A focused neuro-pattern enhancer was a device that allowed those without psychic powers to communicate telepathically.
Gilles Lemaitre built one in 1910s Haiti, calling it his mind mirror. He gained the knowledge on how to construct it from Cthulhu and various occult texts he had collected. He used the enhancer to communicate privately with others, control the zombies he was creating, and mentally influence people.
When the Seventh Doctor found it, he made adjustments to create a feedback loop which would trap anyone using it as long as their focus was not broken. Lemaitre was caught in the loop, but Henri Duval pulled him away which freed him.
Lemaitre had also created a larger enhancer in which he could place the one in his villa. He planned to use the combined enhancers to channel enough mental energy during the Wete Mo Nan Dlo ceremony to reunite Cthulhu's mind and body. Before Lemaitre could bring them together, he was killed in an explosion when he triggered the motion sensor of one of Ace's bombs. (PROSE: White Darkness)