- "The implant works. Hostile Seventeen can't hurt any living creature, in any way, without intense neurological pain."
- ―Riley Finn[src]
Behavior-Modification Circuitry, largely known simply as "the chip", was a behavior modification device used by The Initiative. It was a chip implanted directly into the brain of certain Hostile Subterrestrials, which prevented them from willfully harming or attempting to harm any non-demon lifeform by causing a painful shock directly into the brain..
The chip had sensors capable of differencing humans from demons, though these were susceptible to the effects of magic, demonstrated when Spike became capable of harming the newly-resurrected Buffy Summers as her resurrection caused a slight molecular shift that was just enough to 'confuse' the chip to allow Spike to hurt her. The chip only activated when the subject actually intended to harm someone, such as 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 so long as he never actually hit them, but preventing him from even pointing a gun at Xander 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.
Spike was the only known victim of the chip: when Adam unleashed hordes of demons within the Initiative complex, the demons were all capable of attacking humans without any problems. To add further confusion, 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, although it may have simply taken time for the chip to be properly calibrated.
Spike used the chip to test whether Tara Maclay was truly a demon by giving her a light punch to the nose which caused the chip to activate, thus proving her humanity. While dealing with his grief and guilt over the restoration of his soul, Spike proved far more willing to push aside the pain when acting for a good cause, hitting Peter Nicols several times when Buffy asked for Spike's help to protect Cassie Newton from her predicted death.
After Spike had lived with the chip in his head for three years, 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; Buffy chose to have the chip removed, convinced that Spike's soul would keep him from harming people.
As the creation of behaviorists, the chip was intended to modify observable behavior and not desires or instincts, which are outside the concerns of Behaviorism as they can't actually be perceived or measured in any way.
- Based on available evidence, it would appear that the chip's activation trigger depended on the amount of pain Spike was intending to inflict, such as the chip activating after Spike simply tried to punch someone but triggering pain before he could fire a gun or bite someone.
- It is unknown how the chip causes localized pain as the brain itself does not actually feel any pain and if the chip is using electricity as Drusilla once described, then Spike should be feeling that pain throughout his entire body, not just his head.
- "The Yoko Factor" (First identified)