Biology 202
1999 Second Web Reports
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First, a discussion of the ear physiology is needed. Vibrating air moving at different frequencies hits the eardrum which causes the middle ear's three bones to move accordingly. The stapes, one of these inner ear bones hits on the oval window of the inner ear, and because the inner ear is filled with fluid, the bulging of the oval window causes this fluid to slosh around. The round window, also in the inner ear, compensates for the increased pressure by bulging outward. The inner ear has two functions, to transduce sound via the cochlea and to maintain a person's vertical position with respect to gravity via the vestibular system (1). . But here, we will only consider the transduction of sound. The cochlea is filled with hair cells that are extremely sensitive and depolarize with only slight perturbations of the inner ear fluid. At the point of depolarization, a neural signal is transmitted and on its way to the brain. This nerve impulse travels to the auditory nerve (8th cranial nerve), passes through the brainstem, and then reaches the branched path of the cochlear nucleus: the ventral cochlear nucleus or the dorsal cochlear nucleus. The nerve signal that passes through the ventral cochlear nucleus will reach the superior olive in the medulla where differences in timing and loudness of sound are compared, and location of the sound's origin is pinpointed (1). The nerve signal that crosses the dorsal cochlear nucleus ultimately is analyzed for sound quality.

As seen in the final step of sound transduction, the information relayed by the neural signal branches and processing occurs at different sights. No consensus has been reached as to where music is processed in the brain. Most researchers agree that the different components of music are processed in different parts of the brain, as exemplified by the branching pathway of the cochlear nucleus which facilitates the separation of sound timing and loudness with the sound quality analysis. But this information is not sufficient to answer the question of where our sense of music originates.

Frackwiak has supplied a small part of the puzzle. He conducted a study in which he had his subjects listened to different music tapes. When they listened to familiar music, the PET scan showed that the Broca's area was activated (2)., an area that is mostly associated with speaking. The rhythm tape also activated the Broca's area which can be explained by the fact that it also processes the cadence of spoken language (2).. When the timbre tape was played the right hemisphere of the brain was activated. This finding is often challenged by studies that show the left brain is activated. The pitch tape stimulated the left back side of the brain in an area called Precuneus which is associated with visual imagery (2). Frackwiak's study does not seem conclusive.

More compelling arguments better integrate the physiology of the ear with specific characteristics of music, and therefore eliminate the overlap of music and language processing. As mentioned in the description of sound transduction, the cochlea houses hair cells which depolarize to transduce sound information to the brain. These hair cells are actually organized like keys on the piano, sensitive to frequencies from low to high. When sound information reaches the auditory cortex, the neurons located there are also organized in this pattern of increasing frequency (3).

The implication of this built-in pattern in the brain is exciting because it suggests that the auditory system in humans is almost a complete replication of the natural phenomena of frequency variations. The twelve sites for music perception in the auditory cortex makes the system equally as complicated as language processing. The part of Frackwiak's study that exposed subjects to familiar music suggests that neurons "learn" to recognize tones and the groups of tones that work together. Depending on the culture in which an individual grows up in, the auditory system learns that culture's reoccurring musical themes (3). Therefore memory is closely related to music appreciation. Because neurons are programmed to remember musical patterns, the brain develops a "musical template". This template provides the listener with certain preconception of what music should be like. Thus a listener will expect certain frequencies to go well together (4). So this means that if a person hears dissonance in music, he/she will expect a following note that resolves the dissonance (4). This expectation of the sequence of notes draws on the frequency memory template in the auditory cortex.

WWW Sources

2)The Neural Orchestra

3)Musical studies provide clues to brain functions

4)The brain knows the score

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