Wired magazine this month has an article written by Steven Gulie, a programmer for Apple. Gulie wrote a first hand account of his brain pacemaker operation and subsequent "tunings" in order to help alleviate his Parkinson's Disorder.
A common purpose of modern brain implants and the focus of much current research is establishing a biomedical prosthesis circumventing areas in the brain, which became dysfunctional (in Gulie's case, the area which was targeted was the subthalamic nucleus, which normally produces dopamine). What was described was a process of inserting two filaments (deep brain stimulator lead wires) which were connected (with some peripheral wires) to a pacemaker surgically implanted into the person's collar.
The pacemaker then releases low volatge shocks which stimulate specific parts of the brain to bring upon the desired effect. Gulie described the pacemaker as being about the size of an Ipod.
Although this article is of interest, what is much more interesting is that recent research has focused on implanting pacemakers into the brains of individuals with Tourrette's Syndrome and depression.
Specifically for depression, the pacemaker is set to affect the vagus nerve; these electrical stimulations affect blood flow to different parts of the brain, and affect neurotransmitters including serotonin and norepinephrine.
Gulie wrote about some side effects after the operation, which were related to the identification of the appropriate voltage and rate of shocks that the pacemaker was to deliver. Very interesting stuff indeed.
As he identified, this operation was part of a very Web 2.0 procedure; previous brain pacemakers were much more crude and more difficult to work with.