Portable real-time eavesdropping on the brain is now working technology, according to a report at the Volokh Conspiracy:

“Consider this – one of my favorite neuroscientists, Jack Gallant, is building a brain-mind map. In one of his extraordinary studies, he showed subjects YouTube videos while monitoring their brain activity using a technique called functional magnetic resonance imaging (fMRI). He then built a computer algorithm “dictionary” of the brain (this brain activity corresponds to this image in the YouTube video, and so forth). Using his brain/mind/dictionary, he showed the subject new YouTube video clips that the computer didn’t get to “see” and the computer had to “guess” what the subjects were seeing. The results are really striking. Of course, an fMRI machine is enormous, so law enforcement isn’t going to use it in the field anytime soon (or ever). By contrast, portable brain monitoring has serious potential. (Full disclosure – I already own two Neurosky portable electroencephalograph (EEG) devices, which I use to hack into my own brain activity to improve my concentration, to meditate, and more.) The Parvizi study used electrocorticography. Since we already have really simple EEG devices (they don’t measure much just yet), you can bet it’s just a matter of time before we get high-quality portable brain monitoring. In fact, DARPA has been gearing up for just that (check out its “Portable Brain Recording Device & App” Project, SB131-002). Is this the future of law enforcement and surveillance?”