The knowledge that electric currents activate muscles and nerves is almost as old as the knowledge of electricity itself. When small currents are delivered through an electrode, the changing electric field drives nearby neurons to fire one or more action potentials. By stimulating in specific temporal sequences across many electrodes, it is possible to create patterns of activity that elicit a desired sensation, for example the feel of an object in the hand or a visual image. Stimulation can also reduce or eliminate the pathological patterns of activity that occur in neurological disorders, such as reducing movement deficits in Parkinson’s disease.