How We Process Sound

Auditory Processing depends on several factors to achieve efficient and accurate transmission of sound

from the cochlea to the brain where it can be perceived and interpreted into meaningful information.

Binaural Hearing

Binaural (two ears) hearing creates an essential foundation for effective auditory processing.

85% of the auditory nerve fibres cross from the ear to auditory neural pathways on the opposite side of the brain, helping to create a "stereo surround-sound experience" for the listener in everyday listening environments.

Hearing sound with both ears allows the brain to make use of timing and intensity cues to locate the position of the sound source. These cues also enable more efficient separation of the target signal from unwanted background noise.

Auditory Neural Pathways

After the signal leaves the inner ear (cochlea) it begins the journey through the central auditory system. It travels along the afferent auditory neural pathways up to the primary auditory cortex. These structures include: the cochlear nucleus, superior olivary nucleus, the inferior colliculus and the medial geniculate nucleus. These structures act as relay stations to further refine and enhance the incoming auditory signal. The auditory system can also send signals back down to the peripheral auditory system via the efferent neural pathways to inhibit and suppress sounds. Together, these pathways play an essential part in the ability to hear and process sound.

The Auditory Cortex

As the signal travels up the afferent auditory pathway, auditory information is directed to the auditory cortex which is located on the upper part of the temporal lobe. The auditory cortex is subdivided into the primary auditory cortex and other belt areas. The primary cortex contains a precise spatial arrangement of where the sounds are processed in the brain (tonotopic organization).

The acoustic stimulus has already been organized tonotopically along the length of the basilar membrane in the cochlea (see How We Hear). This organization continues in most of the ascending auditory structures between the cochlea and the cortex.

Within the primary cortex is also a striped arrangement that contributes to binaural hearing. The neurons in one stripe are excited by both ears (and are therefore called EE cells), while the neurons in the next stripe are excited by one ear and inhibited by the other ear (EI cells). The EE and EI stripes alternate.

The sorts of processing that occur in the belt areas of the auditory cortex are not well understood, but they are believed to be important to higher-order processing of natural sounds, including those used for communication. It appears that some areas are specialized for processing combinations of frequencies, while others are specialized for processing modulations of amplitude or frequency.

Information for this page has been adapted from:

Neuroscience. 2nd edition.Purves D, Augustine GJ, Fitzpatrick D, et al., editors.Sunderland (MA): Sinauer Associates; 2001.