Eye Drive
The Eye Drive was a device used by the Silence to allow selected individuals, such as Madame Kovarian, to remember the memory-proof alien species running the order. 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 existence of the Silents.
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)
Madame Kovarian was always seen wearing this device to help her remember her masters, The Silence. Until the events of Area 52, the function of the device was unknown. (DW: Day of the Moon,DW: The Curse of the Black Spot, DW: The Rebel Flesh / The Almost People, DW: A Good Man Goes to War, DW: Closing Time, DW: The Wedding of River Song)
After examining Madame Kovarian's eye-drive in an alternate time line, Amy Pond and her army managed to create their own versions. These were worn by all of them, including The Doctor. Kovarian's drive seemed to be a slightly different colour to the duplicates, most probably because hers was made by The Silence, not Amy Pond's "army". (DW: The Wedding of River Song)
