Having a computer chip in your head controlling or monitoring your thoughts and actions is a cliched type of schizophrenic delusion, just like thinking you're Jesus Christ. But now this paranoid delusion has become reality.
Neuroscientists have successfully implanted a chip in the brain of a quadriplegic.(footnote 1) Its 100 gold sensors detect neural signals and feed them through wires thinner than a hair out through the scull to a computer interface. The quadriplegic can direct a mouse cursor by thought alone. Similar experiments have monkeys moving robotic arms by the power of thought. A large plug in the skull is the current uncomfortable and risky technique but scientists hope to develop wireless in the near future.
The focus of these developments is on enabling disabled people to regain the use of their limbs or cybernetic replacements - mechanical arms and legs. Feasibly the brain, remarkably adept at learning and coordinating movement in the world, could manipulate any kind of machine. You might be able to drive your car, fly a plane, hack into NASA, tell the security door your password, drive a tank on the other side of the world or fire off nuclear bombs on a bad day - all by thought alone. This is all focused on output from the brain and will probably in fact be used benevolently with useful cybernetic devices. In the current model there is no input from the chip back into, and influencing the brain.
But some related papers advocating this technology suggest it can be used to help not only physically disabled people, but people with depression.(footnote 2) The obvious suggestion is that such a chip could detect depressive activity in the brain and generate input to the brain to 'correct' it. If this is possible, then it is easy enough to see that people with schizophrenia might also be treated this way. The terrifying twist is that if someone fears that a computer chip has been implanted in their brain controlling their thoughts, the treatment for this might be to implant a computer chip in their brain to control their thoughts. So if you are worried that the military-industrial complex has already been using this technology since the 50's - watch out, they might cure you, or maybe they already have.
(This is leaving aside the whole question as to whether people with mental illness are in fact 'sick' and need to be 'cured' or whether they just have an artistic temperament, or think differently. While I acknowledge that some people might need help, and possibly treatment, I have serious concerns about over diagnosis, especially for depression. Statements are made by health authorities saying 1 in 5(footnote 3) or similar numbers suffer from depression or mental illness - to me being depressed is a perfectly sane reaction to this world.)
The good news is that while it is, in principle, easy to give people enhanced cybernetic motor control - it is very difficult for this technology to control emotions or types of thought, and practically impossible to report what someone is thinking. The brain is quite adept at learning the consequences of various neural activity and adjusting the links in its network to produce the right output. As babies we learn to move our limbs in an almost trial and error way. Our brains learn that certain activity produces certain limb movements and when learned, the successful configuration of connections and weights in the neural network is retained - though remains flexible as we need to respond differently to the changing world around us. To the brain it makes little difference if what responds to it's activity is a flesh and blood limb, or some robotic device.
Inputting to the brain to control it's activity is a different story. There is some degree to which this is possible. Electromagnetic fields have been applied to mentally ill patients' brains with some success, and the corresponding changes in brain activity at a broad scale are visible using brain scans. Mood changing drugs that affect the brain chemically have been with us for a long time. The point is that these are very general applications and effects and, in principle, they can only ever be general.
The neural networks of our brain do not store specific thoughts or functionality in localised easily discernable sites. The information in a brain, and it's functioning, is distributed widely across many nodes, or neurons, synapses, axons, dendrites and so on. To put this more simply, when someone thinks of a horse, there is not a particular compartment of the brain that corresponds to 'horse'. A whole series of activity and associations takes place, throughout the brain, and this activity constantly changes as the 'meaning' of a horse constantly changes in it's nuance and in different contexts. Thinking about a horse may be localised to some broad region of the brain, but this region would also be active for countless other thoughts and actions. Also, the configuration of one person's neural network when thinking of a horse, built up over time, may be entirely different to somebody else's regardless of how similar their response is, so that even if such a thought could be localised in one person, this does not apply to anyone else so it is just not feasible to attempt to detect it in anyone.
Remember also, that the brain naturally strengthens those network connections that are good and inhibits those that are bad (even if we have learned to do things we know are bad for us, it is because we derive some sort of satisfaction from them, or it is the way we have learned to get by in the world, whatever the social or cultural influences may have been). The brain will naturally inhibit inputs it doesn't like - though this applies to signals direct from the chip to the brain it leaves open the possibility of an overwhelming flood of brain affecting chemicals which cannot be resisted.
If delusional thinking is marked by a degree of over activity in some part of the brain, this could be detected by such a chip, which when processed through a program could lead to another component either outputting a certain type of drug, or some how changing the electrical activity in that broad area of the brain. So, in principle, it is relatively easy for a chip to gather output from the brain to control cybernetic devices, but difficult, though possible, for these chips to even broadly affect mood and types of thinking.
This potential is certainly something we should all be worried about, but it remains that yours and my thoughts, in their details as you and I experience them, remain our own. While the chip in my brain might one day tell the Pentagon I'm feeling kinky, no-one will ever know I am day dreaming of riding a horse in the Swiss alps while wearing a little French maid outfit.