Accepted.
2300 char max (not incl. spaces) - 500 for figure = 1800
San Diego or bust.
Temporal characteristics of cold pain perception.
Mark S. Bolding, Jon Grossman, Luke Stoeckel and Michael A. Frölich
The cold pressor task (CPT) is an experimental technique for inducing pain in humans (Wolf and Hardy, 1941). Recently, the cerebral representation of cold pain has been investigated using block design fMRI (Seifert and Maihofner, 2007; Davis et al., 1998). However, it is not clear whether cold pain
perception follows the boxcar function assumed in a block design. Therefore, we studied how human volunteers
rate the sensation of cold vs. tepid blocks over time.
After obtaining informed consent, human volunteers had their right foot immersed alternately in ice water or tepid water. Participants used an electronic slide algometer to continuously rate the intensity of their cold perception. The rating was recorded at 5Hz. The immersion sequence was either 5 blocks of 15 seconds cold/30 seconds tepid or 5 blocks of 30 seconds cold/30 seconds tepid. Each sequence was repeated twice. Peak cold ratings were analyzed with multivariate analysis of variance. Cold rating timecourses were summarized by aligning the cold immersions across runs and averaging the response curves. Peak cold ratings did not differ significantly across immersions (p=0.1072). However, peak cold ratings were significantly related to immersion duration (15 vs. 30 sec, p=0.008) when controlling for between subject variability. The time course of cold ratings are represented in figure 1. Cold pain ratings did not follow a boxcar function. Rather, the subjects reported a gradually increase of cold sensation during ice water immersion. After change to tepid water, cold perception declined gradually and, with repeated ice water immersions, the baseline (tepid water control) ratings appeared to increase (30 sec trials). Considering these results, a traditional on-off block design model may be inappropriate for the fMRI analysis of cold pain. We suggest that fMRI data should be analyzed using an observed psychometric function as it captures the gradual increase and decline of cold sensation and the carryover from immersion to immersion.
upper row: overlay, lower row: across subject average.