The next civil rights and privacy battle may be over the mind

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Trolling down the street in Manhattan, I suddenly hear a woman's voice.

"Who's there? Who's there?" she whispers. I look around but can't figure out where it's coming from. It seems to emanate from inside my skull.

Was I going nuts? Nope. I had simply encountered a new advertising medium: hypersonic sound. It broadcasts audio in a focused beam, so that only a person standing directly in its path hears the message. In this case, the cable channel A&E was using the technology to promote a show about, naturally, the paranormal.

I'm a geek, so my first reaction was, "Cool!" But it also felt creepy.

We think of our brains as the ultimate private sanctuary, a zone where other people can't intrude without our knowledge or permission. But its boundaries are gradually eroding. Hypersonic sound is just a portent of what's coming, one of a host of emerging technologies aimed at tapping into our heads. These tools raise a fascinating, and queasy, new ethical question: Do we have a right to "mental privacy"?