4. THE BRAIN
AUDITORY CORTEX
Bilateral: situated in both left and right temporal lobes.
Comprised of three layers of neurons: primary, secondary & tertiary organized in concentric circles. The primary layer (in the center) consists of columns of neurons tonotopically arranged.
TONOTOPICAL MAPPING: Frequential locations on the basilar membrane (inner ear) are mapped onto locations in the primary auditory cortex.
Higher frequencies are recognized near the center of the Primary Auditory Cortex; lower frequencies near the boundaries.
Cells in the Secondary (BELT) and tertiary (PARABELT) auditory regions only respond weakly to neuronal signals when pure tones are detected by the cochlea but respond to complex tones that distinguish what from where (e.g. identifying the sound of instrument independently of its location in space).
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“When we pay attention to or focus on a sound, this temporarily enlarges its cortical representation, and it becomes sharper and clearer, at least for a second or two.” (Oliver Sacks)
B. EFFERENT PATHWAY (brain to ear)
Not as well understood!
It is responsible for the cocktail party effect (the ability to pick out a specific sound in a confused mass). Cochlear function is effectively modulated by the brain.
It is also tasked with:
- the detection of signals in noise;
- protection in noise-induced cochlear damage;
- assisting in the processing of complex auditory signals;
- sending inhibitatory messages to the outer hair cells in the cochlea to control stiffness and membrane vibration.
NOTE: The above-information cf. Psychology of Music (Pfordesher, Harré, Tan) & Google Images