In my case, I have noticed that my brain behaves more like it did when I was a child--both good and bad.;-)
I was a hyperactive child and had what I would call a hyperactive brain in that I had to work hard to stay focused. Once I did learn to focus, there were benefits that came from utilizing what was both a weakness and a potential strength. Long story, short, I noticed that my brain is once again mildly hyperactive.
Plus, I noticed significant improvement in memory and I was already a heavy exerciser.
As an aside, my HIIT consists of pushing almost to exhaustion in the intensity portions of my program. I have done Crossfit (stopped that recently due to back problems that cropped up), I do a HIIT classes at my local Wellness Center (4 times a week), and a couple of times a week I do a HIIT routine in which I do a rotation through my gym's rower, ski-erg, and Schwin Airdyne bike (usually 3-6 rounds which consist of 10-20 calories--1+ or 2+ minutes respectively--at each machine).
If you’re doing it FOUR times per week it is absolutely NOT HIIT.
A 20year old man could do it twice a week.
We’re talking exercise at the extremes of capability. Heart rates exceeding 150bpm for folks over 60 years.