|This article is about the the chip implanted in Spike's brain. For the the one implanted in Riley's chest, see Behavior Modifier.|
- “The implant works. Hostile Seventeen can't hurt any living creature, in any way, without intense neurological pain.”
- ―Riley Finn[src]
Behavior-modification circuitry was a behavior modification device used by the Initiative. It was a chip implanted directly into the brain of certain Hostile Subterrestrials, with the intention to prevented them from willfully harming or attempting to harm any non-demonic life form, by causing a painful shock directly into the subject's head.
- “What, that chip in your head? That's not change. That's just… holding you back. You're like a serial killer in prison!”
- ―Buffy Summers[src]
When the vampire Spike was captured by the Initiative, they had the chip implanted on him before he woke up in his cell. The chip did not hinder Spike in any way until he tried to bite Willow Rosenberg, as he was able to successfully fight off the Initiative's personnel during his escape from the complex. Spike was the only known victim of the chip; when Adam eventually unleashed hordes of demons within the Initiative complex, the demons were all capable of attacking humans without any problems.
Spike continued to live with the chip even after the organization was shut down. He used its proprieties to test whether Tara Maclay was truly a demon: when he gave her a light punch in her nose, the chip was activated, a response that would only work on humans. Although, the device's sensors were susceptible to the effects of magic, as Buffy Summers' resurrection caused a slight molecular shift enough to “confuse” the chip, and allow Spike to hurt her.
The chip only activated when the subject actually intended to harm someone, thus allowing Spike to demonstrate a particular combat move to Buffy because he knew he wouldn't actually be able to make contact, or practice combat with the Potential Slayers as long as he never actually hit them, but it prevented him from even pointing a gun at Alexander Harris when he thought the gun was real. The need for intention meant that, when Spike was brainwashed by the First Evil to become its sleeper agent and kill again, the chip did not hinder him as he was not actively intending to kill others himself.
Spke also demonstrated to be capable of putting aside the pain the chip caused. Furious that the Scooby Gang had concealed from him their plan of resurrecting Buffy, Spike grabbed and briefly manhandled Xander, being so angry that the chip had little effect on him. After the restoration of his soul, Spike was also able to hit Peter Nicols several times, despite his own continuous pain.
Spike lived with the chip in his head for three years, until it eventually degraded, randomly activating and threatening to kill Spike. Buffy contacted Riley for help, and in response, he sent a group of soldiers to Sunnydale to do so, but told them that whether or not they were to remove or repair the chip was up to Buffy. Her final choice was to have the chip removed, convinced that Spike's soul would keep him from harming people.