A neurosurgical procedure in which indwelling electrodes are sited within specific areas of the brain, which when activated either suppress or enhance neuronal activity. It is done with the patient awake so that they can guide the surgeon as to the effects so as to help him or her place the electrodes accurately. A device analogous to a pacemaker is also implanted which can be adjusted to ‘switch on and off’ the electrodes as necessary. Currently used in some centres for PARKINSONISM, there are currently nearly 200 trials in progress assessing efficacy and safety of the technique to treat conditions such as EPILEPSY, DEPRESSION, GILLES DE LA TOURETTE'S SYNDROME and others.