We already know what happens to prisoners placed in solitary confinement; the effects on mental health are quite damaging over the long term. There have also been studies on partial sensory deprivation [0] and impairment of cognitive abilities and hallucinations began occurring within days. I can't find the appropriate references at the moment but total sensory deprivation, which is closest to the brain-in-a-jar scenario, is even worse than that IIRC.
[0] https://www.motherjones.com/politics/2012/10/donald-o-hebb-e...
> The title of the game/short novel comes to mind: "I Have No Mouth Yet I Must Scream."
Zelazny's "Go Starless in the Night" would be more appropriate and perhaps only slightly less horrifying.