I've been so dehydrated (hiking, not biking) that I too was hallucinating. I kept seeing tables set with white tablecloths and a big glasses of icewater that I couldn't get my hands on. They were following me. It was scary. I could hear other people drinking and getting their glasses refilled, too (though no one else in my group was drinking either!). You don't want to see the water we ended up drinking to save our own lives. Ick.

Anyway...

I believe that HEED is designed to have enough electrolytes for shorter workouts. For longer ones, Hammer recommends Perpeteum (or however it's spelled) because HEED does not provide enough electrolytes beyond 2 hours (or something like that). Of course, HEED supplemented with shot bloks should have been fine had you been eating them.

And yes, overheating can definitely cause fuzzy thinking. It happens to me particularly because I'm not the most prolific sweat producer. If you aren't either, that probably contributed to your issues.