This is somewhat related. I remember watching an episode of "Stan Lee's Super humans" on the Discovery channel. They were with a man who was also able to do lightning fast calculations, not of the complexity as described in the article, but he could easily recall dates, multiply 5 digit numbers faster than someone with a calculator etc.
They put this man under an MRI scanner while giving him problems to solve. What they noticed was an unusually high activity in a part of the brain that is responsible for eye movement. So – perhaps not very scientifically responsible – the neurologist offered the explanation that he was using a part of his brain that is normally used to solve complex mathematics subconsciously in order to position our eyes properly throughout the day. Apparently this motor movement is very complex and he was leveraging the ability of this area to solve these arithmetic problems and engage in pattern matching in order to recollect past dates.
Reminds me of Joe Haldeman's scifi short None So Blind. I won't spoil it but I'll give the first 2 sentences as a teaser:
It all started when Cletus Jefferson asked himself
"Why aren't all blind people geniuses?" Cletus was only
13 at the time, but it was a good question, and he would
work on it for 14 more years, and then change the world forever.
They put this man under an MRI scanner while giving him problems to solve. What they noticed was an unusually high activity in a part of the brain that is responsible for eye movement. So – perhaps not very scientifically responsible – the neurologist offered the explanation that he was using a part of his brain that is normally used to solve complex mathematics subconsciously in order to position our eyes properly throughout the day. Apparently this motor movement is very complex and he was leveraging the ability of this area to solve these arithmetic problems and engage in pattern matching in order to recollect past dates.