Strong emotions are infectious, causing other people to share similar feelings to the ones you have. Next time you are feeling incredibly happy, you may be thanking the person sitting next to you.

Strong positive or negative emotions can trigger a similar response across individuals. Seeing someone smile or frown can trigger similar responses in the observer. This is more than an outward gesture, as the brain's activity is altered by emotional responses.

The study, led by researchers from the Aalto University and Turku PET Centre, discovered the brain-synchronizing powers of emotions in humans using MRI's to measure brain activity. Participants watched movies that featured unpleasant, neutral and pleasant emotions twice, once while being scanned with an MRI and a second time that had participants rate their levels of pleasantness-unpleasantness and arousal-calmness.

Based on the MRI, strong unpleasant emotions affected and synchronized the frontal and midline regions of the brain, which are a part of the brain's emotional processing networks. Strong pleasant emotions synchronized parts of the brain are a part of the visual network, attention network and affect sense of touch.

Researchers believe emotional synchronicity may play a role in society and social interactions. By judging emotions, be it happiness or anger, a person can feel those same feelings and act accordingly. The synchronizing between can inform people about each other's intentions or actions which can affect group processes.

Strong negative emotions synchronize the emotional processing networks of the brain. In a group, this can help interpret the behavior of an individual or can be used to understand the actions of an individual as well as determine possible risks or rewards. Strong negative emotions affect the thalamus, which plays a role in relaying motor signals and controlling alertness, as well as the ventral striatum, which is associated with motivational behavior and the insula, which also is associated with motor control and perception. Strong negative emotions also affect the medial prefrontal cortex which allows us to organize thought and judge actions based on goals and rewards.

Strong positive emotions affect parts of the brain that play a role in sensation which could help direct attention to environmental cues related to those feelings. Seeing someone smile may increase sensational awareness which may help a person recognize their surrounding environment and hone in on what is causing those positive emotions, such as a child play or a cool breeze on a hot day.

Emotions are an important part in group dynamics and how individuals relate to one another. Synchronicity may play an important role in society, so the next time you see someone who looks sad, throw a smile their way; it's contagious.

The study was published in the Proceedings of the National Academy of Sciences.