Hunger makes soft caresses less pleasurable

Assuming you're up for it, soft caressing can be a very pleasant sensation. This kind of gentle motion against the skin stimulates the reward system in the brain, and triggers the release of oxytocin, a hormone that (in this positive social context) boosts feelings of wellbeing and trust. 

According to new research, though, soft caresses are more likely to achieve these positive social effects after a meal. In a new, pre-registered study in Psychophysiology, Daniela M. Pfabigan at the University of Oslo and colleagues report that when participants were hungry, soft stroking actually felt less pleasant. Their investigations point to a 'hunger hormone' as playing a key role in this effect. 

The hormone in question, ghrelin, is secreted by cells in the stomach when we haven't eaten for a while. It triggers feelings of hunger and, through effects on the brain's reward system, makes fatty and sugary food in particular seem more appealing, which then in turn motivates us to eat them. Ghrelin levels also rise in response to pictures of food, explaining why watching Bake Off can get our tummies rumbling.

However, recent research has also hinted that ghrelin might affect how we respond to other sorts of stimuli that we normally find rewarding. This work suggests that by making some non-food stimuli less rewarding, ghrelin might encourage us to focus on the priority task (as far as survival is concerned) of getting something to eat. So, Pfabigan and colleagues decided to explore how ghrelin levels might affect the pleasantness of touch. 

To do this, they studied 60 adults who came to the lab for tests twice on separate days. They arrived at around 3pm on both days, after fasting for six hours. On the 'food' test day, they were given a liquid, milk-based 'meal' and then later a banana, which blood tests confirmed decreased their ghrelin levels. On the other day, they weren't offered this food until the end of the testing session. Blood tests confirmed that on this 'fasting' day, their ghrelin levels remained high during testing.

On both days, two hours after arrival the lab, the participants were put in an fMRI scanner with their right shin exposed. While their brain was scanned, an experimenter brushed their shin with a soft brush at a variety of speeds: very slowly (0.3cm/second), quickly (30cm/second), and also at 3cm/second, a speed that designed to activate the C-tactile 'caress receptors' in their skin. Over various trials of different stroking speeds, the participants reported on how intense the touch felt, alongside how pleasant it was. 

As expected, the team's analysis of their responses showed firstly that the participants generally found the 3cm/second 'caressing touch' more pleasant than the very slow and quick touches. However, pleasantness ratings were a little higher on the 'food' test days than on the 'fasting' days. Though the effect of hunger on touch pleasantness was small, "the finding that touch is less pleasant when hungry is novel," the authors stress. 

The team didn't find a simple link between the concentration of ghrelin in the participants' blood and touch pleasantness ratings. But, they did find that during caressing touch specifically, larger ghrelin decreases after the liquid meal were associated with greater activation of a region of the brain called the right medial orbitofrontal cortex (mOFC). Earlier work has linked this region with judgements of pleasantness and value from various types of stimuli. The team also found that this mOFC activation was linked to relatively higher pleasantness ratings. 

Overall, higher ghrelin concentrations seem to be linked to weaker 'reward' from caressing touch. This supports the idea that, as well as stimulating hunger, ghrelin levels are able to influence the value of positive social stimuli, making food seem relatively more appealing when we need to focus on finding a meal. 

If further research supports the team's conclusions, there may be implications for understanding disorders characterised by chronically high ghrelin levels, such as anorexia. If people with anorexia get less of a reward from positive social interactions, this might affect their responses to care and treatment. Dieters who follow strict calorie-controlled meal plans could also experience similar effects. 

Though the effect noted in the study was small, these findings do offer one everyday insight. Before attempting to use loving touches to woo your partner, it may be wise to feed them first.

Read the paper in full: https://doi.org/10.1111/psyp.14443