The Eye Drive was a device used by the Silence to allow selected individuals, such as Madame Kovarian, to remember the memory-proof Silents running the order. An Eye Drive looked like an eye patch and was worn over the 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. (TV: 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. (TV: 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 Eleventh 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". Unfortunately, the soldiers were unaware of the built-in fail safe in the design. When the Silents broke out of confinement, they triggered the fail safes, killing many of the soldiers. (TV: The Wedding of River Song)
Behind the Scenes
The eye drives in The Wedding of River Song were one of the tributes to Nicholas Courtney written into the episode after he passed away earlier in the year. Courtney frequently retold an anecdote from the filming of Inferno at fan conventions. For the serial's reveal of the alternate universe Brigadier, who wore an eyepatch, he began with his back facing the camera and then spun around in the office's swivel chair. When Courtney spun the chair around, the entire cast and crew had also put on eyepatches but he managed to continue as if nothing had happened. "Everybody was wearing an eyepatch" became closely associated with Courtney.