A neural inhibitor was a Time Lord device that allowed users with regenerative dissonance to maintain control over the different personalities within their mind. Though the old personalities were essentially "locked away safely" within the current incarnation's mind, the user was still able to "access" them.
The High Council gave the Twelve one as she went on a mission for them. (AUDIO: Planet of the Ogrons [+]Loading...["Planet of the Ogrons (audio story)"])