As for the dystopian applications, I would not be suprised if there are some dr Mengele wannabies in some dark corners of the world experimenting with how to control prison inmates or similar "disposable" people using such tech.
I wonder what's feasible in the near future. I recall excitedly watching a Gabe Newell interview on BCI. Having struggled with major depression and anxiety, his speculation on using BCI to control sleep/mood/etc seemed like a mental panacea. Of course, with that level of control over one's brain, my delight about potential emotional stabilization feels akin to lionizing computers as a newfangled bookkeeping tool -- while true, it's comically myopic.
Here's a hard turn into wild speculation for ya: The Great Filter is either 1) endosymbiosis or 2) inventing BCI.
Not much beyond the article I linked above. I've been aware of such research for a few decades. It appears that inducing happiness is super easy. But also way more addictive than the hardest drugs. There term Wirehead was a term from SciFi (and later Cyperpunk) to describe someone addicted to such stimulation.
Early use was limited by how to control the mood. Manual control would be impossible, since anyone with access to their own happiness would turn it up to max pretty much at once, with no ability to turn it back down. With it at max setting, people would simply stop functioning, not even able to eat, have sex, etc, so unsupervised it would probably be leathal pretty soon.
So for early application, one would have to set it at some constant offset, which probably had some downsides. (Possibly poor reaction to normal stimuli, I don't remember.)
Later on, maybe about 10 years ago, brain research and computer tech started to allow more sophisticated control of the level, where it would regulate the happiness-level in a way similar to how normal/healthy brains do. (The patiens would be treatment resistent MDS)
Still, the potential downsides are obviously immense, potentially making fentanyl, crack and meth seem like child's play.
A self regulated version as this would be so deadly that I think very few knowing its risks would dare use it. But one _could_ imagine people setting up arrangements where they grant the power to regulate the level, according to some principles.
For instance, let's say you're bored at work, and procastinating by reading HN, at a level that reduces your performance. Let's say that, instead of getting hold of ritalin or microdosing shrooms, you go to a shady lab that installs one of these things, and controls it remotely by lowering happiness just a bit when you're not doing what you "should" and rewards you slightly when coding (by monitoring your laptop), with additional rewards when pull requests are approved.
Now, imagine your manager (or a CPP rep, if you're in China) finding out, and bribes the lab to add some more "features" to your profile, including loyalty to her personally as well as a more aggressive level of rewards for workplace performance.
As for the dystopian applications, I would not be suprised if there are some dr Mengele wannabies in some dark corners of the world experimenting with how to control prison inmates or similar "disposable" people using such tech.