The neuro biotracer chip was implanted in John Crichton's brain by Scorpius to allow Scorpius to keep track of Crichton and ensue that he could be discovered by the Peacekeepers in future, in order to preserve the wormhole knowledge that he possessed. It also contained a 'neural clone' of Scorpius's personality, this clone appearing to John as a hallucination that only he could see and hear, possessing Scorpius's knowledge while becoming adapted to John's personality by drawing on his knowledge of Earth to communicate with John in a manner that he could relate to. When initially implanted, the chip allowed the neural clone to control John's body, initially merely altering his memory and temporarily controlling his physiology- such as temporarily 'killing' Crichton to distract a Scarran torturing him or accelerating his metabolism to burn through a drug he had been infected with-, it reached a point where the clone could take control of John completely, the chip having spread out to take control of most of his brain. After the chip was removed- requiring Crichton to receive a brain tissue transplant to compensate for the sections that had been damaged by its extraction, scrambling his ability to communicate-, the neural clone of Scorpius was left in Crichton's mind- now incapable of taking direct control of John while still able to appear to him and offer him advice and information-, while the chip itself acquired a neural clone of Crichton's personality. However, the wormhole knowledge on the chip was lost when the chip overheated during Scorpius's attempt to convince Crichton's clone to help him, the Crichton clone disintegrating itself rather than help Scorpius, with the two clones of Harvey 'dying' when their goal to protect the wormhole knowledge was no longer relevant.