Operant conditioning is the means by which we learn to control our environment. It works by generating variable behavior ("trying out") and then looking, which of the variables in the environment ("stimuli") vary in the same way. For example, imagine a computer game where you don't have any instructions. You move the mouse, joystick and hit the keys until you know what you have to do in order to play the game. Once you've figured it out, you've operantly conditioned yourself
This control of our environment works like a reward. If we are denied this control, we become depressed. Therefore, being confined in prison, for example, is a punishment: we are not free to chose our environment.
For probably similar reasons, pain which we are able to control feels less severe than the same intensity and amount of pain which we cannot control. In a recent study in the Journal of Neuroscience, researchers from the University College London report that they have discovered the brain region involved in this pain-reducing effect: the anterolateral prefrontal cortex. The prefrontal cortex is part of a feedback loop involving the basal ganglia which has long been implied in the generation of behavior and the assessment of its consequences. This work provides one more piece of evidence needed to unravel the mechanisms of operant behavior and how the brain uses it to perform adaptive behavioral choice. ScienceDaily also has a release on this story.