Eye Drive

The Eye Drive was a device created by the Silence to allow selected individuals, such as Madame Kovarian, to remember the alien species running the order for them when they were not present. The Eye Drive looked like a black eye patch and was worn over a person's eye. 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)