This article was first published in my blog on the Scientific American Website.  The sound clip of the rhythm used in these experiments could not be included in that post, but it is available here:   .

R. Douglas Fields—New Orleans, October 16, 2012

You walk into a bar and music is thumping.  All heads are bobbing and feet tapping in synchrony.  Somehow the rhythmic sound grabs control of the brains of everyone in the room forcing them to operate simultaneously and perform the same behaviors in synchrony.  How is this possible?  Is this unconscious mind control by rhythmic sound only driving our bodily motions, or could it be affecting deeper mental processes?

The mystery runs deeper than previously thought, according to psychologist Annett Schirmer reporting new findings today at the Society for Neuroscience meeting in New Orleans.  Rhythmic sound “not only coordinates the behavior of people in a group, it also coordinates their thinking—the mental processes of individuals in the group become synchronized.”

This finding extends the well-known power of music to tap into brain circuits controlling emotion and movement, to actually control the brain circuitry of sensory perception.  This discovery helps explain how drums unite tribes in ceremony, why armies march to bugle and drum into battle, why worship and ceremonies are infused by song, why speech is rhythmic, punctuated by rhythms of emphasis on particular syllables and words, and perhaps why we dance.

Schirmer and her graduate student Nicolas Escoffier from the University of Singapore first tested subjects by flashing a series of images on a video monitor and asked them to quickly identify when an image was flipped upside down.  While participants focused on this task, a synthetic drumbeat gently tapped out a simple four-beat rhythm in the background, syncopated by skipping the fourth beat of each measure. 

The results showed that when the image was flashed on that missed beat, the subjects identified the inverted image much faster than when the image was flashed at times out of synch with the beat or when the images were presented in silence.  Somehow, the brain’s decision making was accelerated by the external auditory rhythm and heightened at precise points in synchrony with the beat.  Since the power of rhythm in boosting cognitive performance was evident on the missing beat when no sound was presented, the effect could not have had anything to do with the sound of the drumbeat acting as a stimulus.  Mental processing must have fallen into a rhythm of heightened expectation and superior performance on the anticipated beat.

Next the researchers attached electrodes to the scalp of such subjects to determine if the brain’s electrical activity was somehow affected by the rhythm of the sound.  The EEG recording detects the combined electrical activity of thousands of neurons working together in the cerebral cortex.  Just like the roar of a crowd at a baseball game, waves of electrical activity in the brain are generated when individual neurons in the cerebral cortex are combined in action.  The EEG recordings showed that the waves of brain activity (alpha and beta waves) became synchronized around the auditory rhythm.  That is, the ongoing oscillations of brain waves became phase shifted so that the peak of the wave always occurred at a precise point relative to the next beat in the drum rhythm.  Rhythmic sound synchronizes brain waves.

The brain wave recordings also revealed a more surprising effect of rhythmic sound on brain function.  Any sensory stimulus, such as seeing a picture or hearing a sound, will generate a brief brain wave in the region of cerebral cortex where such information is received and processed, much like the crack of a bat at home plate causes an eruption of cheers in a stadium.  The researchers found that the sensory-evoked brain wave measured at the back of the skull over the region where vision is processed, peak each time the image was presented, but when the image was presented simultaneously with the missing drumbeat, the electrical response evoked by the picture was bigger than when the image was presented out of rhythm or flashed on the screen in silence.  These visual circuits are more responsive when the image appears in synch with the auditory rhythm.

This region of the brain processes the earliest steps in vision, the circuits that detect visual input.  This means that our perception of the external world entering our mind through our eyes is affected by the rhythm of what we hear.  Something seen at a point precisely in beat with an auditory rhythm is more likely to be perceived than if it appears out of synch with the rhythm.  This gating of visual input by auditory rhythm does not require a prolonged meditation on the rhythm to cause the person to enter into some sort of a trance-like state; the effects are nearly instantaneous.  “Within a few measures of music your brain waves start to get in synch with the rhythm,” Schirmer says.

Steven Pinker has said that music is “auditory cheesecake,” with no particular advantage in the evolution of our species.  Schirmer feels their new results do not support that view.  “Rhythm facilitates our interpersonal interactions in term of not only how we move, but how we talk and think,” she concludes.  “Rhythm facilitates people interacting by synchronizing brain waves and boosting performance of perception of what the other person is saying and doing at a particular point in time.”  Rhythm, whether the lyrics to a song or the meter of a poem facilitates language processing, she concludes, and she is now undertaking new experiments to further test this idea.  “When people move in synchrony they are more likely to perceive the world in synchrony, so that would facilitate their ability to interact.”