Neurobiology of Harmony

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Neurobiology of Harmony

How sound waves produced by instruments become sensible representations in the brain, and how the perceptions become meaningful are interesting questions for neurobiology to ask, as well as necessary ones if knowledge of the brain is to account for all behavior. The brain is able to discern harmony because the inner ear is capable of differentiating between different frequencies. The brain's differentiation between pitches and chords corresponds to the physical, "real," differences between notes and chords, although our sense of music built from perception of harmonies through time, is more subjective and variable.

Our faculty of hearing derives from the anatomy of the inner ear and the brain, as well as from the existence of external stimuli in the outside world. Sound is both the mechanical energy of waves and the sensation produced by receptors in the brain (1). Each wave has an amplitude and a frequency. The amplitude of a vibration corresponds to its volume and is measured by decibels on a logarithmic scale. Frequency is logarithmic, as well, but corresponds to differences in pitch. Greater frequency results in a higher pitch. Mathematically, pitch is represented as the number of vibrations per second (1) (2) .

Vertebrates hear sound through their neurobiological makeup. The ear's tympanic membrane, or eardrum, vibrates as a result of being subjected to sound waves. The waves then travel to the inner ear or cochlea which is the site of sound's transduction into chemical energy. Within the cochlea, sound waves travel through fluid which stimulates the stereocilia, small hair-like projections of hair cells along the basilar membrane. The actions of the stereocilia cause the release of K+, potentially depolarizing the cell (1). The flexibility of the basilar membrane allows stereocilia to move back and forth in response to the waves in the Cochlear fluid. Each stereocilium is linked to another through structures called "tip links" (1) , (3) As the stereocilia move towards the tallest ones, the tip links cause ion channels to open, depolarizing the cell and allowing free K+ to move into the cell (1). Importantly, the stereocilia move in direct response to the sound waves and are cumulative rather than spiking. Neurotransmitter release corresponds to the frequency and amplitude (pitch and volume) of a sound input. Sounds must be sufficiently loud and within a given range in order to cause action potentials. Different sounds will produce different outputs, allowing for discrimination of harmony on a neural level (1).
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