Scientists could soon be able to read people's minds by scanning their brains, researchers have claimed.
While this is not yet possible, technology is rapidly advancing and poses moral dilemmas that must be debated, a pair of University of Cambridge neuroscientists have said.
It includes whether brain scanning should be used for a Minority Report-style justice system in which "criminals" are singled out before they commit any crimes.
Other possible uses of functional magnetic resonance imaging (fMRI) technology include lie detectors in courtrooms, anti-terrorist screening at airports and even so-called neuro-marketing - which could allow big business to determine how much a consumer wanted a product, and adjust prices accordingly.
This vision of the future is set out in a new book titled Sex, Lies & Brain Scans, which is written by Professor Barbara Sahakian and Julia Gottwald.
"Even though fMRI cannot read minds yet, we need to start deciding how and why we might want to use it, where screening might help, and where it might violate privacy," Ms Gottwald, a PHD student at St John's College at the University of Cambridge said. "Because the technology is advancing so rapidly, these kinds of questions are becoming more and more pressing.
"The aim of the book is to give readers the basics about an exciting and emerging field.
"When the public read about fMRI in the media, they get basic facts, but often out of context.
"People need to understand its potential and limitations, so that together we can make better judgments about this technology, and how far we want it to go.
"These questions concern all of us."
The fMRI technology enables researchers to observe which areas of the brain are most active when people perform particular tasks, experience specific emotions, or imagine certain scenarios. Although it has important limitations, the technology is improving all the time, and before long the researchers suggest that it could make ideas that once seemed science fiction a reality.
It has already been used to identify conscious activity in patients who were thought to be in a vegetative state.
And this week a study in the journal Public Library of Science Biology said four patients with completely locked-in syndrome (CLIS), who were unable to speak, move or blink, reported that they were "happy" after a successful attempt was made to read their thoughts.
Scientists had asked the patients to think "yes" or "no" in response to specific personal questions, and this triggered changes correlating with brain activity that could be translated by a computer.
As it stands, most complex processes cannot be traced back to a brain region that does not serve another purpose, making many fMRI results somewhat ambiguous.
Our sense of morality, for example, which we might wish to assess in order to identify potential criminals, appears to be linked to the same brain areas as emotional and social cognition.
Contrastingly, highly influential characteristics such as self-control are not uniformly linked to a single region at all.
The new study urges caution in efforts to use fMRI for advanced applications such as lie detectors in court - as its accuracy is still far less than 100%.
For instance, fMRI can only reflect a subject's beliefs, and therefore does not account for circumstances where a witness may have misremembered an event, or simply imagined it.
Ms Gottwald added: "Given where the technology is going, it is not unrealistic to say that we will see some very useful applications becoming available in the not too distant future.
"We will need clear guidelines to ensure that this is implemented to the public's best advantage."