Disentangling the spinal mechanisms of illusory heat and burning sensations in the thermal grill illusion

ABSTRACT

The thermal grill illusion (TGI), a phenomenon in which the juxtaposition of innocuous warm and cold temperatures on the skin elicits a burning sensation, offers a unique perspective to how pain occurs in response to harmless stimuli. We investigated the role of the spinal cord in the generation of the TGI across 2 experiments (total n = 80). We applied heat and cold stimuli to dermatomes, areas of skin innervated by a single spinal nerve, that mapped onto adjacent or nonadjacent spinal segments. Enhanced warm and burning ratings during the TGI were observed when cold and warm stimuli were confined within the same dermatome. Furthermore, we found the spatial organisation of warm and cold stimuli within and across dermatomes affected TGI perception. Perceived warmth and burning intensity increased when the cold stimulus projected to the segment more caudal to the warm stimulus, whereas perceived cold during the TGI decreased compared with the opposite spatial arrangement. This suggests that the perception of TGI is enhanced when cold afferents are projected to spinal segments positioned caudally in relation to those receiving warm afferents. Our results indicate distinct interaction of sensory pathways based on the segmental arrangement of afferent fibres and are consistent with current interpretations of the spread and integration of thermosensory information along the spinal cord.

Thermosensory predictive coding underpins an illusion of pain

The human brain has a remarkable ability to learn and update its beliefs about the world. Here, we investigate how thermosensory learning shapes our subjective experience of temperature and the misperception of pain in response to harmless thermal stimuli. Through computational modeling, we demonstrate that the brain uses a probabilistic predictive coding scheme to update beliefs about temperature changes based on their uncertainty. We find that these expectations directly modulate the perception of pain in the thermal grill illusion. Quantitative microstructural brain imaging further revealed that individual variability in computational parameters related to uncertainty-driven learning and decision-making is reflected in the microstructure of brain regions such as the precuneus, posterior cingulate gyrus, cerebellum, as well as basal ganglia and brainstem. These findings provide a framework to understand how the brain infers pain from innocuous thermal inputs, with important implications for the etiology of thermosensory symptoms under chronic pain conditions.

Thermal contrast enhancement predicts paradoxical heat sensation

Paradoxical Heat Sensation (PHS) is the remarkable feeling of warmth or heat pain while the skin is cooling. Despite its initial documentation over 100 years ago, a unified explanation for this perplexing experience remains elusive. Here we apply contrast enhancement principles, known for their instrumental role in understanding visual illusions, to the domain of thermosensation. Contrast enhancement describes the amplification of two contrasting visual features, such as the enhanced perception of an edge between a light and dark bar. In thermosensation, this encompasses an enhancement of the difference between sequential warming and cooling of the skin, and is defined as the normalised difference between successive temporal warm and cold temperatures. Remarkably, thermal contrast predicts the occurrence of PHS. Our findings reveal compelling evidence supporting the role of thermal contrast in the generation of PHS, shedding light on its underlying mechanism and offering a framework for broader encoding principles in thermosensation and pain.

Assessing individual sensitivity to the Thermal Grill Illusion: A two-dimensional adaptive psychophysical approach

In the Thermal Grill Illusion (TGI), the spatial alternation of non-noxious warm and cool temperatures elicits burning sensations that resemble the presence of noxious stimuli. Previous research has largely relied on the use of specific temperature values (i.e., 20 °C and 40 °C) to study this phenomenon in both healthy individuals and patient populations. However, this methodology fails to account for inter-individual differences in thermal sensitivity, limiting the precision with which TGI responses can be evaluated across diverse populations. To address this gap, we created a Two-Dimensional Thermal Grill Calibration (2D-TGC) protocol, enabling an efficient and precise estimation of the combinations of warm and cool temperatures needed to elicit burning sensations tailored to each individual. By applying the 2D-TGC protocol in 43 healthy participants, we demonstrated that the TGI can be thresholded using an adaptive psychophysical method, and that multiple combinations of warm and cool temperatures can elicit this phenomenon. More specifically, the protocol facilitated the identification of temperature combinations that elicit TGI with varying levels of probability, intensity, and perceived quality ranging from freezing cold to burning hot. Finally, our results indicate that TGI responsivity can be quantified as a continuous variable, moving beyond the conventional classification of individuals as responders vs. non-responders based on arbitrary temperature values in the innocuous temperature range. The 2D-TGC offers a comprehensive approach to investigate the TGI across populations with altered thermal sensitivity, and can be integrated with other methods (e.g., neuroimaging) to elucidate the mechanisms responsible for perceptual illusions in the thermo-nociceptive system. PERSPECTIVE: This study reveals that the Thermal Grill Illusion can be accurately measured using psychophysical methods. The innovative Two-Dimensional Thermal Grill Calibration protocol allows for personalized temperature assessments, enhancing our understanding of thermal sensitivity variations and perceptual illusions in the thermo-nociceptive system across different populations.

The Wearable Cardioverter/ Defibrillator: retrospective analysis of efficacy, safety and adherence

Background

The use of the Wearable Cardioverter Defibrillator (WCD) is recommended in national, European and American guidelines. However, there are almost exclusively data from the manufacturer's own data network. Independent data on the experience with the WCD are rare.

Objective

The aim of the retrospective study from one cardiologic department was to record efficiency, safety and compliance of the WCD.

Patients and methods

The study included all patients, to whom a WCD was described between 1.11.2010 and 1.5.2018 at one cardiologic department. Clinical data were obtained from the patients' records and the data about the WCD from the information network of the manufacturer.

Results

This study enrolled 66 patients, 51 males (77%) and 15 females (23%). The median age was 55 years (IQR: 45–63). They suffered from ischemic cardiomyopathy (n=33; 49%), dilated cardiomyopathy (n=12; 18%), myocarditis (n=7; 11%), explantation of an implantable cardioverter/defibrillator (ICD; n=5; 8%) and other indications (n=6; 9%).

The median wearing time of the WCD was 73 days (interquartile range-IQR: 39–126), with median daily use of 22.91 h (IQR: 19.58–23.61). Among 38 patients with LVEF ≤35%, LVEF improved to ≥35% in 19 patients (50%) during WCD therapy. Over 1600 times the WCD detected a VT falsely. Four patients (8%) suffered from 212 non-sustained VT. One patient was successfully shocked because of ventricular fibrillation (appropriate shock rate: 1.5%). There were no inappropriate shocks. All patients, who wore the WCD, survived and one patient died when he did not wear the WCD.

At the end of therapy 32 patients (48%) received an ICD. In terms of wearing time and events (shocks, arrhythmias, artifacts) there were no significant differences between patients receiving ICD and those who did not receive an ICD. Patients who received an ICD had a significantly lower LVEF after 3 months than patients who did not receive an ICD.

Conclusion

Our data confirm, that the WCD is safe and that the patients, who wear a WCD, have a high adherence. More than half of the patients with reduced LVEF improved their systolic function during WCD therapy, thus obviating the need for ICD implantation. Questions about the effectiveness of the detection algorithm remained open.

Funding Acknowledgement

Type of funding source: None