Good excerpt at Delanceyplace today, from the book Final Jeopardy: Man vs. Machine and the Quest to Know Everything.
Most of what humans experience as perception is actually furnished by the memory. This is because the conscious brain can only process a trickle of data. Psychologists agree that only one to four 'items,' either thoughts or sensations, can be held in mind, immediately available to consciousness, at the same time. Some have tried to quantify these constraints. According to the work of Manfred Zimmerman of Germany's Heidelberg University, only a woeful fifty bits of information per second make their way into the conscious brain, while an estimated eleven million bits of data flow from the senses every second. Many psychologists object to these attempts to measure thoughts and perceptions as digital bits. But however they're measured, the stark limits of the mind are clear.
(That's a small excerpt of the excerpt.)