Brain and Behavior

Sensation and Perception

How does the brain process sound?

The ear is composed of three sections, the outer ear, the middle ear, and the inner ear. The outer flap that we normally think of as the ear is called the pinna. That and the ear canal, the long inner tunnel where our wax collects, comprise the outer ear. The ear canal ends in the ear drum, a thin membrane that stretches across the back end of the ear canal. The middle ear consists of three tiny, delicate bones that transfer sound vibrations from the ear drum to the cochlea, a fluid-filled spiral that translates sound vibrations into neural activity. The cochlea and the vestibular apparatus, which senses balance and motion, comprise the inner ear. Sound vibrations are captured by the pinna, channeled back to the middle ear by the ear canal, and communicated to the cochlea via the bones of the middle ear.

The fluid-filled cochlea is lined with hair cells, a type of sensory neuron that responds to sounds of specific frequencies. High frequencies (which translate into high pitched sounds) are recorded by hair cells at the opening of the spiral cochlea, middle frequencies in the middle of the cochlea, and low frequencies by hair cells at the end of the spiral. The hair cells send information to the spinal cord, which then connects to several regions in the brain stem. Here the auditory information is processed for the timing and intensity. These neurons connect to the inferior colliculus in the mid-brain, where some further analysis takes place. The inferior colliculus connects to the gate-keeping thalamus. Finally, the auditory information enters the cortex at the primary auditory cortex (A1), located in the superior and posterior (upper rear) temporal lobe. Association areas are nearby, including the regions that deal with language.


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