* Illustration: Paula Scher/Pentagram * 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"?

"We're going to be facing this question more and more, and nobody is really ready for it," says Paul Root Wolpe, a bioethicist and board member of the nonprofit Center for Cognitive Liberty and Ethics. "If the skull is not an absolute domain of privacy, there are no privacy domains left." He argues that the big personal liberty issues of the 21st century will all be in our heads — the "civil rights of the mind," he calls it.

It's true that most of this technology is still gestational. But the early experiments are compelling: Some researchers say that fMRI brain scans can detect surprisingly specific mental acts — like whether you're entertaining racist thoughts, doing arithmetic, reading, or recognizing something. Entrepreneurs are already pushing dubious forms of the tech into the marketplace: You can now hire a firm, No Lie MRI, to conduct a "truth verification" scan if you're trying to prove you're on the level. Give it 10 years, ethicists say, and brain tools will be used regularly — sometimes responsibly, often shoddily.

Both situations scare civil libertarians. What happens when the government starts using brain scans in criminal investigations — to figure out if, say, a suspect is lying about a terrorist plot? Will the Fifth Amendment protect you from self-incrimination by your own brain? Think about your workplace, too: Your boss can already demand that you pee in a cup. Should she also be allowed to stick your head in an MRI tube as part of your performance review?

But this isn't just about reading minds; it's also about bombarding them with messages or tweaking their chemistry. Transcranial magnetic stimulation — now used to treat epilepsy — has shown that it can artificially generate states of empathy and euphoria. And you've probably heard of propranolol, a drug that can help erase traumatic memories.

Let's say you've been assaulted and you want to take propranolol to delete the memory. The state needs that memory to prosecute the assailant. Can it prevent you from taking the drug? "To a certain extent, memories are societal properties," says Adam Kolber, a visiting professor at Princeton. "Society has always made claims on your memory, such as subpoenaing you." Or what if you use transcranial stimulation to increase your empathy. Would you be required to disclose that? Could a judge throw you off a jury? Could the Army turn you away?

I'd love to give you answers. But the truth is no one knows. Privacy rights vary from state to state, and it's unclear how, or even if, the protections would apply to mental sanctity. "We really need to articulate a moral code that governs all this," warns Arthur Caplan, a University of Pennsylvania bioethicist.

The good news is that scholars are holding conferences to hash out legal positions. But we'll need a broad public debate about it, too. Civil liberties thrive only when the public demands them — and understands they're at risk. That means we need to stop seeing this stuff as science fiction and start thinking about how we'll react to it. Otherwise, we could all lose our minds.

Email clive@clivethompson.net.

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