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Auditory Physiology
TONOTOPY
The topographic organization of sound:
- Cochlea: Base - high-frequency sounds, Apex - low-frequency sounds.
- Cochlear nuclei (ventral and dorsal): Anterior - low-frequency sounds, Posterior - high-frequency sounds.
- Transverse temporal gyri (Heschl's gyri, primary auditory cortex): Lateral - low-frequency sounds. Medial - high-frequency sounds.
- Each cerebral hemisphere receives similar input from each set of cochlear nuclei, so when there is a unilateral lesion to any of the steps in the central auditory pathway, hearing is preserved from the duplication of auditory information in the contralateral cerebral hemisphere.
- Tonotopy informs the brain about sound frequency but not location. Sound reaches each ear (the near ear and far ear) at slightly different times and intensities.
- Two main interaural (between ear) differences are the interaural time difference and the interaural level difference - (Time refers to the time the sound reaches each ear. Level refers to the sound intensity level at each ear.)
- Sound reaches the near ear before it reaches the far ear.
- Difference best detected for low-frequency sounds
- The head attenuates sound before it reaches the far ear.
- Difference best detected for high-frequency sounds.
- Low-frequency sounds: Encode within the spherical bushy cells of the ventral cochlear nucleus, which project bilaterally to the medial superior olivary nuclei - sensitive to interaural time differences.
- High-frequency sounds: Encode within the ventral cochlear nucleus in both the spherical bushy cells and globular bushy cells; both cell types project to the lateral superior olivary nucleus - sensitive to interaural level differences.
- Spherical bushy cells send exclusively ipsilateral, excitatory projections to the lateral superior olivary nucleus.
- Globular bushy cells send excitatory projections to the contralateral medial nucleus of the trapezoid body, which in turn sends inhibitory projections to the ipsilateral lateral superior olivary nucleus (thus the globular bushy cells inhibit the contralateral lateral superior olivary nucleus via the medial nucleus of the trapezoid body).
- Relies on the head-related transfer function, which shapes sound waves as they propagate toward the eardrum. The head, torso, and pinna distort sound waves into spectral patterns that the central auditory system uses in the localization of sound.