 1987, Dr. Francine Shapiro went for a walk.
As she walked, she noticed that her eyes were moving from side to side, and that some disturbing thoughts she’d been having were easing.

She said: “The thoughts weren’t as bothersome. I wanted to see if it would work if it was deliberate, so I brought up something that bothered me, moved my eyes in the same way and saw the same thing happening.”
What she had discovered was something that many of us “know” instinctively– going for a walk can help us to process difficult material, so that its power over our psyche is less painful or pervasive.
EMDR is a treatment that works by alternating left-right brain stimulation to help process and integrate difficult memories. It is often used to successfully treat traumatic stress. In EMDR therapy, the client recalls emotionally disturbing memories while the therapist directs them through an external bilaterally stimulating (left-right) process. The therapist may use finger movements, hand tapping, or sounds.