The brains of nonhuman primates respond to gentle, pleasurable touch in the same way as humans, even without awareness, researchers at the California National Primate Research Center have found. This study also revealed age-related differences in the way touch is processed by the brain. The study, conducted by [Ph.D. Candidate] Joey Charbonneau and CNPRC core scientist Eliza Bliss-Moreau’s lab, poses fascinating questions about the role of consciousness in pleasurable touch, as well as age-related changes in the brain’s processing of touch. The work was published April 22nd in Proceedings of the National Academy of Sciences.
Why do we study touch?
Social touch is important in generating and maintaining social bonds that hold families and societies together. Perception of touch changes in a wide variety of human conditions, including autism spectrum disorder, eating disorders, anxiety and neurodegenerative disorders like Alzheimer’s Disease or frontotemporal dementia.
The study revealed similarities between human and monkey neurobiology that can be leveraged to answer questions about touch perception not feasible in other animal models. “If we want to figure out why touch perception is abnormal in many human diseases and disorders, we need to carry out work in an animal model like monkeys where these touch signals travel from the body to the brain along very similar pathways to humans,” Charbonneau said.
To understand the brain’s response during slow touch – typically reported as being pleasurable by people – the researchers anesthetized 33 monkeys and used functional magnetic resonance imaging to record brain activity. In addition to slow touch (the pleasurable or affective touch condition), monkeys also experienced faster touch (referred to as discriminative touch), interspersed with resting periods.
Surprising findings
The team’s research showed that slow touches activated the same brain network that activates during slow pleasurable touch in humans, including the insula, anterior cingulate cortex (ACC), amygdala and secondary somatosensory cortex. Given that the animals were anesthetized, researchers were surprised to find that the brain activity in monkeys mirrored that of awake humans reporting a pleasurable quality of the touch. These findings indicate the rewarding aspects of affective touch, at least as they are represented in the brain, do not require conscious awareness.
In addition to confirming the evolutionary conservation of the brain’s response to affective touch, the work identified age-related changes in the representation of touch in the brain. In old monkeys, fast, discriminative touches were equally represented in some of the brain areas which responded specifically to affective touch in the younger animals.
“The effect in the data for aging was somewhat surprising. It’s not that [older animals] are blunted to the effect of touch or affective touch; it’s that discriminative touch is being equally represented in the structures that represent affective touch,” Bliss-Moreau said. Age-related differences were specific to the anterior cingulate cortex and insula and were not observed in the somatosensory cortex or the amygdala.
Nonhuman primates are an important model for researchers to gain a deeper understanding of how affective touch is represented and communicated to the brain, Bliss-Moreau said.
Why primates?
“If we can capitalize on the ability to manipulate this system in monkeys, it may shed light on mechanisms that allow us to more clearly focus our study in people,” she said.
Future studies in the lab will delve into age-related changes in affective touch perception across the lifespan in hopes to develop treatments and interventions for age-related diseases that impact affective processing. Bliss-Moreau is also working to combine the current findings with previous work illustrating other physiological measures of pleasurable states in nonhuman primates to understand if this type of tactile stimulation is “pleasant” for the monkeys in a way that translates to the human experience.
This work was completed with assistance from coauthors Anthony Santistevan, Erika Raven, Jeffrey Bennett, and Brian Russ and contributions from the National Institute of Health, the UC Davis Chancellor’s fellowship, and the base grant of the California National Primate Research Center.
Written by Logan Savidge
Media Contacts:
Joey Charbonneau | jcharbonneau@ucdavis.edu
Eliza Bliss-Moreau | eblissmoreau@ucdavis.edu
