Previous studies with adults have shown that when the skin is stroked, a specific type of touch receptor is activated in response to a particular stroking velocity, leading to the sensation of "pleasant" touch.
Cognitive neuroscientist Merle Fairhurst of the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany, and colleagues hypothesised that this type of response might emerge as early as infancy.
For the study, Fairhurst and colleagues had infants sit in their parents' laps while the experimenter stroked the back of the infant's arm with a paintbrush.
The experimenter varied the rate of the brushstrokes among three defined velocities (0.3, 3, or 30 cm per second). The experimenters gauged the infants' responses through physiological and behavioural measures.
The results showed that the infants' heart rate slowed in response to the brushstrokes but only when the strokes were of medium velocity; in other words, the touch of the medium-velocity brush helped to decrease their physiological arousal.
The infants also showed more engagement with the paintbrush during the medium-velocity brushstrokes, as measured by how long and how often they looked at the brush while they were being stroked.
Interestingly, infants' slower heart rate during medium-velocity brushstrokes was uniquely correlated with the primary caregivers' own self-reported sensitivity to touch, researchers said.
That is, the more sensitive the caregiver was to touch, the more the infant's heart rate slowed in response to medium-velocity touch.
The researchers noted that this link between caregiver and infant could be supported by both "nurture" and "nature" explanations.
"One possibility is that infants' sensitivity to pleasant touch stems from direct or vicarious experience of differing levels of social touch as a function of their caregiver's sensitivity to social touch," said Fairhurst.
"Another possibility is that social touch is genetically heritable and therefore correlated between caregivers and infants," Fairhurst said.
According to the researchers, the findings "support the notion that pleasant touch plays a vital role in human social interactions by demonstrating that the sensitivity to pleasant touch emerges early in human development."
The study is published in Psychological Science, a journal of the Association for Psychological Science.