Mind Control is a conspiracy theory in Grand Theft Auto: San Andreas.
The conspiracy states that the government is developing, or has already developed, technology that allows them to read people's minds and even control their actions. The conspiracy is linked with Water Pollution, both acting as nefarious government plots, as well as being presented by the same characters: Marvin Trill, The Truth, and homeless pedestrians.
Marvin Trill, the paranoid host of the Area 53 radio show, is the biggest proponent of the mind control conspiracy. The first mention of mind control is during the outro of episode 2, where Marvin mentions that "mind control devices" will be featured in the next episode. During the episode, a caller phones in to discuss the EXsorbeo gaming system, which he accuses of being a Russian plot to reduce productivity. After, Marvin goes on a rant about video games and claims that the Degenatron planted messages in people's heads during the 80's.
The Truth, a paranoid hippie, mentions mind control only once by name but the concept is still prevalent in all his missions. During Are You Going to San Fierro?, CJ asks Truth why he stores so much aluminum foil in his Camper's back seat, which Truth claims is "protection against mind control." He then goes on to elaborate how the scheme works, being "images, sound, or emotion" inducted by one of 23 government satellites used for watching the populace.
Carl Johnson: What's with all the aluminum foil, man?
During Wear Flowers in Your Hair, there are multiple references to mind control. When CJ arrives at the hospital, a gray van will appear and drive onto the street below. The Truth will warn the others to cover their faces and think about a yellow rubber duck. When CJ questions Truth about this, he simply asks "Do you know what a subdermal neureophone is?" to which Carl cannot answer. "Subdermal" means underneath the skin, "neuro" means mind, and "phone" means transmitter, so the subdermal neurophone could be some type of mind reading device. Later in the mission, when the gang arrives at the Police Station, the same gray van drives by. Truth again demands that Carl keeps his eyes closed and to think of a pink golf ball. Jethro and Truth refuse to disclose what the van could be, instead distracting Carl with other conspiracy theories. It's most likely that The Truth demanded CJ to think of absurd objects in his head to prevent whoever was driving the van to read his actual thoughts.
The last mention of any mind experiments is during the mission Don Peyote, where the Truth explains that the government has developed algorithms to best intrude intro people's thoughts.
Carl Johnson: Truth, is that you?
Some homeless pedestrians in San Fierro can be seen wearing aluminum wraps on their head with a cardboard box hat reading "GET OUT!" The complex hats could be protection from perceived mind control, and the text would be directed towards whoever was doing the controlling.
Degenatron is an old school arcade game that appeared in Grand Theft Auto: Vice City, but is still referenced in GTA San Andreas. Degenatron is a reference to the real world urban legend called Polybius, a video game supposedly designed by the US government used for datamining it's players. The urban legend also claims that gameplay would induce extreme psychoactive effects, and even hallucinations in long time players. In GTA, the bizarre nature of Degenatron is referenced when Marvin Trill accuses the game of planting messages in player's heads. It's possible that this means it was used as a form of mind control.