Thought extractor

 was a machine used by Zaadur and Quendril to extract the Second Doctor's thoughts in order to gain information on how to invade the planet Earth.

The machine had the ability to read the thoughts of an individual connected to it and display their thoughts visually on a screen. However, in the process, it caused the individuals extreme pain.

Zaadur ordered the machine to be used on the Doctor to extract his information about planet Earth so she could use it to prepare for an invasion. She chose this method over verbal interrogation because she claimed the Doctor would have no possibility to lie to her this way.

The machine could also destroy a person's conscious if they were left linked up to it for too long. Zaadur attempted to kill Quendril this way after he betrayed her and let the Doctor escape.

The Doctor used the machine to persuade Quendril that Zaadur's plan to kill her race was true by displaying images he had seen of the mine including old, abandoned equipment. (AUDIO: Lords of the Red Planet)