[mtlmtlmtlmtl]

Well, to look for impairment at any specific task, you want to just do that task, of course.

I agree there's really no meaningful concept of "general impairment" unless you're talking about something more extreme like a brain disorder or heavy alcohol intoxication with global involvement leading to something like a coma or gross movement disorder.

These various neuropsych tests try to test something as specific as possible. Working memory for instance. Their development is a complicated field all on its own. And this is psychology, so it's good to keep the reproducibility crisis in mind and adjust one's credences accordingly.

There's a really cool painter whose name I forget who mainly does self portraits while under the effect of various substances. The one where he huffed computer duster for instance,is just a few meaningless lines on a mostly white canvas. That's a pretty good example of global impairment right there. And there's also sorts of paintings on psychedelics, showing varying degrees of visual impairment. Really recommend googling the guy if you want a very visceral way of visualising the various impairments brought on by psychoactive drugs.

This is becoming an unfocused rant so I'll cap it off here I guess.