Interesting work from Charbonneau et al. in macaque monkeys on the affective (gentle, pleasant) touch pathways that in humans use a different neural network than pathways of discriminative touch:
Significance
Affective
touch is thought to be a critical substrate for the formation of the
social relationships which exist as a foundation for primate societies.
Although grooming behavior in monkeys appears to recapitulate features
of affective touch behavior in humans, it is unknown whether affective
touch activates the same neural networks in other primate species and
whether this activation requires conscious perception or changes across
the lifespan. We stimulated lightly anesthetized macaques at affective
(slow) and discriminative (fast) touch speeds during the acquisition of
functional MRI data. We demonstrate evolutionarily conserved activation
of interoceptive neural networks which change significantly in old age.
Abstract
Affective
touch—a slow, gentle, and pleasant form of touch—activates a different
neural network than which is activated during discriminative touch in
humans. Affective touch perception is enabled by specialized
low-threshold mechanoreceptors in the skin with unmyelinated fibers
called C tactile (CT) afferents. These CT afferents are conserved across
mammalian species, including macaque monkeys. However, it is unknown
whether the neural representation of affective touch is the same across
species and whether affective touch’s capacity to activate the hubs of
the brain that compute socioaffective information requires conscious
perception. Here, we used functional MRI to assess the preferential
activation of neural hubs by slow (affective) vs. fast (discriminative)
touch in anesthetized rhesus monkeys (Macaca mulatta). The
insula, anterior cingulate cortex (ACC), amygdala, and secondary
somatosensory cortex were all significantly more active during slow
touch relative to fast touch, suggesting homologous activation of the
interoceptive-allostatic network across primate species during affective
touch. Further, we found that neural responses to affective vs.
discriminative touch in the insula and ACC (the primary cortical hubs
for interoceptive processing) changed significantly with age. Insula and
ACC in younger animals differentiated between slow and fast touch,
while activity was comparable between conditions for aged monkeys
(equivalent to >70 y in humans). These results, together with prior
studies establishing conserved peripheral nervous system mechanisms of
affective touch transduction, suggest that neural responses to affective
touch are evolutionarily conserved in monkeys, significantly impacted
in old age, and do not necessitate conscious experience of touch.
