Eye Drive

The Eye Drive was a device created by the Silence to allow selected individuals to remember them when they were not present. The device formed a neural link with the user's mind and continually reminded them of the Silence.

The Silence built a fail-safe into the devices that caused pain or death when activated, depending on the individual. This fail-safe was integral to the functioning of the device; any duplicates included it. (DW: The Wedding of River Song)