Brain States

Fear Perception Depends on Heartbeat Cycle Timing

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It always amazes me how much our reactions to the world depend on our internal states, even when those states are outside of our conscious awareness. Though we don’t typically consider it a sense like hearing or seeing, the visceral sense is real: there are tons of receptors inside us, monitoring our internal physiology at all times.

One example is the baroreceptors that monitor the pressure inside our arteries as the heart pumps blood through them. The baroreceptors have a major role in the regulation of blood pressure, as one might expect. However, recent research has revealed that information from these internal receptors is also being used when we process emotional stimuli.

Sarah Garfinkle and colleagues investigated this phenomenon recently by showing people images of human faces, either displaying the emotions of fear, disgust, or happiness, or in a neutral state of repose. The facial expressions were embedded in a stream of scrambled images, and were flashed by so quickly that they were at the limit of conscious perception: each frame occupied the screen for a mere 70 milliseconds, or less than a tenth of a second. The facial expressions were timed to coincide with the cardiac cycle, either landing at systole (between the “lub” and “dub” of a heartbeat, when arterial pressure is high) or at diastole (between heartbeats, when arterial pressure is low).

Afterward, participants were asked to identify which face they had seen in a lineup of three faces. In general, the participants were pretty bad at this test, getting it right only about half the time. But for fearful faces especially, participants were much better at picking out the face they had seen when they saw it in the middle of the heartbeat, rather than between them. Being at the “right” time with respect to the heartbeat pushed detection of the fearful face over the edge from subconscious into consciousness.

So people were better able to detect a fearful face when it came in the middle of a heartbeat, but what about how people thought of the face they saw? The scientists assessed this in a separate experiment, at the same time doing functional neuroimaging on the participants to see what their brains were doing. Again, they showed participants a fearful face, either timed to coincide with systole or diastole. This time, the face was on the screen for a bit longer, so the participants were sure to detect it. Afterward the experimenters asked the participants to rate the intensity of the emotion they saw.

Again, the results depended on the heartbeat cycle timing: if the face was presented in the middle of a heartbeat, participants rated it as more fearful than if it was presented between heartbeats.  The difference was not just perceptual, but also was evident in the activity of a brain structure called the amygdala. The amygdala is known be involved in the processing of emotion, and this area was more active when the participants saw the fearful face at systole than at diastole.

Interestingly, this effect was less marked for individuals who were anxious. People with high anxiety conditions like PTSD are more responsive to fearful stimuli, regardless of when it comes with respect to the cardiac cycle.

This work is just one example of the dynamic feedback between our physiological and emotional states. For decades, neuroscientists have debated the nature of emotions: does nervousness cause butterflies in the stomach, or do butterflies in the stomach create the feeling of nervousness? Which comes first, the physiological or the psychological? We may never have a clear-cut answer to this question, because the there may not be a clear-cut answer. It seems that the road between the body and the mind is a two-way street.

Reference: Fear from the Heart: Sensitivity to Fear Stimuli Depends on Individual Heartbeats. (2014) Sarah N. Garfinkel, Ludovico Minati, Marcus A. Gray, Anil K. Seth, Raymond J. Dolan, and Hugo D. Critchley. Journal of Neruoscience 34(19): 6573-6582

 

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