This is an interesting study from the University of Princeton’s Neuroscience Institute: http://www.ncbi.nlm.nih.gov/pubmed/21228167
Multiple stimuli present in the visual field at the same time compete for neural representation by mutually suppressing their evoked activity throughout visual cortex, providing a neural correlate for the limited processing capacity of the visual system.
If you’ve got a bunch of clutter and shit all over the place, it limits your ability to focus. This should apply as much to the physical world as it does your screen real estate. Just like you’re going to have trouble focusing if you’ve got a desk stuffed with tschotskes and photos, you’re going to have trouble focusing if you’ve got a million tabs open, or some crazy background image, or email, web, music and docs all open in front of you.
Just like you restrict your internet use or shut down your email when you’ve got to focus and get something done, you should clean out your workspace, both physical and digital.