Nancy, an airline pilot, arrived promptly for a routine physical. She'd had exams before, but this time was different. She was asked to lie down and place her head in a large metallic torus, while a video screen flashed a series of images before her eyes—the inside of a 747 cockpit, a view of a target seen through a rifle's scope, a chemical formula for polyester, a photo of Bill Clinton. In an adjacent room, a technician watched as colorful images of Nancy's brain appeared on his computer screen, lighting up like brushfires with different hues in response to the pictures. As the test ended, the technician forwarded the results to Nancy's employer.

Reporting for work the next day, Nancy was confronted by her supervisor and an official from the U.S. Federal Aviation Administration. They informed her that the brain images showed Nancy might develop schizophrenia, and had a surprising familiarity with assault rifles as well. The agency revoked her pilot's license. The airline promptly fired her.

This scenario is fiction. But the basics of the technologies it alludes to already exist. New ways of imaging the human brain and new developments in microelectronics are providing unprecedented capabilities for monitoring the brain in real time and even for controlling brain function.

The technologies are novel, but some of the questions that they will raise are not. Electrical activity in the brain can reveal the contents of a person's memory. New imaging techniques might allow physicians to detect devastating diseases long before those diseases become clinically apparent. And researchers may one day find brain activity that correlates with behavior patterns such as tendencies toward alcoholism, aggression, pedophilia, or racism. But how reliable will the information be, how should it be used, and what will it do to our notion of privacy?

Meanwhile, microelectronics is making access to the brain a two-way street. The same electrical stimulation technologies that allow some deaf people to hear could be fashioned to control behavior as well. What are the appropriate limits to the use of this technology? In an age of overcrowded prisons, might society be tempted to release criminals if behavior-modifying brain implants could guarantee that they would pose no further threat?