Over the past 20 years, there has been a concerted effort to expand our 
understanding of the neural circuitry involved in the pathogenesis of 
psychiatric disorders. Distinct neuronal circuits and networks have been 
implicated in obsessive compulsive disorder (OCD) and major depressive disorder 
(MDD) involving feedback loops between the cortex, striatum, and thalamus. When 
neurosurgery is used as a therapeutic tool in severe OCD and MDD, the goal is to 
modulate specific targets or nodes within these networks in an effort to produce 
symptom relief.Currently, four lesioning neurosurgical procedures are utilized 
for treatment refractory OCD and MDD: cingulotomy, capsulotomy, subcaudate 
tractotomy, and limbic leucotomy. Deep brain stimulation (DBS) is a novel 
neurosurgical approach that has some distinct advantages over lesioning 
procedures. With DBS, the desired clinical effect can be achieved by reversible, 
high frequency stimulation in a nucleus or at a node in the circuit without the 
need to produce an irreversible lesion. Recent trials of deep brain stimulation 
in both OCD and MDD at several neuroanatomical targets have reported promising 
early results in highly refractory patients and with a good safety profile. 
Future definitive trials in MDD and OCD are envisaged.