Notes on sound

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.

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.