We don’t have a mind reading machine. But what if we one day did? The technique of functional MRI (fMRI), which measures changes in localized brain activity over time, can now be used to infer information regarding who we are thinking about, what we have seen, and the memories we are recalling. As the technology for inferring thought from brain activity continues to improve, the legal questions regarding its potential application in criminal and civil trials are gaining greater attention.

Last year, a Maryland man on trial for murdering his roommate tried to introduce results from an fMRI-based lie detection test to bolster his claim that the death was a suicide. The court ruled (PDF) the test results inadmissible, noting that the “fMRI lie detection method of testing is not yet accepted in the scientific community.” In a decision last year to exclude fMRI lie detection test results submitted by a defendant in a different case, the Sixth Circuit was even more skeptical, writing (PDF) that “there are concerns with not only whether fMRI lie detection of ‘real lies’ has been tested but whether it can be tested.”

So far, concerns regarding reliability have kept thought-inferring brain measurements out of U.S. (but not foreign) courtrooms. But is technology theonly barrier? Or, if more mature, reliable brain scanning methods for detecting truthfulness and reading thoughts are developed in the future, could they be employed not only by defendants hoping to demonstrate innocence but also by prosecutors attempting to establish guilt? Could prosecutors armed with a search warrant compel an unwilling suspect to submit to brain scans aimed at exploring his or her innermost thoughts?

The answer surely ought to be no. But getting to that answer isn’t as straightforward as it might seem. The central constitutional question relates to the Fifth Amendment, which states that “no person … shall be compelled in any criminal case to be a witness against himself.” In interpreting the Fifth Amendment, courts have distinguished between testimonial evidence, which is protected from compelled self-incriminating disclosure, and physical evidence, which is not. A suspected bank robber cannot refuse to participate in a lineup or provide fingerprints. But he or she can decline to answer a detective who asks, “Did you rob the bank last week?”

So is the information in a brain scan physical or testimonial? In some respects, it’s a mix of both. As Dov Fox wrote in a 2009 law review article, “Brain imaging is difficult to classify because it promises distinctly testimonial-like information about the content of a person’s mind that is packaged in demonstrably physical-like form, either as blood flows in the case of fMRI, or as brainwaves in the case of EEG.” Fox goes on to conclude that the compelled use of brain imaging techniques would “deprive individuals of control over their thoughts” and be a violation of the Fifth Amendment.

But there is an alternative view as well, under which the Fifth Amendment protects only testimonial communication, leaving the unexpressed thoughts in a suspect’s head potentially open to government discovery, technology permitting. In a recent law review article titled “A Modest Defense of Mind Reading,” Kiel Brennan-Marquez writes that “at least some mind-reading devices almost certainly would not” elicit “communicative acts” by the suspect, “making their use permissible under the Fifth Amendment.” Brennan-Marquez acknowledges that compelled mind-reading would raise privacy concerns, but argues that those should be addressed by the Fourth Amendment, which prohibits unreasonable searches and seizures.

That doesn’t seem right. It would make little sense to provide constitutional protection to a suspected bank robber’s refusal to answer a detective’s question if the thoughts preceding the refusal–e.g., “since I’m guilty, I’d better not answer this question”–are left unprotected. Stated another way, the right to remain silent would be meaningless if not accompanied by protection for the thinking required to exercise it.

And if that weren’t enough, concluding that compelled brain scans don’t violate the Fifth Amendment would raise another problem as well: In a future that might include mature mind-reading technology, it would leave the Fourth Amendment as the last barrier protecting our thoughts from unwanted discovery. That, in turn, would raise the possibility that the government could get a search warrant for our thoughts. It’s a chilling prospect, and one that we should hope never comes to pass.