Introduction: The RIII reflex is used in fundamental and clinical pain research. Here we introduce a continual reflex threshold tracking algorithm to facilitate investigations of the time courses of influences on the threshold. Methods: First we investigated the probability of reflex occurrence at the threshold estimated by the continual algorithm and the changes of the threshold over the time during continual recordings of 100 min duration in 10 subjects. Secondly we compared the threshold estimates of the continual algorithm with those of a standard algorithm of threshold estimation in 52 subjects and compared the differences between the two methods with the test-retest-variability of each method. Results: The average probability of reflex occurrence at the threshold estimated by the continual algorithm was 48.7% (SD = 3.2%). Changes of the RIII reflex threshold over the time were not significant (Friedman test, p > 0.05). The variability between the thresholds determined by the different algorithms (test: SD = 2.50 mA, retest: SD = 1.80 mA) was lower than the variability between test and retest (standard algorithm: SD = 4.32 mA, continual algorithm: SD = 4.44 mA). Discussion: The continual algorithm can be used for a continuous estimation of the reflex threshold at the 50% probability of reflex-occurrence. No evidence of habituation was detected. This allows for investigations of the time courses of pharmacological and physiological influences on the reflex threshold by using this algorithm. The lower variability between the continual algorithm and the standard algorithm compared to the variability between tests and retests of the methods allows for interchangeable conclusions drawn from data obtained with both methods. © 2009 Elsevier B.V. All rights reserved.
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