In the context of pain memory, the research of Mannheim scientists led by PD Dr. Dieter Kleinböhl and Prof. Dr. Rupert Hölzl is significant: in one experiment, the pain sensitivity of healthy study participants could be significantly increased without them being aware of it. Conversely, sensitivity could be lowered in the same way, depending on the consequences that followed the perceptual reactions.
The experiment
For their study, which was funded by the German Research Foundation, the researchers were awarded the second prize of 3,500 euros in the basic research category of the 2006 Pain Research Award at the German Pain Congress in Berlin. The trial went like this: The test subjects received heat stimuli on their hands via a so-called thermode. They were allowed to regulate the temperature themselves.
Their task was to keep the perceived stimulus intensity constant. “In healthy subjects, just painful stimuli normally resulted in habituation, i.e. they regulated the temperature higher over time to keep the sensation the same,” explains Dr. Kleinböhl.
“In contrast, in chronic pain conditions, such as back pain, you don’t find habituation to such stimuli – here sensitization occurs, i.e., an increase in subjective pain sensation.” The question was whether such altered pain perception could result from unconscious learning processes. To find out, the researchers studied healthy individuals under two conditions. The task of keeping the sensation intensity of the heat stimuli constant remained.
The result
In one group, a sensitization response was “enhanced” by a subsequent further reduction in temperature. The habituation response, however, was “punished” by a subsequent increase in temperature.
In the second group, the situation was reversed: here, habituation was reinforced and sensitization was punished. It was found that in the group in which the sensation of pain was increased, sensitization reactions to heat stimuli occurred more frequently, while in the other group habituation reactions were found more frequently. In the group with learned sensitization, it was also demonstrated that as stimulus intensities decreased, personal sensation intensity remained the same.
The participants were not aware of this gradually increasing sensitivity to pain over the course of the experiment.
