I learned in an intro to cognitive psychology class that it seems that our brain is wired to recognize sets of 1, 2, and 3 things directly, but that after that we construct the numbers. For instance when we see 5 things it is actually 3+2 that we see. The experiment was based on response time + accurateness when showing people sets of geometrical shapes for very short periods of time, sometimes using the same shape for all elements, sometimes random shapes, and sometimes groups of similar shapes. I don't recall the precise reference though, maybe someone else knows it?
To get back to the keyboard, I would like to see a demo video of someone actually typing something with it. The current video doesn't give any sense of what it is like to use this keyboard, and doesn't make me want to try it at all.