Aged couple, man in grey sweater on couch with women in white sweater

Aged and unfazed: Older monkeys exhibit blunted nervous system response to arousing stimuli

Aged rhesus macaques appear unfazed, at least when it comes to their autonomic nervous system (ANS), compared to their younger counterparts. In an experiment monitoring peripheral nervous system responses to mood-inducing  videos, researchers from Eliza Bliss-Moreau’s lab at the California National Primate Research Center found the videos elicited a response from the monkeys based on content, but the response appeared blunted in older monkeys. This research, published in the Journal of Psychophysiology, is a critical step in understanding how age-related changes in the ANS mediate changes in emotional well being across the lifespan. 

The ANS is the part of your nervous system responsible for regulating bodily functions necessary for keeping you alive (e.g., breath, heart rate, digestion). ANS activity is a primary building block of social, emotional, and affective experiences. There are known disruptions to ANS function as humans age, but results from these studies have been inconsistant. The Bliss-Moreau lab set out to test ANS in action by observing if age has an impact on emotional response to video stimuli. 

How’d they do it?

The Bliss-Moreau lab monitored cardiac activity of monkeys from two age groups, middle aged or adult aged. The monkeys were trained to passively observe curated videos while their data was recorded noninvasively.

Researchers measured the two branches of ANS activity through two cardiac measures respiratory sinus arrhythmia (RSA) and pre-injection period (PEP) which reflect parasympathetic and sympathetic nervous system activity, respectively.  RSA is a measure of synchronicity between heart rate variability and breathing rate. RSA values increase when there is a greater variation in heart rate across the breathing cycle. PEP is the timing between when the heart is electrically signaled to beat and when blood is actually ejected from the aorta – an index of how the heart’s ventricles are functioning. 

Findings showed the middle-aged subjects’ cardiac activity varied depending on the mood-related content of the video. The older aged group, however, did not exhibit variation in cardiac activity and appeared to have a blunted response to the video stimuli. For example, younger monkeys’ RSA increased or decreased with the valence of the video. In line with their hypothesis, aged animals’ RSA remained consistent across mood-states.  

Modern medicine has doubled the expected lifespan over the last century and science is just beginning to catch up. The Bliss-Moreau lab is continuing to study interactions between the parasympathetic nervous system and altered social and affective behavior as animals age.  As humans strive to live longer, research like Bliss-Moreau’s can shed light on possible mechanisms for healthier aging. Future work will utilize the newly established animal model to test if altering parasympathetic activity can directly impact an animal’s behavior thus exploring ways to manipulate socioaffective experiences through the autonomic nervous system.


Written by Logan Savidge


Written by Logan Savidge