Comparison of thermal threshold tests to assess small nerve fiber function: limits vs. levels

Objective: The method of limits (MLI) and the method of levels (MLE) are psychophysical stimulus procedures most commonly applied to quantify warm and cold sensation thresholds in humans. This paper evaluates basic methodological properties of both methods and investigates the correspondence between the methodʹs results. Methods: Warm sensation threshold was measured in 20 healthy participants using the psychophysical MLIs and MLEs. Two differently shaped kind of levels stimuli were used with triangular (TRIANG) and trapezoid (TRAP) temperature-time profile. Results: A linear model of temperature response, based on threshold level-crossing, quantifies sensation threshold, independent of the MLI inherent ‘reaction-time’ artifact. It results from modeling MLI responses to warm stimuli with different rates of temperature change. The model also quantifies the reaction-time delay in the physiological system from thermal stimulus presentation until manual response. This study shows that using the reaction-time independent MLE, TRAP should preferably be used for optimal quantification of sensation threshold. Conclusions: Statistical testing shows that model-based MLI threshold equals MLE threshold provided MLE TRAP stimuli are used. Recommendations for optimal MLI and MLE stimulus configurations and properties are given in relation to application of quantitative sensory testing.