Somnor Disc

Somnor discs seem to make the person with the disc on their forehead susceptible to the mesmerent, a hypnotic pulse created by the Morpho. The Doctor is also susceptible to the hypnosis, however in all cases the effect was broken when Susan attacked and presumably killed the Brains of Morpho.

The name of the devices are never said on-screen, however they are in the script as revealed by the Info Text on the BBC DVD release.