As predicted, performance seemed to be enhanced by thinking with eyes closed, as in both the general knowledge questions and math problems, participants got a higher percentage correct when they did their thinking with their eyes closed.

Why?

Research in this area suggests that cognitive performance takes a hit when we are looking at distracting, task-irrelevant visual stimuli, because some of our brainpower is diverted to the task of suppressing or ignoring what we’re looking at.

Normally, it’s pretty important that we be tuned into our surroundings. That we notice the smell of smoke (cookies burning in the oven), be aware of cars running red lights (instead of texting while crossing the street), or recognize that strange silence which often signifies that the dog is up to some sort of mischief (taking dirty clothes out of the hamper to make a bed for himself).

But when it comes to performing, much of what’s in our immediate environment is actually not relevant to the task at hand (unless we’re talking about an orchestral performance, of course, in which case your conductor might beg to differ). So when we engage visually with the audience, puzzle over the meaning of the piece of tape on the floor in front of us, or marvel at how our hands are moving across the keyboard, this diverts precious resources from creating and shaping the phrase we are actually playing at the moment, potentially leading to a performance that’s slightly less awesome than it could have been.

(via)