It will become freaky now, folks. Apart from using an EEG to record brain activity in order to control a BCI, one can, in principle, record electrical activity directly from nerve cells, using implantable electrodes as sensors to target individual cells. This approach has certain advantages (more) and some rather obvious problems, which have to do with the necessity of an implantation.
First experiments with microelectrodes in the brain were performed as early as in the 1960s, but only in recent years microelectrodes became sophisticated enough to produce a stable interface with the brain without moving or causing scar tissue to grow (more).
As far as BCIs are concerned, by now, using a systems with implantable electrodes, monkeys have managed to move cursors on a screen and even operate a robot arm via the internet. In some individual cases, severely handicapped humans with lock-in-syndrome (more) have also received electrode-based systems, generally with encouraging results.