by Norman M. Weinberger
All known societies throughout the world have had music. Infants as young as two months will turn toward consonant, or pleasant, sounds and away from dissonant ones. And when a symphony's denouement gives delicious chills, the same kinds of pleasure centers of the brain light up as they do when eating chocolate, having sex or taking cocaine.
Therein lies an intriguing mystery: Why is music universally beloved and uniquely powerful in its ability to wring emotions? On the other hand, is music just "auditory cheesecake" - a happy accident of evolution that happens to tickle the brain's fancy?
Neuroscientists don't yet have the ultimate answers. But in recent years we have begun to gain a firmer understanding of where and how music is processed in the brain.
Studies have uncovered no specialized brain "center" for music. Rather music engages many areas distributed throughout the brain, including those that are normally involved in other kinds of cognition. The active areas vary with the person's individual experiences and musical training.
Imaging studies have given us a fairly fine-grained picture of the brain's responses to music. The processing of sounds, such as musical tones, begins with the inner ear (cochlea), which sorts complex sounds into their constituent elementary frequencies. The cochlea then transmits this information along separately tuned fibers of the auditory nerve as trains of neural discharges. Eventually these trains reach the auditory cortex in the temporal lobe.
Different cells in the auditory system of the brain respond best to certain frequencies; neighboring cells have overlapping turning curves so that there are no gaps. Indeed, the auditory cortex forms a "frequency map" across its surface.
The response to music per se, though, is more complicated. Music consists of a sequence of tones, and perception of it depends on grasping the relationships between sounds.
At one time, investigators suspected that cells tuned to a specific frequency always responded the same way when that frequency was detected.
But in the late 1980's Thomas M. McKenna and I, working in my laboratory at the University of California at Irvine, found that cell responses (the number of discharges) depended on the location of a given tone within a melody; cells may fire more vigorously when that tone is preceded by other tones rather than when it is the first.
Moreover, cells react differently to the same tone when it is part of an ascending contour (low to high tones) than when it is part of a descending or more complex one. The pattern of a melody matters: processing in the auditory system is not like the simple relaying of sound in a telephone or stereo system.
Most research has focused on melody, but rhythm, harmony and timber are also of interest. Studies of rhythm have concluded that one hemisphere is more involved, although they disagree on which hemisphere. The problem is that different rhythmic stimuli can demand different processing capacities. For example, the left temporal lobe (the center for technical and motor skills,) seems to process briefer stimuli than the right temporal lobe, and so would be more involved when the listener is trying to discern rhythm while hearing briefer musical sounds.
The situation is clearer for harmony. Imaging studies find greater activation in the auditory regions of the right temporal lobe (the center for emotional perceptions,) when subjects are focusing on aspects of harmony. Timber also has been "assigned" a right temporal lobe preference.
Brain responses also depend on the experiences and training of the listener. Even a little training can quickly alter the brain's reactions.
Learning retunes the brain so that more cells respond best to important sounds. This cellular adjustment process extends across the frequency map so that a greater area of the cortex processes tones.
The retuning is remarkably durable: it becomes stronger over time without additional training.
These findings initiated a growing body of research indicating that oe way the brain stores the learning importance of a stimulus is by devoting more brain cells to the processing of that stimulus.
The long-term effects of learning by retuning may help explain why we can quickly recognize a familiar melody in a noisy room and also why people suffering memory loss from Alzheimer's can still recall music that they learned in the past.
Even when incoming sound is absent, we all can "listen" by recalling a piece of music. Think of any piece you know and play it in your head. Where in the brain is this music playing? In 1999, McGill University scanned the brains of non-musicians who either listened to music or imagined hearing the same piece of music. Many of the same areas in the temporal lobes that were involved in listening to the melodies were also activated when those melodies were merely imagined.
Studies of musicians have confirmed the brain's ability to revise its wiring in support of musical activities. Just as some training increases the number of cells that respond to a sound when it becomes important, prolonged learning produces more marked responses and physical changes in the brain. Musicians show such an effect - their responses to music differ from those of non-musicians; they also exhibit hyper-development of certain areas in their brains.
Christo Pantev, at the University of Munster in Germany, found that when musicians listen to a piano playing, about 25 percent more of their left-hemisphere auditory regions respond than do so in non-musicians. This effect is specific to musical tones and does not occur with similar but non-musical sounds.
Moreover, this expansion of response area is greater the younger the age at which lessons began. Studies of children suggest that early musical experience may facilitate development.
Musicians may display greater responses to sounds, in part because their auditory cortex is more extensive - 130 percent larger. The percentages of increase were linked to levels of musical training, suggesting that learning music proportionally increases the number of neurons that process it.
In addition, musicians' brains devote more area toward motor control of the fingers used to play an instrument. The brain regions that receive sensory inputs from the index to pinkie fingers of the left hand were significantly larger in violinists; these are precisely the fingers used to make rapid and complex movements in violin playing. In contrast, they observed no enlargement of the areas of the cortex that handle inputs from the right hand, which controls the bow and requires no special finger movements. Non-musicians do not exhibit these differences.
Further, the brains of professional trumpet players react in such an intensified manner only to the sound of a trumpet - not, for example, to that of a violin.
Musicians also must develop greater ability to use both hands, particularly for keyboard playing. Thus, the anterior corpus callosum, which contains the band of fibers that interconnects the two motor areas, is larger in musicians than in non-musicians. Again, the extent of increase is greater the earlier the music lessons began.
The actual size of the motor cortex, as well as that of the cerebellum - a region at the back of the brain involved in motor coordination - is greater in musicians.
Beyond examining how the brain processes the auditory aspects of music, investigators are exploring how it evokes strong emotional reactions.
Pioneering work in 1991 revealed that more than 80 percent of sampled adults reported physical responses to music, including thrills, laughter or tears.
A 1997 study recorded heart rate, blood pressure, respiration and other physiological measures during the presentation of various pieces that were considered to express happiness, sadness, fear or tension. Each type of music elicited a different but consistent pattern of physiological change across subjects.
An imaging experiment in 2001 sought to better specify the brain regions involved in emotional reactions to music. OPET (positron emission tomography) imaging conducted while subjects listened to consonant or dissonant chords showed that different localized brain regions were involved in the emotional reactions. Consonant chords activated the orbitofrontal area (part of the reward system) of the right hemisphere and also part of an area below the corpus callosum. In contrast, dissonant chords activated the right parahippocampal gyrus. Thus, at least two systems, each dealing with a different type of emotion, are a work when the brain processes emotions related to music.
In the same year, when they scanned the brains of musicians who had chills of euphoria when listening to music, they found that music activated some of the same reward systems that are stimulated by food, sex and addictive drugs.
Overall, findings indicate that music has a biological basis and the brain has a functional organization for music. Many brain regions participate in specific aspects of music processing, whether apprehending a melody or evoking emotional reactions.
Musicians appear to have additional specialization, particularly hyper-development of some brain structures.
These effects demonstrate that learning retunes the brain, increasing both the responses of individual cells and the number of cells that react strongly to sounds that become important to an individual.
(edited by David Van Alstyne)
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