Doctor Who and the Brain: Regeneration as a Path Less Taken
We know that when the Doctor undergoes his regeneration, every cell of his body changes, including his brain. He experiences gaps in memory and alterations in personality, to some extent. But how does his former personality relate to his new one? Psychology professor Travis Langley, author of Batman and Psychology: A Dark and Stormy Knight, discussed the Doctor's relationship with his former personalities from a neuroscientific and psychological perspective.
During regeneration, the Doctor's brain undergoes changes, which one could argue makes him a "different person." But the definition of identity is very fluid, and as long as there is a continuity in consciousness and his new brain is simply an altered version of his old brain, he can be considered to be the "same person," or more accurately, in some kind of murky gray area between the same and different. This gray area is not relegated to the realm of science fiction; Langley offers several real-life examples that are comparable to the Time Lord's regeneration in this way: "It makes sense that, at least at first, he feels disconnected from his old self. People who've had serious facial reconstructive surgery can experience a depersonalization effect... The temporary, partial amnesia that usually happens during regeneration can also enhance this. People who've suffered amnesia or brain damage will sometimes refer to who they were before the change as a different person. As his memories settle back into place, he feels more connected to his past."
Langley also discussed the ways in which the psychology of the Doctor may impact his regeneration process. In the case of the First Doctor, he was conflicted about his own abrasive tendencies, and so during the first regeneration changed into a much more charming, impish version of himself. He still retained his core principles, but was generally a more gregarious person. Similar occurrences can be seen throughout the series; for example, the iterations after the War Doctor became increasingly innocent and childlike, presumably as a reaction to the violence perpetrated by the War Doctor. Professor Langley analyzed this phenomenon: "Each new Doctor seems to be a reaction to the last one. . . . As he's looking back on his last life just before regeneration, thinking about how he could've been different, it could be like a person focusing on something just before sleep and then having that idea become an influence in their dreams."
This hypothesis seems to be in keeping with the characterization of Peter Capaldi's Twelfth Doctor, at least from what we've seen so far. He appears to be reacting against his previous incarnations who cared very much about humans and their pursuits, as he "doesn't quite understand human beings or really care very much about their approval," according to Capaldi. Early buzz has suggested that, unlike his counterparts, this Doctor will be cold and calculating, without very much sympathy for human-centric perspectives. This reaction against his previous personalities was arguably made explicit in the trailer for series 8, when he says that he has "made many mistakes" and will attempt to rectify them.