It's very interesting that you'd ask this! Yes, if bloodsugar is extremely high for a longer period of time you can go into Hyperosmolar Hyperglycemic State (HHS), where one can experience hallucinations and other altered mental states due to extreme dehydration.
I find it so interesting because the other night I experienced my first "diabetic hallucination" after I ate a very sugary snack, forgot insulin, and went to bed. I woke up several times in the night and saw some pretty crazy stuff (floating, flaming Greco-roman buildings?) I was completely unfazed at the time, though definitely fully awake. I know my BS must have been at least 600 at the time and had been for several hours. When I woke up the next day, I did some research because I had no idea what had happened!
It is a different state than DKA or high ketones which is more common in T1, because your body is still getting SOME insulin. So the typical vomitting and extreme weakness are not present.