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. An Eye Drive looked like a black eye patch and was worn over a person's eye. The device formed a neural link with the wearer'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 duplicate included it. (DW: The Wedding of River Song)

Madame Kovarian was always seen wearing this device. Until the events of Area 52, the function of the device was unknown. (DW: Day of the Moon, The Curse of the Black Spot, The Rebel Flesh / The Almost People, A Good Man Goes to War, Closing Time, The Wedding of River Song)

After examining Madame Kovarian's eye-drive in an alternate time line, Amy Pond and her army created their own versions. These were worn by all of them, including The Doctor. Kovarian's drive seemed to be a colour slightly different from the duplicates, most probably because hers was made by The Silence, not Amy Pond's "army". (DW: The Wedding of River Song)