I can explain but there might be spoilers to The Brain of Morbius
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During the final confrontation between the Doctor and the Frankensteinish-resurrected Morbius, they both decided to do a type of mental duel, using some metal as a conduit for their mind battle. During the mind-contest, several faces mentally appear, some of them are the Doctor's known incarnations and the the Morbius monster, and some unknown faces.
Some viewers think of those unknown Doctor's as the Doctor's previous incarnations)
