Effects of short and prolonged transcutaneous vagus nerve stimulation on heart rate variability in healthy subjects.

The vagus nerve is strategically located in the body, and has multiple homeostatic and health-promoting effects. Low vagal activity predicts onset and progression of diseases. These are the reasons to activate this nerve. This study examined the effects of transcutaneous vagus nerve stimulation (t-VNS) on a main index of vagal activity, namely heart rate variability (HRV). In Study 1, we compared short (10min) left versus right ear t-VNS versus sham (no stimulation) in a within-subjects experimental design. Results revealed significant increases in only one HRV parameter (standard deviation of the RR intervals (SDNN)) following right-ear t-VNS. Study 2 examined the prolonged effects of t-VNS (1h) in the right ear. Compared to baseline, right-t-VNS significantly increased the LF and LF/HF components of HRV, and SDNN in women, but not in men. These results show limited effects of t-VNS on HRV, and are discussed in light of neuroanatomical and statistical considerations and future directions are proposed.

Discrepancy between implicit and explicit preferences for food portions in obesity.

We investigated the implicit preference in terms of food portion in obesity using the affective priming paradigm. Primes representing different portions of fast food (small, medium and large) were used to assess participants' readiness to respond to a positive or negative target word. A self-reported affective rating scale of food portion and a portion judgment task were administered to determine the explicit preference for food portion and portion misperception, respectively. The results of the affective priming paradigm showed an implicit preference for large food portions in the obese group. No implicit preference in terms of food portion was found in the non-obese group. The explicit preference measure of food portion demonstrated a rather negative attitude for large portions in the obese group, whereas the non-obese group reported no explicit preference in terms of food portion. Thus, unlike the non-obese group, the obese group showed clear discrepancies between implicit and explicit preferences in terms of food portion: obese participants demonstrated an implicit, but not an explicit preference for large food portions. These results could not be attributed to a misperception of food portion, as revealed by the portion judgment task. The current findings suggest that social desirability might conceal self-reported preference in terms of food portion and/or that obese individuals are less aware of their internal preferences.