"Feelings are contagious" has a literal meaning for dementia and Alzheimer's disease (AD) patients, who are more likely to mimic the feelings of their friends and family, according to a study published today in the Proceedings of the National Academy of Sciences (PNAS). The authors tie this behavior to the loss of brain cells involved with recognizing emotions.

When you feel happy around happy people or sad around those who are upset, research suggests you are reading and imitating the emotions in the facial expressions and body language of others. Neuroscientists refer to this empathy reflex as "emotional contagion,"as the feelings of others can cause a physical reaction in — or 'infect' — your mind.

We begin this habit almost from birth: infants will often mimic the facial expressions while playing. One study found newborns were more likely to cry after hearing the wails of other infants, but not after hearing a tape recording of their own cries. After a while, we start to develop a clearer sense of self and individuality, and as a result, become less of a mimic.

But if you skip ahead 60+ years to the early stages of Alzheimer's disease, behaviors revert. Alzheimer's is thought to harm the brain regions that govern the ability to recognize and process emotions. It is unknown, however, how this neurological disease impacts emotional contagion.

Researchers from the University of California, San Francisco used a psychological survey to test 120 patients with Alzheimer's or mild cognitive impairment, a form of dementia, for their levels of emotional empathy. One hundred and eleven healthy controls were also recruited. The study found that patients with dementia, especially those with Alzheimer's, were prone to mimicking the emotions of their peers. Men and women were equally as likely to participate in this behavior.

Structural damage was discovered — via magnetic resonanance imaging (MRI) — in part of the brain network that controls emotional reactions, the medial temporal lobe. Although both sides of the brain have medial temporal lobes, emotional contagion was linked to atrophy in the right hemisphere. Damage was also recorded in the hippocampus, a brain area that controls memory recall and known to degenerate in Alzheimer's disease.

Changes in emotional behavior are common in dementia, with three out of four Alzheimer's patients displaying symptoms of psychiatric disease, namely depression. The presence of depression, however, did not correlate with emotional copycatting in these patients, suggesting the two are governed by different brain regions.

On the other hand, emotional mimicry may have good and bad consequences for people with dementia. While the investigators only measured negative feelings of anxiety and discomfort in this study, happiness is thought to be 'infectious' as well. Their finding suggests that adopting a cheerful attitude when visiting an afflicted family member with could help relieve anxiety.

In order to retain their social graces and interpersonal relationships, emotional copycats may also mask their declining mental health by feeding off the demeanors of others. The authors conclude that further research is needed to determine if 'emotional contagion' can be used as a possible readout for dementia.

Source: Sturm VE, Yokoyama JS, Seeley WW, Kramer JH, Miller BL, and Rankin KP. Heightened emotional contagion in mild cognitive impairment and Alzheimer's disease is associated with temporal lobe degeneration. PNAS. 2013