Synaptic pattern displacement is a process whereby an individual's consciousness can be completely transferred to another. This process was often performed by Vulcans in order to preserve their katra, through use of a mind meld.
In 2369, Kobliad criminal Rao Vantika utilized synaptic pattern displacement in order to transfer his consciousness to Doctor Julian Bashir, as Vantika's body was on the verge of death. The process worked, and for a time Vantika was able to use Bashir's body toward his own ends. Vantika was eventually thwarted, however, when Lieutenant Dax was able to design an electromagnetic pulse to disrupt Vantika's neural energy patterns, allowing Bashir to regain control of his own body. (Star Trek II: The Wrath of Khan; DS9: "The Passenger")