Monday, October 29, 2007

Your Heart Tells What Your Brain Knows

One of the academic groups to which I belong, the Society for Psychophysiological Research, started as the Society for Polygraph Research.

It follows that much of the technologies that we use in the lab have some foundation in lie detection.

An important tenet of lie detection is that your body reacts differently depending upon what you know.

Although media portrayals frequently portray lie detection as catching someone telling an untruth, it is more reliable to use the guilty knowledge test.

In this test, an individual is told various aspects of a crime, for example. Some of the details are untrue, and some of the details are things that only someone who had been at the crime scene would know. Thus, if you respond to the proper location of the murder weapon, it suggests that you saw it in that location.

For the first time, my lab is attempting to correlate physiology and memory. I will avoid the details of the experiment here, but we tested memory following presentation of a media-related stimulus. During the memory test, participants heard sound clips that they previously heard during the experiment, and they heard sound clips that they did not hear.

Their job was to say "Yes" they had heard it during the earlier experiment or "No" they hadn't.

We recorded their physiology while they were listening and for several seconds afterward while the screen was black.

We wanted to compare the physiological results based upon their memory performance. Now, I had no idea how much work this would be. Really, it was an insane data analysis. It took forever. There are now more than 12,000 variables in the heart rate data set alone.

In the end, it was worth it, if only for the "cool" value.

The figure above shows a cardiac response curve (CRC) for trials where participants correctly recognized a sound clip (i.e., hits, shown in blue). The second cardiac response curve is for trials where they correctly failed to recognize a clip (i.e., correct rejections, shown in green).

At first, both CRCs show an initial deceleration at the beginning of the trial. This is an orienting reflex elicited by the onset of the trial. However, for the correct rejections, there is sustained cardiac deceleration. We typically associate this sustained deceleration with continued cognitive effort. As your brain tries harder, your heart slows down. Cool, huh?

This makes sense, and it fits with our (and others') model(s) of memory. In a recognition task, you can stop trying as soon as you find a match. That is, when the recognition prime matches a memory, you can confidently feel that you recognized it. Since these trials were correct recognitions, we can assume that the match occurred relatively quickly.

Conversely, when there is no recognition, the brain has to keep trying for matches until you give up. This takes longer, obviously, and should require more cognitive effort. This is the exact picture that we see.

In each case, after we stopped collecting data (which went on for 2 seconds after this figure), participants used a computer mouse to make their recognition decision. Thus, these physiological data precede the recognition decision.

To early lie detectors, these data must seem trivial. Of course there is a difference. To me, however, it is fascinating that your heart beats differently when you recognize something than when you don't.

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Anonymous Anonymous said...


Thanks for posting this research. I really enjoyed reading it.

- Tim

10:24 PM  
Anonymous Anonymous said...

Do you have enough participants who incorrectly guessed that they had heard the audio or that they hadn't so that you could plot those 2 heart rate lines?

- Tim

10:56 AM  

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