Temporal characteristics of cold pain perception

Adaptation to a sustained stimulus is an important phenomenon in psychophysical experiments. When studying the response to an experimental task, the investigator has to account for the change in perceived stimulus intensity with repeated stimulus application and, if the stimulus is sustained, for the change in intensity during the presentation. An example of a sustained stimulus is the cold pressor task (CPT). The task has been used both as an experimental pain task and to study cardiovascular physiology. In functional imaging research, the CPT has been used to evaluate cognitive processing of a noxious stimulus. Investigators typically model the stimulus in a block design as a categorical (on-off) stimulus and do not account for a temporal change in stimulus perception. If the perceived stimulus changes over time, the results may be misleading. Therefore, we characterized the time course of cold pain in human volunteers and developed a model of the temporal characteristics of perceived cold pain. Fifteen healthy participants underwent cold pain testing by immersing their right foot into a container filled with ice water (2 degrees C) for 30s alternating with a 30s immersion into a container filled with tepid water 32 degrees C (control). Participants rated the pain intensity using an electronic slide algometer. Using a mixed general linear model (effectively a polynomial regression model), we determined that pain ratings follow a crescendo-decrescendo pattern that can be described well using a quadratic model. We conclude that the time course of quantitative perception differs fundamentally from the time course of stimulus presentation. This may be important when looking for the physiological correlates of perception as opposed to the presence of a stimulus per se.