Jan. 9, 2015

By Medical Discovery News

Have you ever noticed when someone in the audience can’t clap along with a beat at a concert? Well, it turns out that beat deafness actually exists. The first case was documented nearly five years ago, identified in a 26-year-old man who could not follow the beat at all when listening to music. Chances are, you don’t have it, though. Beat deafness is a form of musical brain disorder that is extremely rare.

Sometimes, audience members get so off beat that performers stop in an effort to get back on track. That in part inspired a group of neuroscientists in Montreal to look for people who felt they had no sense of the beat. After screening dozens of people, only one, Mathieu, was found to have true beat deafness.

Mathieu loves music, studies guitar, and once had a job as an amusement park mascot that involved dancing, which by his own admission did not go so well. “I just can’t figure out what’s rhythm, in fact,” Mathieu said. “I just can’t hear it, or I just can’t feel it.” However, he can follow the beat if he watches someone else. He could also follow the beat of a metronome, indicating that he did not have a movement disorder. In one test, Mathieu was asked to bounce or bend his knees to the beat of different kinds of music while holding a Wii controller that logged his movements. His results were compared to normal people who could identify the beat. After being tested with merengue, pop, rock, belly dancing, and techno music, he was only able to follow the distinct and obvious beats of techno music.

Rhythm appears to be sensed by a widespread network in the brain, not in a defined region like speech. Rhythm itself consists of several temporal elements such as pattern, meter, and tempo. Meter is the repeating cycles of strong and weak beats, pattern is the intervals at each point in time, and tempo is the frequency of underlying pulses. Each of these appears to be sensed differently and has been mapped to neural systems within the brain through positron emission tomography (PET) scans. It also appears that only humans can process meter, whereas other species may be able to process pattern and perhaps tempo. Distinct and distributed neural systems are also involved in sensing and processing other elements of music such as melody, harmony, and timbre.

When it comes to dancing to music, though, neural processing of rhythm is only the beginning. Orchestrated or planned movements start in the motor cortex, which is divided into sections that each govern a different part of the body. Signals from the motor cortex travel down 20 million nerve fibers in the spinal cord to an arm or finger, telling it to respond in a particular way.

To achieve a rhythmic, well-coordinated style of dance, the brain must coordinate all these efforts for joints to act in proper order and muscles to contract to the perfect degree. So as complex as all this is, perhaps it is not all that surprising that some people are better dancers than others.

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