An MRI can now show, with 78% accuracy, whether you're thinking about a hammer or thinking about a pair of pliers. Apparently, even a few months ago, the same test could barely distinguish between major categories (e.g, whether the subject was thinking about "places or faces.")

This a major step forward, but I think -- all questions aside as to whether we actually want mind-reading technology -- there is still a long way to go. What we're seeing here is more a step forward in lie detection than actual mind reading. What goes on inside our brains involves a highly complex set of relationships between words, symbols, and images. At best, we're only able to articulate an approximation of what we're thinking via the spoken or written word (or visual media.)

The point is that we often don't really know what we're thinking. How then could a machine know, much less someone reading the output from the machine?