Neural activity can be monitored from outside the head using non-invasive techniques such as EEG. With these techniques, each channel records the summed activity of millions of neurons, which means the details are blurred away. Imagine experiencing a sports event through a microphone placed outside the stadium. From the roars or groans of the crowd you can tell when something good or bad happens to the home team, but you'll have a hard time distinguishing whether they scored or made a great defensive play. And you certainly wouldn’t be able to hear what individual people were saying about the game. The same is true for recording from the brain: recordings made at a distance provide some useful, high-level information, but to access fine-scale information, you need to be close to the source. Here, that means recording action potentials, or voltage “spikes,” from individual neurons. Currently, that can only be done by placing electrodes inside the brain.

The code that electric currents activate muscles and nerves is almost as old as knowledge of electricity itself. When small currents are delivered through an electrode, the changing electric field drives nearby neurons to fire one or more action potentials. By stimulating in the right temporal sequences across many electrodes, it is possible to create patterns of activity that elicit a desired sensation, for example the feel of an object in the hand or a visual image. Stimulation can also reduce or eliminate the pathological patterns of activity that occur in neurological disorders, such as reducing movement deficits in Parkinson’s disease.