Regenerative dissonance

Regenerative dissonance was a mental illness seen in Time Lords. It occurred when the personalities of a Time Lord's previous incarnations lived on inside the body of their current incarnation, exerting just as much influence on the body as the personality of the current incarnation. Time Lords known to suffer from this illness often committed suicide as they couldn't cope with the voices. (AUDIO: World of Damnation) It was normal for a Time Lord's past personalities to continue existing within them, but they usually remained relatively inert; Rassilon was said to have imposed the twelve-regeneration because Time Lords could not handle the "multiplicity of psyches" beyond that point. (PROSE: Head Games)

The Time Lord calling himself the Eleven by their eleventh incarnation suffered from this. (AUDIO: The Eleven) Other than him, the Time Lord who lasted the longest with this condition shot out both his hearts with a staser in his eighth body. (AUDIO: World of Damnation)

While the various incarnations could interact with each other, they were still fundamentally dependent on their other selves to create a psychological balance. When the Eleven attempted to extract some of his past personas and transfer them into cloned bodies, even uploading the One into his TARDIS's telepathic circuits, this began to cause a psychological breakdown in the Eleven himself. The Sixth Doctor also observed that the extracted incarnations were regressing to a more basic level, such as the Six and the Nine only capable of thinking of murder or theft when they usually had the potential to explore other interests. The Sixth Doctor was able to negotiate an alliance with the One and the Nine where they would be transferred into his mind long enough for him to return them to the Eleven. (AUDIO: One for All)

The regenerative dissonance altered the nature of the energy released by the afflicted Time Lord when they regenerated, with the result that they were essentially immune to the Ravenous, a race who essentially fed on regenerative energy. When the Nine and the Eleven briefly met, their two versions of the Four actually argued about the Nine's current plan, prompting the Eleven to observe that it would appear that regeneration actually affected the past personalities as well, speculating that they 'filtered' their perception of their past selves through their current persona. (AUDIO: The Odds Against) It would appear that the condition essentially manifested a copy of the old personalities rather than just allowing them to remain 'active' on some level, as the Eleven found himself talking to the Eleven in his subconscious after he was shot by the Masters but before he actually regenerated into the Twelve. (AUDIO: Day of the Master)

Neural inhibitors allowed users with regenerative dissonance to maintain control over the different personalities within their mind. The High Council gave the Twelve one as she went on a mission for them. (AUDIO: Planet of the Ogrons) Meditation also provided a way to suppress different personalities but as shown with the Eight, the process was a temporary and, at times, painful solution. (AUDIO: The Eighth Piece, The Doomsday Chronometer) It was also a technique that couldn't be effectively repeated by succeeding incarnations such as when the Nine tried using meditation to hold back his past selves but the Eight claimed the former's skill in meditation wasn't as strong as his. (AUDIO: The Crucible of Souls)

Degeneration energy could also be used to suppress the different personalities of regenerative dissonance as seen by The Union however, suppressing the past incarnations left the current incarnation taking on several of their predecessor's traits, as The Doctor noted that the Union shared: the Nine's kleptomania, the Seven's technical expertise, the arrogance and smarm of the Four, the Ten, or the Eleven, and the Six's insanity. It also blocked off memories from the different personalities as the Union had no recollection of meeting River Song despite the Nine and the Eleven encountering her numerous times. (AUDIO: The Union)