Pain Modulates Responses to Emotional Stimuli

Capsaicin-induced pain increases neural responses to low-calorie non-spicy food cues: An ERP study

Capsaicin, the main spicy ingredient in chili, can activate pain receptors on the human tongue and skin. Although some studies have determined that pain influenced preference for high-calorie foods, little is known whether pain can modulate the individuals' preference for spicy foods and its neural mechanisms. After 30 participants underwent painful (topical capsaicin cream) and control (hand cream) treatments, an event-related potential (ERP) study was conducted to investigate the modulation of capsaicin-induced pain on food preference with food images. Results showed that both P3 and late positive potential (LPP) amplitudes during the painful treatment were significantly larger than those during the control treatment for low-calorie non-spicy food cues. However, for the other three categories of food cues, there were no significant differences between the two treatments. The present study suggests that capsaicin-induced pain increases individuals' neural processing of low-calorie non-spicy food cues, which provides empirical evidence on the relationship between pain and neural responses to food cues to help optimize dietary interventions for patients experiencing pain.

Gut feelings: vagal stimulation reduces emotional biases

The vagus nerve is a key physical constituent of the gut-brain axis. Increasing attention has recently been paid to the role that the gut, and the microorganisms inhabiting it, play in emotion and cognition. Animal studies have revealed the importance of the vagus nerve in mediating communication between the gut microbiome and the central nervous system, resulting in changes in emotional behaviour. This has renewed interest in understanding the role of vagal signalling in human emotion, particularly since human studies have also shown that alterations in gut microbiome composition can affect emotion. While stimulating the vagus nerve can help treat some cases of severe depression, here we investigate whether vagal afferent signalling can influence emotional processing in healthy subjects. We use the dot-probe task to determine the effect of transcutaneous vagus nerve stimulation on attentional biases towards emotional stimuli in 42 volunteers. Participants received both active and sham treatments using a within-subject design. We show that transcutaneous vagus nerve stimulation reduces the emotional bias towards faces expressing sadness and happiness, indicating a decrease in emotional reactivity. While our novel findings reveal the effect that vagal signalling can have on emotional biases in healthy subjects, future studies should seek to develop our understanding of the ways in which the microbiome interacts with, and stimulates, the vagus nerve. Since we find a reduction in emotional bias, most notably towards sadness, this may partly account for the effective use of vagus nerve stimulation in treatment-resistant depression. While its clinical application currently involves surgical stimulation, our results support the potential benefit of transcutaneous vagus nerve stimulation as a non-invasive, intermittent adjunctive therapy for patients with depression given its frequent association with emotional biases.