The thing that confuses us about what percent of our brain is in use is the fact that most of what the brain does is not conscious or detectable by our consciousness. Visual processing is immensely complex, involving simultaneous serial and parallel processes. For example, how do we recognize the separateness of objects from each other and the surrounding environment just by looking? We have only teased out some of the first stages of visual processing, finding neurons that respond preferably to edges at particular angles, or light spots surrounded by darkness or vice-versa, or changing vs. stationary patterns. We know some similar details for auditory discrimination, for example that it works by frequency analysis, whereas vision works by spatial analysis. While this can be localized to certain not insignificant portions of the brain, how does it get translated into conscious seeing and hearing? (Optical and auditory illusions may offer some insight, but as "black box" experiments, are difficult to interpret as to what part of the brain is causing them.) It seems much of the remaining portion of the brain we haven't detailed must be needed for translation to conscious appreciation, so it seems reasonable that conscious reasoning (which I think is what is implied in those "10%" statements) may use little of the total brain "machinery."