Can Looking at P300 Brain Waves Really Reveal a Person’s Secret Thoughts?

The only thing that P300 brain waves reveal is whether you recognize something that’s meaningful and significant. We developed a new 300 protocol for detecting concealed information. We developed it specifically to be resistant to countermeasures, which are attempts to control the size of P300 waves in one way or another.

In a P300 test, there are two types of stimuli that are presented. There are probe stimuli, which are meaningful or guilty-knowledge items—such as the gun you used to kill someone or rob a store. Then, there are a bunch of other items called “irrelevant” items. They might be other guns that have no relation to the issue you’re being asked about. So you are presented with a series of irrelevant items and occasional probes, and if you’re not playing any tricks, you respond to the meaningful ones and not the irrelevant ones.

But there is a trick you can play, which is to secretly make one of the meaningless items meaningful. You could do this by secretly thinking of your mother’s face or your own name every time you see, for example, a certain weapon that is not the one you used. If you do this every time you see that stimulus, that particular weapon, you’ve secretly made that weapon meaningful because you’ve assigned a secret task to it. And if you can make the P300 to your self-instructed stimulus big enough, it becomes just as big as the P300 for the guilty-knowledge item, and then we can’t tell the probes from the irrelevants, which means the test is beaten.

However, in our version of the test, we do other things beyond giving probes and irrelevants. It’s meant to be resistant to countermeasures, so that—although the irrelevant responses do get larger—the probe responses are still much larger in most people. So even though you might have made the irrelevants larger, they’re not large enough to beat the test.

We did this by altering the original version of the P300 test, which I first published in 1987. That test consisted of three kinds of stimuli: a probe, an irrelevant, and also a target. A target is an irrelevant item, but you are assigned to make a unique response to that item; in other words, you press a button when you see it. And the idea behind this is just to be sure you’re paying attention. The problem with that kind of a system—which works very well if nobody’s trying to beat the test—is that it doesn’t work when people try to beat the test the way I told you about. The reason is that their attention becomes divided between probe recognition and target recognition. When this happens, the P300s get smaller. So, in that old protocol, the P300 was never as large as it really could be if someone’s attention were undivided.

Now, a perp doesn’t want to pay attention to the probe, but it happens automatically. And when you split the target task from the probe recognition task, the test becomes virtually (albeit not totally) impossible to beat. About 5 percent of the time, someone may be able to beat it. We also look at a person’s reaction time to the stimulus. If you’re thinking of which countermeasure to make, then your reaction time is going to be longer. And even if you beat the test of the brain waves, we know that you’re doing something funny, and we can report noncooperation.

And there’s still another way to prevent countermeasures, which we haven’t studied for that long yet, but which is looking better and better. Basically, if a person is using countermeasures, it produces a different kind of brain wave, which we call a P900 brain wave.

So, as you can see, it becomes harder and harder to beat this test. Remember, more than 90 percent of the time, we can catch someone with just the P300 test. But for those cases when we can’t, we can look at reaction time and P900 brain waves.

Peter Rosenfeld is a professor of psychology at the Institute for Neuroscience at Northwestern University.