Why are the little fingers not used on Braille devices?

I would like to know why the little fingers are not used as input on Braille display hardware devices? After all, the top faces only have positions for the thumbs and the the fingers adjacent to them. On a regular keyboard for non-blind and blind users (without text input feedback), the little fingers might not used very frequently, but they are used.

Or am I getting mixed up? Perhaps the two keys on the horizontal surface below the Braille refreshable keys are what the thumb uses, or perhaps the thumbs switch back and forth between the upper and lower horizontal surface positions?