Richter, Ward R., Zouhar, Raymond L., Tatsuno, Jiro, Smith, Robert H. and Cullen, Stuart C. Electron microscopy of the macaca mulatta brain after repeated applications of electric current. Anesthesiology. 36(4):374-377, 1972.

2 immature Rhesus monkeys and a control were studied to assess safety of electric currents applied to the brain. 1 monkey received only sine wave current up to 50 mA, another received only square wave, up to 13 mA. They received 10, 1 hour treatments. Following the course of treatment, the monkeys were sacrificed and brain tissue was analyzed. They examined neurons, neuroglia, myelinated and nonmyelinated fibers, capillaries and nearby astrocytes, synaptic endings and the general architecture of the neurophil. Special attention was given to the structure of the synapse and the size, location, and distribution of synaptic vesicles. The authors concluded that all of these structures were within normal limits, that there is no generalized evidence of injury, and that they have greatly increased their confidence in the safety of repeated application of electric currents.