Contagious Yawning is Hard-wired

It probably comes as no surprise that yawning is contagious. If you hear or see someone else yawn, there’s a good chance that you’ll want to yawn yourself. More specifically, contagious yawning is a type of echophenomena in which actions are imitated without awareness. Furthermore, contagious yawning is not only observed in humans, but in chimpanzees, monkeys, and dogs.

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In a 2017 study published in Current Biology, Brown and co-authors employ transcranial magnetic stimulation (TMS)—a noninvasive procedure that uses a magnetic field to activate the brain—to uncover a neural basis for contagious yawning. Of note, TMS has been used to treat depression, anxiety, psychosis, and other conditions.

Hypotheses

It’s been hypothesized that contagious yawning is linked to the disinhibition of the mirror neuron system (MNS), which is a group of neurons that mirror the actions and behaviors of other people. The MNS plays a role in social cognition, understanding, language, empathy, and synchronization of behaviors, in addition to different neuropsychiatric disorders. When evaluated using brain imaging studies, however, this hypothesis has received mixed support.

Alternatively, it’s hypothesized that contagious yawning and other echophenomena are tied to individual differences in the balance between motor excitability and physiological inhibition within the primary motor cortex. Brown and colleagues tested this second hypothesis.

Study design

The researchers showed 36 adults videos of people yawning. The viewing period was broken up into four blocks. During these blocks, the participants were asked either to yawn freely or suppress their yawns. The participants were recorded so that the number of full and stifled yawns could later be counted. Participants were also asked to record their current urge to yawn.

During the experiment, TMS was used to measure motor cortical excitability. During the last two blocks, the participants received transcranial electrical stimulation to the cortex. This stimulation increased the urge of participants to yawn.

Findings

Overall, the researchers found “that individual variability in the propensity for contagious yawning is determined by cortical excitability and physiological inhibition in the primary motor cortex.” In other words, the propensity to engage in contagious yawning is hard-wired into an individual’s brain. Furthermore, TMS measurements of motor excitability and physiological inhibition can be used to predict contagious yawning and accounted for about half the variability in contagious yawning.

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The researchers also showed that being instructed to resist the urge to yawn actually results in more yawning. This instruction also results in more yawns being stifled than fully expressed.

Notably, although the researchers found that TMS measures of cortical excitability and physiological inhibition are predictors of the propensity to yawn, they do not drive the urge to yawn, which may indicate the role of upstream brain areas, such as the anterior insular cortex and cingulate motor area.

Implications

The findings from this study help scientists better understand the connection between motor excitability and ecophenomena. Ecophenomena has been observed in various neurocognitive illnesses, including epilepsy, Tourette's syndrome, autism and dementia, which are all linked to increased cortical excitability as well as decreased physiological inhibition. Armed with this knowledge, experts may someday be able to reverse the pathology of such conditions using personalized TMS treatments to balance brain networks.

In a press release, Stephen R. Jackson (one of the study’s authors) states the following:

“If we can understand how alterations in cortical excitability give rise to neural disorders we can potentially reverse them. We are looking for potential non-drug, personalized treatments, using TMS that might be affective in modulating imbalances in the brain networks.”