An image forming medium including at least a polymer and a photochromic compound such as spiropyran embedded in the polymer, wherein spiropyran molecules of the spiropyran compound are chelated by a cation. Specifically, a colon stimulation system applies one or more electrical stimuli to the colon of the patient in order to induce peristaltic activity in the colon to relieve chronic constipation. The rate control signal may be sent as a response to a rate change request signal received from the UE by the first entity.