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. 2019 Nov:57:101368.
doi: 10.1016/j.infbeh.2019.101368. Epub 2019 Sep 12.

Social touch alters newborn monkey behavior

Affiliations

Social touch alters newborn monkey behavior

Elizabeth A Simpson et al. Infant Behav Dev. 2019 Nov.

Abstract

In humans, infants respond positively to slow, gentle stroking-processed by C-tactile (CT) nerve fibers-by showing reductions in stress and increases in eye contact, smiling, and positive vocalizations. More frequent maternal touch is linked to greater activity and connectivity strength in social brain regions, and increases children's attention to and learning of faces. It has been theorized that touch may prime children for social interactions and set them on a path towards healthy social cognitive development. However, less is known about the effects of touch on young infants' psychological development, especially in the newborn period, a highly sensitive period of transition with rapid growth in sensory and social processing. It remains untested whether newborns can distinguish CT-targeted touch from other types of touch, or whether there are benefits of touch for newborns' social, emotional, or cognitive development. In the present study, we experimentally investigated the acute effects of touch in newborn monkeys, a common model for human social development. Rhesus macaques (Macaca mulatta), like humans, are highly social, have complex mother-infant interactions with frequent body contact for the first weeks of life, making them an excellent model of infant sociality. Infant monkeys in the present study were reared in a neonatal nursery, enabling control over their early environment, including all caregiver interactions. One-week-old macaque infants (N = 27) participated in three 5-minute counter-balanced caregiver interactions, all with mutual gaze: stroking head and shoulders (CT-targeted touch), stroking palms of hands and soles of feet (Non-CT touch), or no stroking (No-touch). Immediately following the interaction, infants watched social and nonsocial videos and picture arrays including faces and objects, while we tracked their visual attention with remote eye tracking. We found that, during the caregiver interactions, infants behaved differently while being touched compared to the no-touch condition, irrespective of the body part touched. Most notably, in both touch conditions, infants exhibited fewer stress-related behaviors-self-scratching, locomotion, and contact time with a comfort object-compared to when they were not touched. Following CT-targeted touch, infants were faster to orient to the picture arrays compared to the other interaction conditions, suggesting CT-targeted touch may activate or prime infants' attentional orienting system. In the No-touch condition infants attended longer to the nonsocial compared to the social video, possibly reflecting a baseline preference for nonsocial stimuli. In contrast, in both touch conditions, infants' looked equally to the social and nonsocial videos, suggesting that touch may influence the types of visual stimuli that hold infants' attention. Collectively, our results reveal that newborn macaques responded positively to touch, and touch appeared to influence some aspects of their subsequent attention, although we found limited evidence that these effects are mediated by CT fibers. These findings suggest that newborn touch may broadly support infants' psychological development, and may have early evolutionary roots, shared across primates. This study illustrates the unique insight offered by nonhuman primates for exploring early infant social touch, revealing that touch may positively affect emotional and attentional development as early as the newborn period.

Keywords: CT fibers; Face perception; Infant behavior; Neonate; Sociability; Social attention; Tactile.

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Conflict of interest statement

Competing Interests Statement: The authors have no competing interests to declare.

Figures

Figure 1.
Figure 1.
Sample (a) video stimulus presentation order, and (b) eye tracking areas of interest (AOIs). Nonsocial (plastic bag floating in the wind) and social (conspecific producing an affiliative/positive facial expression) video pairs played concurrently for 10 seconds, then the side of the social and nonsocial were switched for a second 10 second presentation, to control for potential side bias effects.
Figure 2.
Figure 2.
Picture array stimuli sample (a) stimulus presentation, and (b) eye tracking areas of interest (AOIs).
Figure 3.
Figure 3.
During caregiver interactions, infants in the No Touch condition (light blue bar) displayed higher internalizing stress composite scores (scratching, time in contact with surrogate [a comfort object], and time moving around [locomotion]), compared to either of the touch groups (dark blue bars), *ps < .05, suggesting touch may reduce infants’ stress levels. Error bars reflect standard error of the mean.
Figure 4.
Figure 4.
Following caregiver interactions, infants’ attention was impacted by condition (No Touch, Hand/Foot [non-CT touch], Head/Shoulder [CT-targeted touch]) (a) Infants looked a significantly greater proportion of time to the non-social relative to the social video in the No Touch condition (light red bar), but looked equally to the social and nonsocial videos (.5 = chance looking, represented by the dashed line) in both touch conditions (dark red bars), suggesting touch may shift infants to attend relatively less to dynamic but not socially relevant stimuli (instead attending more equally to dynamic socially relevant and nonsocial stimuli). (b) Infants were significantly faster to look to the image array in the Head/Shoulder Touch condition (far right dark purple bar) compared to the both the No Touch and the Hand/Foot Touch conditions (left two purple bars), suggesting CT-targeted touch may facilitate infants’ attention capture. Notably, this later effect was not specific to social stimuli (face photos), but was a more general effect for orienting to all photo stimuli (social and non-social). Error bars reflect standard error of the mean, *ps ≤ .054.

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