The synaptic scan was an experimental technique developed by Noonian Soong. Synaptic scanning allowed Soong to archive the memories of Juliana Tainer, and to then transfer her "consciousness" into an android body. Soong was not certain that the technique would be successful. When Juliana awoke, however, she fully believed that she was still a Human being. (TNG: "Inheritance")
It is likely that Soong learned of this technique (or at least some principles behind it) from Ira Graves, who used a similar procedure to implant his consciousness into Lieutenant Commander Data in 2365. (TNG: "The Schizoid Man").