Half a heartbeat can chill out our response to threat.

Whether we are breathing in or breathing out can have a pronounced effect on our threat detection threshold. Meditation regimes and stress performance training (as for Navy Seals) emphasize prolongation of exhalation as a calming technique. During exhalation, measurements have shown a relative increase in parasympathetic and vagal activity, a relative decrease in amygdala reactivity, and lower reactivity to possible threats. Now work of Garfinkel and colleagues, reported at the recent meeting on the Assoc. for the Scientific Study of Consciousness in Brighton, U.K. (meeting abstracts here, 4.7 MB download) shows that the cardiac cycle can influence our emotional response to scary stimuli. Here is a clip from the writeup in The New Scientist:

In one experiment...people were asked to look at a stream of flashing images and highlight when they spotted a face. Some of the faces looked fearful, others looked neutral...Unbeknown to the volunteers, images were time-locked to appear in sync with their heart-beat. Sometimes the images were synced with the systole phase - the part of the cardiac cycle where the heart muscle contracts to squeeze blood out of the heart, at other times they were linked to the diastole phase - the stage where the heart relaxes and fills after contracting...people were better at spotting fearful faces compared with neutral faces, but only when the pictures were timed to appear at the systole phase.

In another study, people saw the same pictures while having their brain scanned using MRI. People had a stronger response in the hippocampus and amygdala - areas of the brain associated with fear - when they were shown fearful faces at systole than when they saw them at diastole. In other words, half a heartbeat was all it took for a person to experience a significantly different response to the same scary stimulus...The finding seems to be mediated by barorecepors - stretch and pressure sensitive receptors in the heart and surrounding arteries which help initiate systole. "When barroreceptors are activated at systole, a flurry of activity is transferred to the brain at that moment," Garfinkel says, which could explain the difference in the brain scans.

It is not at all clear whether this is a functional adaptation, but other studies show heartbeat can mediate other emotional functions, such as empathy and overt fear responses.