Optogram

An optogram was the image of the last thing that a person saw, retained in a dead eye. Both Madame Vastra and the Eleventh Doctor dismissed the notion as "silly superstition", with the Doctor particularly ascribing the notion to the Romany people. Nevertheless, the Doctor allowed for the possibility of optograms if the body's chemistry were disturbed. The presence of red leech toxin was sufficient disturbance to allow for optograms, as evidenced by the fact that the image of the Doctor was indeed retained by Edmund's eyes. (TV: The Crimson Horror)

The Fourth Doctor referred to this belief on the Nerva Beacon while constructing a device which would enable him to tap into the Wirrn hive mind and learn how the Wirrn queen died, although he did not mention the belief's name. (TV: The Ark in Space)