A method for controlling electrical conditions of tissue in relation to a current stimulus. A first current produced by a first current source is delivered to the tissue via a current injection electrode. A second current drawn by a second current source is extracted from the tissue via a current extraction electrode. The second current source is matched with the first current source so as to balance the first current and the second current. A ground electrode which is proximal to the current injection electrode and the current extraction electrode is grounded, to provide a ground path for any mismatch current between the first current and second current. A response of the tissue to the current stimulus is measured via at least one measurement electrode.
