Scientists and doctors have long used electricity to both study and treat the brain. But a report in the August 27, 2009, issue of Neuron indicates that the brain's response to electricity is exceptionally complex. Using a new type of optical imaging, Harvard Medical School researchers observed neurons as they were stimulated by an electrode. Instead of activating a small sphere of surrounding neurons as expected, the electrodes caused sparse strings of neurons to flre across the brain. The flnding suggests that brain surgeons and the designers of neural prosthetics have a much smaller margin of error than previously thought)shifting an electrode even slightly could activate an entirely different set of neurons