I went fifteen months without consuming any caffeine or heavily acidic foods to get my GERD back in control. I don't think I've ever felt the same mental stimulation away from coffee. I understand that since I've been drinking coffee for such a long time it could have long-term effects, but I can't find anything that supports caffeine having long-term effects on the brain other than some mood and sleep patterns.

So, I'd be inclined to believe my baseline levels are generally around where I am now (a cup of coffee every few days). Taking from that, I do have a lot of faith that coffee is still a great mental stimulant for me, and it's not just bringing me back to my old normal.

Of course, I know I can't provide anymore evidence of this other than personal experience. I don't happen to have an MRI machine in my closet.