Synaptic Transfer is a process by which two people can possess a shared dream-state, thus granting an individual access to an unconscious person's mind. The process can be used to communicate directly with a person in a coma and search their memories, and Walter Bishop even claims to have used the technique to extract information from a corpse - although he also claims this is only possible if the corpse has been dead for less than six hours.
To allow access to the unconscious person's mind, a conscious individual must have an electromagnetic probe inserted in the base of their skull and be put into a drug-induced coma by a mixture of Ketamine, Neurontin, LSD and other substances. The individual is then immersed in water, whilst wearing as few clothes as possible, and allowed to fall unconscious. The probe is attached to a relay which allows it to share information with the probe inserted into the unconscious subject, and the brain patterns of both are monitored through a computer and data recording equipment.
The searcher often experiences hallucinations before reaching a shared dream-state with the unconscious subject; evidence shows these are often the searcher's own memories and may not have any apparent connection with the subject or information they are trying to reach. Once the searcher's brainwave patterns synchronize with the unconscious subject they are able to communicate directly and can access the unconscious subject's thoughts and memories; it is unknown whether or not the subject can do the same with the searcher's memories.
Harvard Lab Edit
Olivia Dunham uses a synaptic transfer to communicate with a comatose John Scott. Assisted by an enthusiastic Walter and a reluctant Peter Bishop, she is hooked up to the equipment in the harvard laboratory and is lowered into the water tank wearing only her underwear. She experiences visions of a scrap yard, a child's room, her uncle's kayak and a graveyard, before finally synchronizing her thoughts with Scott's and communicating directly with him. He is able to show her the face of Richard Steig, the man who knew the contents of the compound which put Scott into a coma. Once Olivia finally gains the information she needs, she awakens from her coma in shock and is immediately pulled from the tank by Walter and Peter. She retains the memory of Stieg's face and is able to accurately put together a reconstruction later. ("Pilot")
Massive Dynamic Edit
A deceased John Scott is brought to the Massive Dynamic building. Nina Sharp appears and asks the attendant how long he has been dead. On learning he has only been dead for five hours, she replies "Question him", implying that Massive Dynamic has access to the Synaptic Transfer process and knows it can be used to communicate with the recently deceased. ("Pilot")
Olivia goes back into it to go through John's memories she's picked up from the first time. The first memory is of their first date, where she says John see's her when Walter says he can't see her. It is shown that he did see her at the end of the episode. The second memory holds the information she needs. This is in Season 1, Episode 9. ("The Dreamscape")