From Scientific American:
Every time that a colored square appeared on the monitor in front of me, I braced for pain. Early into the 10-minute session as a subject of this experiment, I learned that about half of the times that I saw that square, I received a low-voltage shock, via a bar strapped to my right wrist. I also learned that every time I saw a square of a different, “good” color, I could momentarily breathe easy. But in the second day's session, as I watched the squares appear in random order, no shocks punctuated either the “bad” or “good” colors. After several minutes I started to relax.
The researcher conducting this fear conditioning experiment at New York University's Department of Psychology, David Bosch, was monitoring my fear responses. Changes in how much I was sweating, which he gauged by measuring the electrical conductance of the skin on my left middle finger, indicated my fear level. On the first day I was conditioned, like Pavlov's dog, to sweat at the sight of the “bad” color. But that response faded over the course of the second, shock-free session as my fear was “extinguished,” a process known as extinction. As Bosch explains, “extinction is learning a new contingency.”