Put a hand on your widow's peak. About an inch below your fingertips in your medial prefrontal cortex is the home of your sense of self. Julian Keenan, director of the Cognitive Neuroimaging Lab at Montclair State University, did a nifty trick: He used what is effectively an electric Ping-Pong paddle to zap this region in healthy subjects, overexciting every neuron within range, and thus for about a fifth of a second, knocking that one-cubic-centimeter area of the brain off the grid.

And while he did this, he flashed pictures of faces. Blasted subjects retained the ability to recognize faces of loved ones or even learned strangers, but for this fifth of a second, they failed to recognize themselves.

Interestingly, there's one type of person who retains sense of self even with the medial prefrontal cortex blasted: narcissists.

When I interviewed him for my book, Brain Trust, Keenan explained that, "in narcissists, more brain areas are dedicated to self-deception." So when a narcissist's medial prefrontal cortex is taken offline, backup generators are in place to maintain that overblown sense of self.

It's a stark enough difference that soon there may be a neuroimaging diagnosis of narcissism. Does your sense of self sit in the medial prefrontal cortex box designed for it, or does it creep out to colonize other areas of your brain?

Neuroimaging knows and should soon settle all doubts as to who is, and who isn't, a narcissist.