The author hypothesizes that fMRI, which reveals brain activity, could someday be used as an effective lie detector. Why? Because in a study where participants told lies while undergoing an fMRI scan, the scanner picked up increased brain activity associated with stress.

Something that's important to notice here is that the study didn't use a control group of people who told the truth during an fMRI scan. That means that we don't actually know whether the stress reaction is connected to lying, or just arises because doing an fMRI study is stressful. In other words, we don't know if there's truly a correlation between lying and the kind of fMRI results that these participants displayed.

The author takes the study's results to mean that fMRI can distinguish when someone is lying—this is crucial for the hypothesis that fMRI can help to detect lies. However, from the stimulus we can't be sure that a correlation between lying actually correlates with a particular fMRI result. A true connection between lying and fMRI results is essential to the conclusion, so we can weaken by undermining that link. For example, this could be by showing that participants in a control group had the same stress reaction.