Last time we discussed Dr. Robert Bartholow, the 19th-century physician who demonstrated that the human brain sends signals via electricity by putting wires into his housemaid's brain and causing her limbs to move.

While Bartholow's callous usage of his housemaid as a guinea pig made him a medical outcast, there are modern situations where a more delicate form of direct electrical stimulation is a must. These instances show us a great deal about how the brain operates.

The situation arises, for example, when patients have brain tumors that must be surgically removed. In this case the surgeon needs to know as much as possible about which parts of the brain near the tumor are still healthy, and what functions they serve. Direct electrical stimulation allows for this, and although it is a much more delicate affair than the experiment run by Dr. Bartholow, the principle is the same.

When surgeons touch different regions of the brain with electrodes through which current is passing, they are able to activate various functions. What the patient experiences during these forced activations depends on the area that was touched.

An area in the middle and front of the brain called the Motor Cortex, for example, makes different muscles contract. Stimulation of the back of the brain produces sensations of light and color. Another area might bring a long-distant memory to mind, or place the patient abruptly into a dream-like state.

Although it's strange to think of ourselves this way, direct brain stimulation shows that, in one sense, your brain is an electrical machine, the most complex and intricate electric device in the world.