Apparently the part of the brain which does calculations semi-shuts down/screens out the memory functions, and vice versa.
The researchers showed that groups of nerve cells in a structure called the posterior medial cortex, or PMC, are strongly activated during a recall task such as trying to remember whether you had coffee yesterday, but just as strongly suppressed when you're engaged in solving a math problem.
Your mind therefore actually switches functions, and these are major functions. The “current math” includes navigation, calculating options, and making coffee. The memory includes remembering where you’re going, related information not in the calculus mode and remembering where you put the coffee you made.
So many people become disoriented by these switches. These are different mental worlds, created by your own knowledge and turned into living functions. You’re not so much “absent-minded” as “elsewhere” when you find yourself dealing with something which you’d expect to be obvious but didn’t notice.
Your mind has good reasons for not wanting to be in too many places at once. In both cases, it screens out what's not required for the more important function. Arguably, there would be a sort of balancing act for some situations where a combination of current and memory logic would be required.
It looks like the mind automatically sections off what it doesn’t need for a particular thought or function. This does make survival sense. It makes more sense to do something and give it proper attention. The other side of the equation is also true- When you’re trying to think, you may not want to be jumping up and down on a trampoline in a rowboat or being run over by a herd of elephants on their way to a disco revival.
Irrelevance is the mind’s saboteur. The mind is quite capable of throwing in a few non-sequiteurs, so a filter for current/math thinking and memory/deep thinking is really a pretty good idea.
Stanford’s logic is also apparently in construction mode:
This suggests, both Parvizi and Foster said, that the PMC is not the brain's "center of self-consciousness" as some have proposed, but is more specifically engaged in constructing autobiographical narrative scenes, as occurs in recall or imagination.
Let’s not leap at definitions of a nicely defined anatomical version of self-consciousness, guys? Think about the inputs, stored memories, and logic alone. Then think about the emotions, etc. Too damn easy to have a single spot, isn’t it? Particularly with a few million synapses coming up with multi value data every second? You want the nice people that publish Gray's Anatomy to have heart failure?
Next time someone tells you you’re out of your mind, just say you’re out having it fixed. They’ll believe that.