How the Motor System Responds to Rhythm
It can be argued that rhythm is the most essential property of music—indeed, while harmonic scales vary across cultures, rhythm can often traverse cultural bounds more easily. Rhythm is also an aspect of music that has a significant effect on the brain. You have probably noticed (and enjoyed) the integral link between movement and rhythm, whether it was while dancing to a heavy bass beat or while tapping your foot to keep better time while playing an instrument.
Learning and Rhythm
One of the most important shared mechanisms between musical and non-musical functions in motor control is rhythm and timing. Timing is essential to learning motor sequences, especially those pertaining to skilled motor activity. Without a sense of timing, a person cannot execute a movement skillfully or gracefully. Rhythm and timing are also essential elements of music. Both dancing and playing a musical instrument are key forms of movement in which the rhythm of music allows one to anticipate the appropriate timing of movement. Wen Jun Gao and Sarah L Pallas at Georgia State University have conducted research which suggests that the re-organization of circuits in the cortex, (essentially the process of learning), is influenced by “patterned sensory activity” such as the rhythmic presentation of clicks (Pallas, 2001).
At five months, a human fetus forms auditory neural circuits and auditory memories. During this time, one of the few stimuli that the baby is exposed to from within the womb is that of sound. The fetus is constantly experiencing the rhythm of the mother’s heartbeat and breathing, rhythms which influence the organization and pruning of the vast number of axonic connections in the developing baby’s brain to form auditory circuits and memories. The effect of a mother’s natural rhythm on the neural circuitry of her baby is thought to prepare the baby to develop its own natural rhythm and subsequent ability to execute motor sequences.
After birth, babies are constantly developing and smoothing out their basic motor abilities, such as reaching/grabbing, crawling, and eventually walking. It is thought that a key component of this learning process is the development of a personal motor cadence (the balanced or rhythmic flow of movement based on measure or beat), which most children maintain for the rest of their lives. This intrapersonal rhythm helps with timing and sequencing of motor processes and thus affects the balance and flow of movements to produce natural body movements.
Because humans tend to internalize rhythms that they hear, motor control (of both injured and noninjured brains) is influenced by external. Brain imaging studies have shown that when an individual makes a repetitive movement with a consistent rhythm (such as tapping fingers at one-second intervals), an area in the prefrontal motor cortex begins to become active at exact intervals in anticipation of the movement. The loss of motor control or the ability to initiate movement experienced by many stroke or Parkinsonian patients is often compensated for with external rhythmic cues, which allow the patients to synchronize motor sequences such as walking. By being attentive to rhythmic cues, patients are often able to walk faster and in a more controlled fashion. They also show improvements in motor activities that require more complex and nuanced control, such as limb coordination.
The most basic level of sound processing involves the amygdala, which produces an arousal (fight or flight) response when a startling auditory cue is perceived. An example of this is the automatic reaction we experience when we are surprised by a loud noise. Michael Thaut at Colorado University has done research in this area (Thaut, 2009). He proposes that our motor systems are so extraordinarily sensitive and attuned to external sounds because such sensitivity was originally an evolutionary adaptation. Because the reactivity of the motor system to sound promotes movement that ensures safety, it endowed an evolutionary advantage on our ancestors and was thus selected for. The connection between audition and the amygdala, which controls our fight or flight response, enables defensive behavior to be easily evoked by external sounds that are indicative of danger!
The auditory system also connects to the brain stem, midbrain, and higher cortical structures, regions which are all also essential to motor functioning. Lower brain regions initiate the signal to move and this signal eventually reaches our higher cortical circuits, at which point we experience the conscious desire or decision to instigate a movement. Based on this pathway we can infer that music may illicit movement by exciting the lower brain regions. Only after this signal travels to our higher cortical structures do we consciously perceive that we are moved to move by music. If you’ve ever noticed that you are tapping your foot in time to music without recalling ever consciously deciding to do so, you have experienced this phenomenon. The significance of this is that when we are tapping our fingers in tempo, the accuracy of our timing is not necessarily a conscious thought process, but rather ‘keeping time’ might be taken care of by our more basic mental faculties. This can be seen in specific cases such as that of one stroke victim names Sam. Sam was in therapy because he had an uneven gait and had to completely concentrate on his body in order to walk. When he started music therapy, music (and later just a consistent tempo) his ability to walk improved dramatically—in fact, at one point he even started dancing! Sam walked perfectly in tempo to music but was not consciously aware of this. Sam’s initial improvement in motor function was on a purely subconscious level; his subcortical functions were evidently activated prior to those regions of his brain involved with conscious motor planning (Tomaino, 2002).