Breathing up with O2

There was a thread started about breathing up on O2 recently. Obviously it would not be wise to breathe up to 1.0 pp O2 and dive down to 20 or even 10 meters, but I was wondering what the effects would be sitting on land.
So, my son and I went down to the shrimp hatchery on Sunday and gave it a shot. We use O2 for shipping baby shrimp to other farms and here.
First off, my personal best on air is only 4 min 5 sec.
I laid down on the floor and breathed up normally, deep and slow for a couple minutes. I then took 5 deep, breaths of pure O2, followed by 4 quick hyperventilations.
I felt a little dizzy at first but just laid there. The first attempt I reached 3 min 40 sec. I started feeling a little dizzy near the end and saw a few stars so I stopped. Also, my diaphragm was slowly spasming.
I waited another 8 minutes or so and repeated the process. This time I reached 6 min and 20 seconds.
Same thing with the spasms.
I think that the CO2 was building up, telling me to breath, thus the spasms. But I had enough O2 left in my blood that I didn't feel the need to breath.
I think I could reach 8 minutes easily. But then, what's the use? This isn't something I would ever apply while diving....or even sticking my head under water in the bathtub.
But, the weather was bad so no spearfishing that day....life in the country.