Whether we are home alone or at a cocktail party, electrical signals are always busy firing across our brain. When examined with electrodes, these signals appear as oscillating patterns—brain rhythms that change depending on what we are doing and thinking. Researchers have recently identified a new rhythm that appears during social interactions, offering a glimpse of the coordination that takes place within, as well as between, brains.

Neuroscientists Emmanuelle Tognoli and J. A. Scott Kelso and their colleagues at Florida Atlantic University simultaneously recorded, for the first time, the brain rhythms of two people sitting across from each other (below) as they each moved one of their fingers up and down. When a barrier prevented the volunteers from seeing each other's actions, there was no trace of the newly identified rhythm, dubbed phi. When the scientists removed the visual obstruction, however, phi appeared, originating from one of the regions associated with mirror neurons. Phi's complex rhythm differed depending on whether the volunteers maintained their independent finger wagging or synced up with the other person. “Phi distinguishes whether a person does their own thing or whether they coordinate with others,” Kelso says.

Brain rhythms arise when groups of neurons in distant regions of the brain synchronize to generate cognitive activities such as information processing, sleep and memory. When incoming electrical messages are absorbed by tens of thousands of neurons simultaneously engaged in the same process, the resulting electrical charge is strong enough that electrodes on the scalp can detect it.

Once scientists determine a brain rhythm's function, they can make predictions about how it will behave in a variety of situations, including when the brain is diseased or damaged. Many patients with neurological disorders are able to monitor and adjust their own rhythms with biofeedback training, which can sometimes alleviate debilitating symptoms. The discovery of phi may help diagnose and treat autism and schizophrenia, diseases in which social interactions prove difficult.